astable multivibrator using ic 555 experiment pdf

Astable Multivibrator using 555 Timer IC

In this article, we are explaining an astable multivibrator, often called a free-running multivibrator.  An astable multivibrator can be designed using different types of components, say using transistors (and associated components) alone or by using op amps (and associated components). In this article, we are designing an Astable Multivibrator using 555 Timer IC.

If you are wondering what all are the practical applications of an astable multivibrator, lets begin by seeing a few. One of the most common application for an astable multivibrator is to generate time delays . Let’s say you want to turn an LED “ ON ” for 1 second and keep it “ OFF ” for 0.5 seconds, then an astable multivibrator is the best circuit you could use to build this application.

Unlike the monostable multivibrator, this circuit does not require any ex­ternal trigger to change the state of the output, hence the name free-running. Before going to make the circuit, make sure your 555 IC is working. For that go through the article: How to test a 555 IC for working An astable multivibrator can be produced by adding resistors and a capacitor to the basic timer IC, as illustrated in figure. The timing during which the output is either high or low is determined by the externally connected two resistors and a capacitor. The details of the astable multivibrator circuit are given below.

So let’s begin to design an astable multivibrators using 555 timer IC. Let’s first draw the 555 timer astable multivibrator circuit.

555 Timer Astable Multivibrator Circuit Diagram

An Astable Multivibrator can be designed by adding two resistors (RA and RB in circuit diagram) and a capacitor (C in circuit diagram) to the 555 Timer IC. These two resistors and the capacitor (values) are selected appropriately so as to obtain the desired ‘ON’ and ‘OFF’ timings at the output terminal (pin 3). So basically, the ON and OFF time at the output (i.e the ‘HIGH’ and ‘LOW’ state at the output terminal) is dependent on the values chosen for RA,RB and C. We will see more about this on the astable multivibrator design section given below.

Note:- The capacitor C2 (0.01uF) is connected to pin number 5 (Control Voltage Terminal) in all 555 IC based circuits in which that particular pin (pin 5 – control voltage terminal)is not used. This capacitor is used to avoid noise problems that could arise in the circuit if that pin is left open.

Take a look @ 555 Ic Pin configuration and 555 block diagram before reading further.

The Connections

Let’s see how the 555 timer astable multivibrator connections are made in the circuit diagram.

Pin 1 is grounded; pins 4 and 8 are shorted and then tied to supply +Vcc , Output ( Vout ) is taken from pin 3 ; pin 2 and 6 are shorted and then connected to ground through the capacitor C to ground terminal , pin 7 is connected to supply + V CC through a resistor R A ; and between pin 6 and 7 a resistor R B is connected . At pin 5 either a bypass capacitor (to bypass noise signals) of 0.01uF is connected or modulation input is applied.

Astable Multivibrator Working

So far we have seen how an astable multivibrator is designed using 555 timer IC, the circuit diagram and how the pin connections are made. Now let’s see the operation and working of an astable multivibrator. To explain the 555 timer astable multivibrator working, we have drawn an internal circuit diagram of 555 timer IC (consisting of two Op Amps, an SR Flip Flop and the transistor connected at the discharging terminal – pin 7) along with the necessary external connections (RA, RB and C). Wave forms from the output terminal (Vout – pin 3) is shown towards the right side of the circuit diagram.  Carefully observe the circuit diagram and the output waveforms before we begin the explanation.

The basic objective of an astable multivibrator is to switch the output status (from HIGH to LOW and from LOW to HIGH) at the desired time intervals, without any external intervention (say an input trigger pulse like in the case of monostable multivibrator). We achieve this (in a 555 IC) by controlling the discharge terminal (pin 7) of 555 IC through a capacitor (C). Inside the 555 IC, this discharge terminal (pin 7) is connected to the collector terminal of a transistor whose base is directly connected to the output terminal (non inverting terminal – Q) of SR flip flop. You have to notice that Vout (pin 3 – output terminal of 555 IC) is taken from the inverting output terminal (Q complimentary terminal) of SR flip flop. So when when flip flop output (non inverting) Q is HIGH, Vout will be LOW and when flip flop output Q is LOW, Vout will be HIGH.

Now let’s see how the automatic switching of ON and OFF state at the Vout terminal is achieved by the capacitor C connected to the dicharge terminal -pin 7.

You may also recall the Block Diagram (given below) and the Working Principle of 555 IC , explained in our previous chapter/tutorial on 555 Timer.

The block diagram of a 555 timer is shown in the above figure. A 555 timer has two comparators (which are basically 2 op-amps), an R-S flip-flop, two transistors and a resistive network.

• The Resistive network consists of three equal resistors (5K Ohms each) and acts as a voltage divider. Notice that the resistor network is designed in such a way that the voltage at the Inverting terminal of Comparator 1 will be 2/3Vcc and the voltage at the Non Inverting terminal of Comparator 2 will be 1/3Vcc.
• Comparator 1 – compares the threshold voltage (at pin 6) with the reference voltage + 2/3 V CC volts.
• Comparator 2 – compares the trigger voltage (at pin 2) with the reference voltage + 1/3 V CC volts.

For the sake of explanation, lets assume the circuit is powered up and right now the status at non inverting flip flop output – Q is LOW. When Q is LOW, Vout will be HIGH (which we call as Timer Output).  You see that Q is directly connected to base of transistor (at the discharge terminal). So when Q is LOW, transistor will be in its cut off state (OFF state). In this state, capacitor C is directly connected to power supply Vcc through resistors RA and RB. So the capacitor will start charging towards the supply voltage Vcc and the charging time constant will be defined by RA and RB values as (RA+RB)*C. The capacitor will charge towards Vcc and this will increase the threshold voltage (voltage across pin 6) of 555 IC. When the capacitor charges upto 2/3Vcc and beyond, the threshold voltage will also surpass 2/3Vcc level and this will force the op amp output (comparator 1) to go HIGH (take note that the reference voltage at – terminal of comparator 1 is 2/3Vcc).  Since the op amp output of comparator 1 is connected ‘S’ (SET input) of SR flip flop, the flip flop will be triggered and the Q output (non inverting output) of flip flop will turn HIGH. You got upto this? You may recall that we have begun this explanation by assuming Q is LOW initially. Now as a result of capacitor charging, Q has turned HIGH automatically from LOW. When Q goes HIGH, Vout will automatically go to LOW as Vout is nothing but a compliment of Q.

When Q is HIGH, the transistor at pin 7 (the discharge terminal) will be turned ON and the transistor will get saturated. When the transistor gets saturated, the pin 7  (discharge terminal) will act as a ground for the capacitor.As a result, a new path is available for the capacitor to discharge from 2/3Vcc level to zero volts. The capacitor will start discharging through the new path (through RB) and this will result in decrease in voltage across the trigger terminal (pin 2) of 555 IC. The discharging time constant is defined by RB*C. Once the capacitor discharges to a level below 1/3Vcc, resulting in the same voltage (voltage of capacitor) across the trigger terminal (notice that the reference input voltage at + terminal of comparator 2 is 1/3Vcc), the op amp output of comparator 2 will go HIGH. Since the output of comparator 2 is connected to ‘R’ – the RESET input terminal of SR flip flop, the Q output of flip flop will go from HIGH to LOW. When Q goes to LOW, Vout will automatically go to HIGH. Thus an automatic transition from HIGH to LOW and then from LOW to HIGH is achieved in an Astable Multivibrator. The cycle repeats.

We have successfully completed the working explanation of an astable multivibrator using 555 IC. You can see the output wave forms in the diagrams given above. The two important parameters that we should understand from the output of timer is ON Time ( T HIGH ) and OFF Time ( T L0W ).

ON Time – is the time for which the timer output Vout has remained in HIGH State. We notate this with T HIGH.

OFF Time – is the time for which the timer output Vout has remained in LOW state. We notate this with T L0W .

ON Time and OFF Time are dependent on the values of  R A, R B  and  C . So we can obtain the desired ON Time and OFF Time at the timer output with the proper calculation of   R A, R B  and  C values.

A stable Multivibrator using 555 – Design Theory

The time during which the capacitor C charges from 1 /3 V CC to 2/3 V CC is equal to the time the output is high and is given as  t c or T HIGH = 0.693 (R A + R B ) C , which is proved below.

Voltage across the capacitor at any instant during charging period is given as, v c =V CC (1-e t/RC )

The time taken by the capacitor to charge from 0 to +1/3 V CC

1/3 V CC = V CC (1-e t/RC )

The time taken by the capacitor to charge from 0 to +2/3 V CC

or t 2 = RC log e 3 = 1.0986 RC

So the time taken by the capacitor to charge from +1/3 V CC to +2/3 V CC

t c = (t 2 – t 1 ) =  (10986 – 0.405) RC = 0.693 RC

Substituting R = (R A + R B ) in above equation we have

T HIGH = t c = 0.693 (R A + R B ) C

where R A and R B are in ohms and C is in farads.

The time during which the capacitor discharges from +2/3 V CC to +1/3 V CC is equal to

the time the output is low and is given as

t d or  T L0W = 0.693 R B C where R B is in ohms and C is in farads The above equation is worked out as follows: Voltage across the capacitor at any instant during discharging period is given as

v c = 2/3 V CC e – t d/ R B C

Substituting v c = 1/3 V CC and t = t d in above equation we have

+1/3 V CC = +2/3 V CC e – t d/ R B C

Or   t d = 0.693 R B C

Overall period of oscillations, T = T HIGH + T LOW = 0.693 (R A + 2R B ) C , The frequency of oscillations being the reciprocal of the overall period  of oscillations T is given as

f = 1/T = 1.44/ (R A + 2R B )C

Equation indicates that the frequency of oscillation / is independent of the collector supply voltage +V CC .

Often the term duty cycle is used in conjunction with the astable multivibrator.

The duty cycle, the ratio of the time t c during which the output is high to the total time period T is given as

% duty cycle, D = t c / T * 100 = (R A + R B ) / (R A + 2R B ) * 100

From the above equation it is obvious that square wave (50 % duty cycle) output can not be obtained unless R A is made zero. However, there is a danger in shorting resistance R A to zero. With R A = 0 ohm, terminal 7 is directly connected to + V CC . During the discharging of capacitor through R B and transistor, an extra current will be supplied to the transistor from V CC through a short between pin 7 and +V CC . It may damage the transistor and hence the timer.

However, a symmetrical square wave can be obtained if a diode is connected across resistor R B , as illustrated in dotted lines in figure. The capacitor C charges through R A and diode D to approximately + 2/3V CC and discharges through resistor R B and terminal 7 (transistor) until the capacitor voltage drops to 1/3 V CC . Then the cycle is repeated. To obtain a square wave output, R A must be a combination of a fixed resistor R and a pot, so that the pot can be adjusted to give the exact square wave.

Although the timer 555 has been used in a wide variety of often unique applications it is very hard on its power supply lines, requiring quite a bit of current, and injecting many noise transients. This noise will often be coupled into adjacent ICs falsely triggering them. The 7555 is a CMOS version of the 555. Its quiescent current requirements are considerably lower than that of 555, and the 7555 does not contaminate the power supply lines. It is pin compatible with the 555. So this CMOS version of the 555 should be the first choice when a 555 timer IC is to be used.

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why the capacitor charges from 1/3 vcc to 2/3 vcc?

it is very useful to me thank to the whole team…

Great! Very Clear explanations. Thanks a lot! 🙂

When we use diode D then capacitor C will not charge through Rb. The diode bypasses the Rb. Then duty cycle will become Ra/(Ra+Rb). here neglected the diode drop. THis is right know..?

Thanks for information of astable mode.

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sir i want to knowthat why in astable multibrator operation in capacitor and output voltage waveform the 0 volts and 1/3 Vcc not lies on the time axis -t? please reply me quickly.

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I m really thankful to circuitstoday.com n all the things mentioned in Abhishek Chandel’s comment r wrong.

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as i under stand the question the answer is, the Astable multivibrator will give continuous oscillation frequency. mono stable will produce an output pulse of definite width (even though the input goes high and stays for a long period the output will go high and stay in that state only for a short defined duration and go low after that, hence a defined pulse width). bistable will change its state from low to high when the first input pulse appears. with the second pulse it will reset to low from high. thus it will have two stable condition one high and one low always till the inputpulse changes the state.

hey i want to know about astable multivibrator as IR transmitter. so plz give me the descriptive working & circuit diagram as soon as possible.

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Experiment No: 14 ASTABLE MULTIVIBRATOR USING IC

Experiment No: 14

ASTABLE MULTIVIBRATOR USING IC 555

To design and set up astable multivibrator of 1000 Hz frequency and 60% duty cycle using IC 555

IC 555 timer is an analog IC used for generating accurate time delay or oscillations. The entire circuit is usually housed in an 8-pin package as specified in figures 1 & 2 below. A series connection of three resistors inside the IC sets the reference voltage levels to the two 2 1 comparators at V and V , the output of these comparators setting or resetting the flip- 3 CC 3 CC flop unit. The output of the flip-flop circuit is then brought out through an output buffer stage. In the stable state the 푄 output of the flip-flop is high (ie Q low). This makes the output (pin 3) low because of the buffer which basically is an inverter.The flip-flop circuit also operates a transistor inside the IC, the transistor collector usually being driven low to discharge a timing capacitor connected at pin 7. The description of each pin s described below,

Pin 1: (Ground): Supply ground is connected to this pin. Pin 2: (Trigger): This pin is used to give the trigger input in monostable multivibrator. When trigger of amplitude greater than (1/3)Vcc is applied to this terminal circuit switches to quasi-stable state. Pin 3: (Output) Pin 4 (Reset): This pin is used to reset the output irrespective of input. A logic low at this pin will reset output. For normal operation pin 4 is connected to Vcc. Pin 5 (Control): Voltage applied to this terminal will control the instant at which the comparator switches, hence the pulse width of the output. When this pin is not used it is bypassed to ground using a 0.01μF capacitor. Pin 6 (Threshold): If the voltage applied to threshold terminal is

greater than (2/3)VCC, upper comparator switches to +Vsat and flip-flop gets reset. Pin 7: (Discharge): When the output is low, the external capacitor is

discharged through this pin Figure 2: IC 555 pin diagram Pin 8 (VCC): The power supply pin

Electronic Circuits Lab, Department of Electrical Engineering, College of Engineering Trivandrum 1

Figure 1: IC 555 Functional block diagram

Astable multivibrator using IC 555

One popular application of the 555 timer IC is as an astable multivibrator or clock Circuit. Figure 3 shows an astable circuit built using 2 external resistors and a capacitor to set the timing interval of the output signal. Capacitor C charges toward VCC through external 2 resistors RA and RB. Referring to figure, the capacitor voltage rises until it goes above V . 3 CC This voltage is the threshold voltage at pin 6, which drives comparator 1 to trigger the flip- flop(푄 low 푄 high) so that the output at pin 3 goes low. In addition, the discharge transistor is driven on, causing the output at pin 7 to discharge the capacitor through resistor RB. The 1 capacitor voltage then decreases until it drops below the trigger level V . The flipflop is 3 CC triggered so that the output goes back high and the discharge transistor is turned off, so that the capacitor can again charge through resistors RA and RB towards VCC.

CIRCUIT DIAGRAM & DESIGN

Take 푉 = 10푉 and f = 1000 Hz and duty cycle = 60 %

Then t = 1 ms, tH = 0.6 ms, tL = 0.4 ms

Electronic Circuits Lab, Department of Electrical Engineering, College of Engineering Trivandrum 2

Figure 3 Astable multivibrator circuit using IC 555

VCC 2 3 VCC 1 3 VCC

Figure 4 Waveforms of voltage across the capacitor and output voltage

Assume C = 0.1 μF

tL0.693  RC B then RB = 5.77 kΩ take RB = 5.6 kΩ

tH0.693  RRC AB  then RA = 3.06 kΩ take RA = 3.3 kΩ

The resistance RA and RB should be in the range of 1k to 10k to limit the collector current of the internal transistor.

PROCEDURE 1. Set up the circuit after verifying the condition of IC 2. Observe the waveforms at pin number 3 and 6 of the IC

RESULT Astable multivibrator using timer IC 555 is designed and setup, and the waveforms are obtained.

Electronic Circuits Lab, Department of Electrical Engineering, College of Engineering Trivandrum 3

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555 Multivibrator Circuits Tutorial – Astable, Monostable, Bistable

by admin · Published December 7, 2018 · Updated July 14, 2019

555 Timer IC

IC 555 timer IC is one of the most popular integrated circuit chip used for a variety of applications such as astable, monostable, bistable multivibrators, timer circuits, oscillators, PWM (Pulse Width Modulation), PPM (Pulse Position Modulation), square wave generator or pulse generator, etc. Astable mode, Monostable mode and Bistable mode are the three modes of operation of IC 555.

Its low price, simplicity, ease of use, and stability still make it a favorite one for beginners and hobbyists.

The IC 556 and IC 558 are 14 pins dual timer and 16 pin quad timer versions of the IC 555 respectively. IC 7555 is the CMOS version of the 555 IC with same pin configuration and function.

Internal Diagram of 555 timer IC

555 internal circuit consists of three series 5K resistors connected between the Vcc and GND. Thus the voltage drop across each resistance in the series network is Vcc/3. Hence, the voltage at the non-inverting input of comparator 1 will be 1/3Vcc and at the inverting input of comparator 2 will be 2/3Vcc of the supply voltage.

The outputs of the comparator 1 and comparator 2 is connected to the R and S input of the SR flip-flop respectively. The Q̄ output of the SR flip-flop is connected to the base of the transistor Q and to the inverting buffer circuit. The buffer circuit inverts the input; output 1 for input 0 and 0 for 1. The output pin 3 of the IC is taken from the output of the buffer circuit.

SR flip-flop Truth table

When the voltage at pin2 and pin6 is less than 1/3Vcc the output states of the Comp 1 and Comp 2 becomes HIGH and LOW or 1 and 0 respectively. S=1 & R=0, Q̄ =0, the output switches to a HIGH state ( inverse of the Q̄ state).

The outputs of both comparators remain in a LOW state when the voltage at Pin 2 and Pin6 is greater than 1/3Vcc but less than 2/3Vcc. S=0 & R=0, Q̄ =Q̄ n , the output remains in the previous state.

When the voltage at Pin2 and Pin6 is greater than 2/3Vcc the Comp 1 and Comp 2 has an output state of 0 and 1 respectively.S=0 & R=1, Q̄ =1, the output switches to a LOW state.

555 timer Pin configuration

Pin1 GND: Ground

Pin2 Trigger: The output switches to the HIGH state, when the voltage at the pin2 becomes less than 1/3 of the supply voltage.

Pin 3 Output: The output pin; either a HIGH state or LOW state output. It can drive a maximum current of 200 mA.

Pin 4 Reset: An active-low input. Resets the IC when the voltage at the pin is below 0.7V (Output switches to LOW). Normally connected to the positive terminal of the supply.

Pin 5 Control Voltage: It is connected directly to the voltage divider; across two series resistors at 2/3Vcc. Hence, the threshold value will be always equal to the voltage at pin 5 (2/3Vcc) and the trigger input (1/3Vcc) will have half of its value.

So, by adjusting the voltage at the control pin the threshold and triggering voltages can be overridden. Pulse width modulation, Pulse position modulation, etc. with the 555 IC are done by using this function of the IC.

Normally, when the control pin is not used a bypass capacitor of value 10nF will be connected across the control pin and ground for eliminating any voltage fluctuations and noises.

Pin 6 Threshold: When the voltage at the pin 6 comes above 2/3 of the supply voltage, the output switches to a LOW state.

Pin 7 Discharge: Pin for the discharge of the capacitor. It is connected to the open collector of the internal transistor. The emitter of the transistor is grounded and the base input is a complement of the IC output. So when the output is a LOW state, Pin7 becomes grounded.

Pin 8 +Ve Supply: Power supply input with a voltage range 4.5V to 15V.

Specifications of 555 IC

Operating Voltage: 4.5 to 16V

Max Output current: ± 200 mA; sink or source

Reset Voltage: 0.4 to 1V

Packages: DIP-8 and SIP-8

555 Timer Astable multivibrator

An astable multivibrator output consists of two quasi-stable states, that is it has no stable states in either HIGH or LOW states.

The output remains in either High state or Low state for a time T1 and then changes to the opposite state for a time T2. And the cycle repeats. The time period of a complete square wave, T = T1 + T2.

The frequency of oscillation f = 1/ T

Duty cycle is the ratio of ON time (duration of HIGH state or pulse width) to the total time period of a cycle. The duty cycle of an Astable multivibrator = TON/ TON + TOFF

Working of astable multivibrator Circuit

Initially, the voltage across the trigger and threshold input is below 1/3Vcc. Hence, the output switches to the high state and remains the state until the threshold input reaches 2/3Vcc. Gradually the capacitor C starts charging towards the supply voltage Vcc through resistor RA and RB. When the voltage across the capacitor charges to 2/3 Vcc, the output switches to the low state and the capacitor starts to discharge through resistor RB. The output state remains in the low state until the voltage across the capacitor discharges to 1/3Vcc. When the voltage reaches to 1/3Vcc the capacitor stops discharging and again the charging begins. The cycle repeats which generates a periodic square wave.

Formula derivation of 555 astable multivibrator

The ON time and OFF time of the astable multivibrator circuit can be calculated by taking the charging and discharging time of the capacitor C. The ON time of a cycle is equal to the charging time of the capacitor from 1/3Vcc to 2/3Vcc and the OFF is equal to the discharge time from 2/3Vcc to 1/3Vcc.

V(t) is the voltage value across the capacitor at a particular instant of time t.

V( t ) = V Final – (V Final – V Initial ) e -t/RC

During charging, the capacitor is charging towards the source voltage Vcc which is the final voltage. And the initial voltage is 1/3Vcc which has across the capacitor during the beginning of each cycle.

2Vcc/3 = Vcc – (Vcc -Vcc/3) e -T1/(RA+RB) C

T1 is the time required to charge the capacitor C through resistance RA + RB from the voltage 1/3Vcc to 2/3Vcc by the source Vcc.

2Vcc/3 = Vcc – 2Vcc/3 e -T1/(RA+RB) C

Vcc – 2Vcc/3 = 2Vcc/3 e -T1/(RA+RB) C

Vcc/3 = 2Vcc/3 e -T1/(RA+RB) C

1/2 = e -T1/(RA+RB) C

Applying natural log function at both sides of the equation.

In (1/2) =In ( e -T1/(RA+RB) C )

-In (2) = -T1/(RA+RB) C                       | In (1/x) = – In (x)

T1 = In (2) (RA+RB) C                           | In(2) = 0.69314

Equation for calculating ON time, T1 = 0.69 * (RA + RB) * C

Similarly, for the OFF calculate the time period for discharge of capacitor from the voltage 2Vcc/3 to Vcc/3.

Vcc/3 = 2Vcc/3 e –T2/RB * C )

T2 is the time required to discharge the capacitor C through resistance RB.

T2/RB * C = In (2)

Formula for calculating OFF time, T2 = 0.69 * RB * C

The total time period, T = T1 + T2 = TON + TOFF

T = 0.69 * (RA + RB) * C + 0.69 * RB * C

T = 0.69 * (RA + 2RB) * C

555 timer formula for frequency of oscillation, f = 1/ 0.69 * (RA + 2RB) * C

f = 1.44/(RA + 2RB) * C

Duty cycle = T1/ T1 + T2

From the above formula of T1 and T2, the duty cycle can be calculated as, D = (RA+RB)/(RA+2RB)

% Duty cycle, D = (RA+RB)/(RA+2RB) * 100

Astable multivibrator using 555 timer with diode

The above 555 astable mutivibrator circuit can only generate an output which has a duty cycle above 50%. Because as the charging resistance is RA+RB and discharging resistance is RB, the T1 or the ON period will be always greater than the OFF period. Thus a duty cycle below 50% is not possible with the normal 555 astable circuit.

So in order to charge and discharge the capacitor through different resistors, a bypass diode is added in the below circuit. Hence, during charging the Diode D bypass the resistance RB and the charging current flows through RA and D. And the discharge as same as normal circuit through the RB.

Astable multivibrator with diode formula derivation

Here the charging and discharging is done through two independent resistors the formula derivation is same as the above 555 astable multivibrator circuit except for the charging resistor.

ON time Formula, T1 = 0.69 * RA * C

OFF time Formula, T2 = 0.69 * RB * C

T = 0.69 * RA * C + 0.69 * RB * C

T = 0.69 * (RA + RB) * C

Here we get a Duty cycle, D = RA/(RA+RB)

Duty cycle in %, D = RA/(RA+RB) * 100

555 Monostable Multivibrator

Monostable multivibrator has only one stable state. The output of the circuit switches to a quasi stable state on receiving the trigger pulse and after a time period T the output return to its stable state. It remains the stable state until next trigger signal is received.

In a monostable multivibrator, an external trigger pulse is required for the transition of output from the stable state to quasi-stable state.

During the LOW state of the output, the capacitor remains fully discharged and the supply voltage drops across the resistance R1 through the discharge pin 7.

When an external negative pulse is applied to the Pin 2 trigger input, it switches the output HIGH and the capacitor C1 starts charging towards the supply voltage Vcc through resistance R1. Once the capacitor reaches the voltage 2/3Vcc the Output switches to a LOW or stable state and the capacitor discharges instantly. The state remains till next pulse is applied.

555 Monostable multivibrator formula derivation

The time period of the quasi-stable state or ON time is the charging time of the capacitor C from 0 to 2/3Vcc through resistance R1 by the source votlage Vcc.

2Vcc/3 = Vcc (1- e -T/ (R1 * C) )

2/3 = 1 – e -T/ (R1 * C) )

1/3 = e -T/ (R1 * C) )

Applying natural logarithm,

In (1/3) =In ( e -T/ (R1 * C) )

-In (3) = -T/(R1* C  )                     | In (1/x) = – In (x)

T = In (3) (R1 *C )

Equation for calculating ON time, T1 = 1.1 * R * C

Bistable Multivibrator using 555 timer

A bistable multivibrator has two stable states. The circuit remains in any one of the two stable states unless external trigger pulse is applied.

Here the trigger and reset input is connected across the switch s1 and switch s2 respectively. One terminal of both switches are connected to the positive of the supply through a resistor (around 10k) and the other terminal to ground.

When the switch S1 is pressed the trigger input is grounded (voltage at trigger input gets below Vcc/3) and the Output switches to a HIGH state. And when S2 has pressed the IC resets which switches the output to LOW. It is represented in the above timing diagram, how the output changes with the input during the switch press of s1 & s2.

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Design and Implementation of Astable Multivibrator using 555 Timer

The 555 timer is widely used as IC timer circuit and it is the most commonly used general purpose linear integrated circuit. It can run in either one of the two modes: Monostable (one stable state) or Astable (no stable state). In the Monostable mode it can produce accurate time delays from microseconds to hours. In the Astable mode it can produce rectangular waveforms with a variable Duty cycle. The simplicity and ease with which both the multivibrator circuits can be configured around this IC is one of the main reasons for its wide use. The state of the art presented in the paper is the design and implementation of an Astable multivibrator using 555 timer IC, generating non-sinusoidal waveform in the form of Rectangular waveform as well as capacitor voltage waveform in the form of ramp waveform.

Related Papers

IJERA Journal

The study was prepared with the aim of implementing a High Quality Monostable and A stable Multivibrators by Using 555 Timer, A commercially available IC. The study established that 555 Timer IC is a critical device, most pronounced economical and applicable in the global technology industry and reliable actual timing device which can apply either as a basic timer to produce single pulses or overhaul delays, or rather relaxation oscillator generating and emitting a trend of stabilized wave-forms of diversified duty patterns trending from 50 to 100%. The method used thereto comprise of practical observation as a methodology. Where from the study the capacitors should have charges of non-other than 1/3 VCC to 2/3 VCC as equivalent to the period the output value is high. Research recommend industries with circuit work oriented tasks to go for this technology as it is economical and well manageable.

International Journal of Scientific Research Engineering & Technology

Md.Tanvir Ahad

The aim of this paper is to design and implementation of astable multivibrator for many application of communication system. Here we used multivibrator circuit design by using different IC such as IC timer 555, V741 as An optional amplifier, flip flop circuits. The work ability proposed circuits is confirmed through Pspice, Simetrix, and Tina pro simulations tools and then the practical work is compared, An astable Multivibrator is constructed for given specification one of which is, it must have a duty cycle 68% or .68 and frequency of oscillation must be larger than 1 KHZ. Here we used the PS-pice simulation technique, which is very fast, and also bears the accuracy of circuit performance and reliability and the results are explored with browsing output data facility.

Ijariit Journal

This paper contains simulation and design of DC voltage multiplier using the principle of charging, discharging and storing properties of a capacitor with the help of different stages of boosting with each stage containing capacitor and diode set, contributing to the particular boosting multiplier. In this project we have used 4 stages of the cascaded multiplier, each stage adding 12 volts, stepping up 12V DC input to 48V DC output. Since charging and discharging of capacitor requires pulsating DC voltage, so a constant DC voltage is first converted into pulsating DC voltage with the help of a 555 timer.

Vivek Mahour

Antonio Buonomo

2007 Ph.D Research in Microelectronics and Electronics Conference

Andrea De Marcellis

Journal of Physics: Conference Series

Gate circuit multivibrator is composed of logic gates and RC circuit. When the vibrator is designed based on FPGA, the pulse frequency measured using frequency meter in FPGA is much higher than theoretic analysis value. If this circuit is used as a pulse signal generator, mistakes will be made easily. In this paper, a gate circuit multivibrator based on FPGA is designed, and its vibrating frequency is measured using the FPGA itself. A frequency error is measured. The reason that causes this error is analyzed, and it is testified through simulation and real signal test. Pulse shaping through Schmitt trigger is an effective solution method. This study shows that clock pulse is very important to FPGA system.

IEEE Transactions on Industrial Electronics and Control Instrumentation

Wasim Ahmed

DAYSI CAROLINA QUIMBITA YUPANGUI

IRJET Journal

In today’s world, the need for electronic gadgets has increased significantly. In a lithium-ion battery, overcharging can create unstable conditions inside the battery, increase pressure and cause thermal runaway. Most phone manufacturers preach “Do not leave your phone connected to the charger for long periods or overnight” and “Keeping your battery level as close to the middle (30% to 70%) as possible can effectively prolong the battery life”. So, to solve this problem, we should have a timing mechanism that automatically shuts the power after the desired time. To make this timing mechanism we used a 555 Timer IC as a monostable multivibrator which has one stable state and a quasi-stable state. The output of the power supply is given to the 555 timer and whenever the trigger pulse is given the timer is activated, which in turn gives power to the load only for that desired time. The time delay of the circuit is determined by the resistor and capacitor value. The potentiometer is used here to vary the resistor value and in turn the time delay. The 16 x 2 LCD will display the time delay at the current instant for the corresponding potentiometer value

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Astable Multivibrator Using 555 Timer – Electronic Circuit

A multivibrator is a one type of electronic circuit , that is used to implement a two state system like flip-flops, timers and oscillators. Multivibrators are categorized by two amplifying devices like electron tubes, transistors and other devices like capacitors and cross coupled by resistors. Multivibrators are classified into three types based on the circuit operation, namely Astable multivibrators, Bistable multivibrators and Monostable multivibrators . The astable multivibrator is not stable and it repeatedly switches from one state to the other. In monostable multivibrator, one state is stable and remaining state is unstable. A trigger pulse is the root to the circuit to enter the unstable state. When the circuit enters into the unstable state, then it will return to the normal state after a fixed time. A bistable mutivibrator circuit is stable that can be changed from one stable to other stable by an external trigger pulse. This multivibrator circuit is also called as flip-flop which can be used to store one bit of data.

Astable Multivibrator Working

This type of multivibrator includes two amplifying stages that are connected with a two capacitive-resistive coupling networks in a positive feedback. The amplifying elements are FETs , JFETs, Op-Amps, vacuum tubes, etc. The astable multivibrator circuit using BJTs is drawn in the form of cross coupled pair. The o/p terminals of the multivibrator can be defined as the active devices, that will have opposite states; one will have low voltage whereas the other has high voltage.

The above astable multivibrator circuit has two unstable states that change alternatively with a max transition rate due to accelerating +ve feedback.

It is employed by the coupling capacitors that suddenly transfer voltage variations because the voltage across a capacitor cannot change quickly. In every state, one transistor is turned on and the remaining one is turned off. So, one fully charged capacitor discharges slowly, therefore changing the time into an exponentially altering voltage. At the same time, remaining empty capacitor charges quickly. The operation of the above circuit is based on the forward-biased BE junction of the turned on BJT, that can provide a route for the restoration of the capacitor.

Astable Multivibrator Using 555 Timer

The designing and working of astable multivibrator using a 555 timer IC is done by using  ransistors and operational amplifiers . The 555 timer IC affords exact time delay from ms to hours. The oscillation frequency can be measured manually by small modification. 555 timer IC is a relatively cheap, stable and user-friendly integrated circuit and that is apt for circuit designers with for both astable and monostable applications. The first 555 integrated circuit was designed in the year 1971 by the corporation of Signetics as the SE555 or NE555. Astable multivibrator using 555 IC is a simple oscillator circuit that generates continuous pulses. The frequency of the circuit can be controlled by shifting the values of resistors R1, R2 ad capacitor C1.

Design of Astable Multivibrator

• The designing steps of the astable multivibrator are given below.
• The Required Components are NE 555 or SE 555, Resistors (1MΩx2, 1KΩ), Capacitors (0.01Fµ, 1Fµ) and LED
• The capacitor in the above circuit charges through the two resistors R1 and R2 and there charging time can be calculated as Tcharges= 0.69 (R1+R2) C1. During this charging time the o/p is high that is 1.38Sec
• The capacitor discharges through the resistor R2 then the discharging time can be Tdischarge= 0.69 R2C1. During this discharging time the o/p is low, that is 0.69 Sec.
• The total time period is T= Tcharges+ Tdischarge => 1.38+ 0.69= 2.07Sec
• The oscillation frequency is 0.483Hz.
• The duty cycle can be calculated in the following manner.
• Duty cycle= Ton /Ton + Toff => 1.38/2.07= 66%

Working of Astable Multivibrator

When the power is turned ON consider the flip flop is cleared initially, then the o/p of the inverter will be high. The charging of the capacitor will be done using two resistors R1& R2. When the voltage of the capacitor goes above 2/3 Vcc, then the output of the higher comparator will be High, it changes the control flip flop.So the control flip-flop’s Q o/p will be a LOW & Q’ will be High. So the final o/p of the Inverter is LOW. At the same time the Q1 transistor switches ON and the C1capacitor starts discharging through resistor R2.

When the voltage of the capacitor is < 1/3Vcc, then the o/p of the lower comparator will be high and control flip flop gets is set to 1. When the discharge transistor Q1 gets off, then the capacitor gets charged and continues this process. According to the status of the o/p, the LED at the output will blink. When the low voltage is applied at the 4th pin (reset pin) of the IC then it resets the IC. When the low signal is applied to the base of the Q2 transistor then it switches ON by the capacitor.

Applications of Astable Multivibrators

The applications of Astable multivibrators involve in radio gears to transmit and receive radio signals and also in time, morse code generators and some systems which require a square wave like analog integrated circuits and TV broadcasts.

Advantages and Disadvantages of Astable Multivibrators

These multivibrators are changes between one stable state to another continuously. This permits multivibrators to power themselves & achieve work at a constant rate without impact from any external forces or actions. Also, these multivibrators are low cost to produce, and simple to design

These multivibrators do not transfer the whole o/p signal to the i/p. This is due to resistance in the circuit, nonexistence of a totally closed loop at the o/p terminals, and the leaning of one transistor/capacitor to absorb energy at a somewhat different rate than the other. Although the amplifier restores the lost energy when it amplifies the signal, the signal will finally be too small to be of any use.

This is all about astable multivibrator using 555 timer, advantages,disadvantages and applications.We hope that you have got a better understanding of this concept, furthermore any doubts regarding this topic please give your suggestions in the comment section below.

Photo Credits:

• Astable Multivibrator by tpub
• Astable Multivibrator Using 555 Timer by visionic
• Working of Astable Multivibrator by circuitstoday

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Heating element temperature rise rate control with ic555 timer as an astable multivibrator, rancang bangun otomasi oxygen flow meter berbasis level spo2, the presence of differential vibration, teaching ic timer through simulation for future stem teacher, practical electronics for inventors, related papers (4), an artificial neural network-based temperature measurement system using a thermistor in astable multivibrator circuit, pneumatic astable multivibrator with positive feedback system, the simulation of the asymmetric multivibrator by the multisim software, transistorized high speed bistable multivibrator for digital counter bit, trending questions (1).

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In this hands-on electronics experiment, build an oscillator using a 555 timer and learn the theory of operation of the 555 timer when configured as an astable multivibrator.

Project overview.

The 555 integrated circuit (IC) is a general-purpose timer useful for various functions. In this experiment, we explore its use as an astable multivibrator or oscillator. Connected to a capacitor and two resistors , as shown in Figure 1, it will oscillate freely, driving the light-emitting diodes (LEDs) on and off with a square-wave output voltage . 

Figure 1. Schematic diagram of an audio oscillator using a 555 timer.

This circuit works on the principle of alternately charging and discharging a capacitor.

Theory of Operation

When the voltage detected at the threshold (THRESH) pin exceeds 2/3 of the power supply voltage (V cc ), the 555 begins to discharge the capacitor by grounding the discharge (DISCH) pin. It stops discharging the capacitor when the voltage detected by the trigger (TRIG) terminal falls below 1/3 of the power supply voltage. Thus, when both the THRESH and TRIGH terminals are connected to the capacitor’s positive terminal, the capacitor voltage will cycle between 1/3 and 2/3 power supply voltage in a sawtooth pattern.

During the charging cycle, the capacitor receives charging current through the series combination of the 1 MΩ and 100 kΩ resistors. As soon as the DISCH terminal on the 555 timer goes to ground potential (a transistor inside the 555 connected between that terminal and ground turns on), the capacitor’s discharging current only has to go through the 100 kΩ resistor. The result is an RC time constant that is much longer for charging than for discharging, resulting in a charging time greatly exceeding the discharging time.

The 555’s OUT terminal produces a square-wave voltage signal that is high (nearly V cc ) when the capacitor is charging and low (nearly 0 V) when the capacitor discharges. This alternating high/low voltage signal drives the two LEDs in opposite modes; when one is on, the other will be off.

Since the capacitor’s charging and discharging times are unequal, the high and low times of the output’s square-wave waveform will also be unequal. This can be seen in the relative brightness of the two LEDs; one will be much brighter than the other because it is on for a longer period of time during each cycle. The equality or inequality between the high and low times of a square wave is expressed as that wave’s duty cycle.

A square wave with a 50% duty cycle is perfectly symmetrical; its high time is precisely equal to its low time. A square wave that is high 10% of the time and low 90% of the time is said to have a 10% duty cycle.

Parts and Materials

• Two 6 V batteries or a 12 V power supply
• One capacitor, 0.1 µF, non-polarized
• One 555 timer IC 
• One 1 MΩ resistor
• One 100 kΩ resistor
• Two 510 Ω resistors
• Audio detector with headphones
• Oscilloscope (recommended, but not necessary)

An oscilloscope would be useful in analyzing the waveforms produced by this circuit, but it is not essential. An audio detector is a useful piece of test equipment for this experiment, especially if you don’t have an oscilloscope.

Learning Objectives

• How to use the 555 timer as an astable multivibrator
• Illustrate working knowledge of the  duty cycle

Instructions

Step 1:  Build the circuit illustrated in the schematic of Figure 1 and shown in the breadboard implementation of Figure 2.

Figure 2. Breadboard implementation of an audio oscillator using a 555 timer.

Step 2: In this circuit, the output waveform has a high time exceeding the low time, resulting in a duty cycle greater than 50%. Therefore the lower LED should be brighter than the upper LED in the circuit. Is it?

Step 3:  Use the audio detector (or an oscilloscope) to investigate the different voltage waveforms produced by this circuit.

Step 4:  Try different resistor values and/or capacitor values to see what effects they have on output frequency or charge/discharge times.

Related Content

Learn more about the fundamentals behind this project in the resources below.

• Multivibrators
• The 555 Timer Experiment Chapter

Calculator:

• 555 Timer Astable Circuit Calculator
• RC Time Constant Calculator
• 555 Timer Oscillator
• Timer Circuits Worksheet
• Time Constant Worksheet
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• 555 Timer , Electronics , Free Project Circuits , Mini Projects

Understanding 555 Timer

• March 22, 2024
• By Ravi Teja

The IC 555 is one of the most popular and most widely used IC’s. It is a versatile and extremely robust integrated circuit that is used in many applications like timers, wave generators (pulse) and oscillators.

The IC555, popularly known as the 555 Timer, was developed by Hans Camenzind of Signetic Corporation in the year 1971.

It was released in two parts: NE 555 and SE 555. The NE 555 parts were of commercial usage with a temperature range of 0 0 C to 70 0 C and the SE 555 parts are designed to meet the military standards with a temperature range of -55 0 C to 125 0 C. It is a monolithic IC and was the first commercially available timer IC’s.

Understanding 555 Timer Features

Some of the important features of the 555 timer are

• The 555 timer can be operated at a wide range of power supplies ranging from 5 V to 18 V .
• It is available in 3 different packages: 8-pin Metal Can package, 8-pin DIP and 14-pin DIP.
• Timing can be anywhere from microseconds to hours.
• It can operate in both astable and monostable modes.
• High output current.
• It has an adjustable duty cycle.
• It is TTL compatible due to its high output current.
• The output can source or sink a current of 200mA to the load.
• It has a temperature stability of 0.005% per 0C.

Know more from 555 Timer datasheet

Different Modes Of Operation in 555 Timer

Generally, the 555 timer can be operated in three modes: Astable, Monostable (or one-shot) and Bistable.

Astable Mode

In this mode, the 555 work as a free running mode . The output of astable multivibrator will continuously toggle between low and high , there by generating a train of pulse, which is why it is known as pulse generator.

It is a best example for a perfect square wave generato r. They are used as an inverter and also used in many of the internal part of the radio. Selecting a Thermistor as a timing resistor allows the use of the 555 in a temperature sensor.

Read : Astable Mode

Monostable Mode

In the monostable mode, as the name suggests, it stays in its stable state until and unless an external trigger is applied. In this mode, the 555 functions as a “one-shot” pulse generato r. The best application of a monostable is to introduce a time delay in to a system.

Applications comprise many things viz., timers, missing pulse detection also included bounce free switches, touch switches as well as frequency divider, capacitance measurement and pulse-width modulation (PWM) and many more.

Read :  Monostable Mode

Bistable Mode

In the bistable mode, the IC 555 acts as a flip-flop as it has two stable states . It can be used to store 1-bit of data . It is not a great choice for implementing a flip-flop.

Read : Bistable Mode

Pin Configuration of 555 Timer

The 555 Timer is available in 8-pin Metal Can Package, 8-pin Mini Dual in-line Package (DIP) and 14-pin DIP. The 14-pin DIP is IC 556 which consists of two 555 timers.

The 8-pin DIP is most commonly used. The pin out diagrams of 555 Timer in both the 8-pin packages is shown below.

Pin 1 – Ground (GND)

Ground reference voltage (low level 0V). All the voltages are measured with respect to this terminal.

Pin 2 – Trigger Terminal

It is responsible for SET and RESET transitions of the flip-flop. The amplitude of the external trigger pulse will influence the output of the timer. The output goes high and the timing interval starts when the input at trigger pin falls below half of Control voltage (i.e. 1/3 of the VCC).

Pin 3 – Output Terminal

Output driven waveform is available at this pin. It is driven to 1.7 V below VCC. Two types of loads can be connected to the output. One is Normally OFF load, which is connected between Pins 3 and 1 (GND) and the other is Normally ON load, which is connected between pins 3 and 8 (VCC).

Pin 4 – Reset Terminal

A negative pulse on this pin will disable or reset the timer. The timer will begin only when the voltage on this pin is above 0.7 V and hence it is normally connected to VCC when not used.

Pin 5 – Control Voltage

It controls the threshold and trigger levels and hence the timing of the 555. The width of the output pulse is determined by the control voltage. The output voltage can be modulated by an external voltage applied to this pin. Generally, it is connected to ground through a 10µF capacitor when not in use in order to eliminate any noise.

Pin 6 – Threshold Terminal

The voltage applied at this terminal is compared with a reference voltage of 2/3 VCC. When the voltage at this terminal is greater than 2/3 VCC, the flip-flop is RESET and the output falls from High to Low.

Pin 7 – Discharge

It is connected to the open collector of the internal NPN transistor which discharges the timing capacitor. When the voltage at this pin reaches 2/3 VCC, the output toggles from high to low.

Pin 8 – VCC or Supply

A supply voltage in the range of 5V to 18V is applied to this terminal.

555 Timer Internal Circuit Diagram

The internal block diagram of a 555 timer is shown below. It consists of the following

• Two Comparators
• An SR flip-flop
• Two transistors
• A resistive network

The comparators are the basic Op-amps. The comparator 1, which provides the R input, compares the threshold voltage with a 2/3 VCC reference voltage.

The comparator 2, which provides the S input to the flip-flop, compares the trigger voltage with a 1/3 VCC reference voltage.

The resistive network of three resistors will act as a voltage divider circuit. The values of these resistors are 5KΩ each. These three 5K resistors are responsible for the name “ IC 555 ”.

Out of the two transistors, one transistor is a discharge transistor. The open collector of this transistor is connected to the discharge pin (Pin 7) of the IC. According to the output of the flip-flop, this transistor either goes into saturation or cut-off.

When the transistor is saturated, it provides a discharge path to the capacitor that is connected externally. The base of the other transistor is connected to the reset terminal (Pin 4) which resets the timer irrespective of the other inputs.

555 Timer Working

The three 5KΩ resistors form a voltage divider network. This network provides two reference voltages to two comparators 2/3 VCC to the inverting terminal of the upper comparator (comparator 1) and 1/3 VCC to the non-inverting terminal of the lower comparator (comparator 2).

The inverting terminal of the upper comparator is connected to the control input. Generally, control input is not used and is connected to 2/3 VCC. The other input of the upper comparator is threshold and its output is connected to the R input of the flip-flop.

When the threshold voltage is greater than 2/3 VC C (i.e. the control voltage), then the flip-flop is RESE T and the output goes LOW. This will turn the discharge transistor ON (transistor goes to saturation) and provides a discharge path to any externally connected capacitor.

A trigger input is connected to the inverting terminal of the lower comparator. When the trigger input is less than the reference voltage (1/3 VCC) , the lower comparator’s output is high.

This is connected to the S input of the flip-flop and hence the flip-flop is SET and the output goes HIGH and the timing interval starts. As the output is high, the discharge transistor is turned OFF and allows charging of any capacitor connected to it externally.

Hence, in order for the output to go HIGH, the trigger input should be less than the reference voltage momentarily. The output is low when the threshold voltage is greater than 2/3 VCC, which resets the flip-flop and hence the output.

Introduction to Time Constant RC

Meeting timing requirements is a high priority task in most of the operations. For example, the heating process of a metal or a material in an industry is time limited.

Hence meeting the specific time requirements can be achieved by timer circuits .

A basic timer circuit is shown below. It consists of a charging circuit, a comparator and an output unit.

The charging circuit consists of a resistor and a capacitor. When a series combination of an RC circuit is applied with a DC voltage, the time taken for the capacitor to charge to the peak value is controlled by the resistor.

The charging time is proportional to the value of the resistance . The rate at which the capacitor charges in an RC circuit is given by Time constant.

RC Time Constant, generally called Tau (represented by the symbol τ), is the time constant of an RC circuit which is the time taken by the Capacitor to charge through the Resistor by approximately 63.2 % of the difference between initial and the final values.

It is also equal to the time taken by the capacitor to discharge to 36.8%. Time constant of an RC circuit is equal to the product of R and C.

As mentioned earlier, when the trigger input falls below 1/3 VCC, the output of the timer goes high and the period for which this stays high is determined by the RC time constant.

The pulse width and the frequency of the output of the 555 timer are determined by the RC time constant.

Choosing Timing Components for RC circuit in Timer

A 555 timer can provide delays from microsecond to hours depending on the values of R and C in the charging circuit. Hence it is very important to choose appropriate values for resistors and capacitors.

When the 555 timer is operating in Astable mode, then it requires an RC circuit consisting of two resistors and a capacitor. And in case of monostable mode of operation, the RC circuit comprises of a resistor and a capacitor.

Timing Capacitor

Choosing capacitors with large capacitances will be a problem. This is because electrolyte capacitors with large capacitances often tend to have wider tolerance limits. So the actual values and the marked values may have a significant difference.

Large capacitance electrolyte capacitors will have high leakage currents which can affect the timing accuracy as the capacitor charges. When choosing capacitors with large capacitance and low leakage current, Tantalum capacitors are a better option.

It is better to avoid electrolyte capacitors that have a high working voltage rating as they do not work efficiently when operated at a voltage 10% less than their rated voltage.

Hence, capacitors with working voltage greater than the VCC of the 555 timer should be chosen.

Timing capacitors with capacitance less than 100pF in order to produce short output pulses may also cause problems.

For capacitors with such low values, stray capacitance around the circuit might affect the capacitance of the timing capacitor.

Timing Resistor

When operating the 555 timer as an Astable multivibrator , the value of the timing resistor should be at least 1 Kilo Ohms. If the idea is to build a low power consumption circuit, then it is better to have higher values for the timing resistors.

But there is a disadvantage in choosing resistors with higher resistances as they lead to inaccuracies in timing. In order to minimize these inaccuracies, the value of the timing resistor shouldn’t be more than 1 Mega Ohms.

Trigger Pulses

The Pin 2 in the 555 timer is a trigger input. When the trigger input goes below the reference voltage i.e. 1/3 VCC, the output of the timer is high and the timing interval begins.

The trigger pulse should momentarily go below the reference voltage and the duration is important as it should not be longer than the output pulse.

Trigger pulses are generally identified by a narrow negative going spike. A differentiator circuit made from a capacitor and a resistor will produce two symmetrical spikes but a diode is used to eliminate the positive going spike.

The duration of the pulse is determined by the differentiator circuit (i.e. it depends on the capacitor and resistor).

Applications

Since the introduction of the IC 555 in the early 70’s, it has been employed in numerous circuits and applications by researchers as well as hobbyists. Some of the important areas of applications of the 555 timer are:

• Pulse Generation
• Time Delay Generation
• Precision Timing
• Sequential Timing
• Pulse Width Modulation (PWM)

The typical applications of a 555 timer can be differentiated by the mode of operation. Depending on the mode in which it is operated i.e. either in astable or in monostable mode, some of the applications of IC 555 are:

• Frequency Divider
• Linear Ramp Generator
• Missing Pulse Detector
• Pulse Position Modulation
• Square Wave Generation
• Pulse Width Modulation
• Tone Burst Generator
• Speed Warning Device
• Regulated DC – to – DC Converter
• Voltage – to – Frequency Converter
• Low Cost Line Receiver
• Cable Tester

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6 Responses

how can a 555 timer be use to build an alarm system

Read the post: Water Level Alarm using 555 Timer to get an idea about how to build an alarm system using 555 timer.

this Block diagram of 555 timer is technically different in each description i have read anywhere in books ,in magazines,in websites posts. But it seems very easy to understand. Nice job done.

Very True , the content is very crisp.

how can a 555 timer be to build astable multivabrator

Thanks for good information

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