IC Based Multivibrator Circuits

In this article, we will discuss monostable and astable multivibrator circuits that can be configured around some of the popular digital and linear integrated circuits.

Digital IC Based Monostable Multivibrator

Some of the commonly used digital ICs that can be used as monostable multivibrators include 74121 (single monostable multivibrator), 74221 (dual monostable multivibrator), 74122 (single retriggerable monostable multivibrator) and 74123 (dual retriggerable monostable multivibrator), all belonging to the TTL family, and 4098B (dual retriggerable monostable multivibrator) belonging to the CMOS family.

Figure 1 shows the use of IC 74121 as a monostable multivibrator along with a trigger input. The IC provides features for triggering on either LOW-to-HIGH or HIGH-to-LOW edges of the trigger pulses.

Figure 1(a) shows one of the possible application circuits for HIGH-to-LOW edge triggering, and Fig. 1(b) shows one of the possible application circuits for LOW-to-HIGH edge triggering.

The output pulse width depends on external R and C. The output pulse width can be computed from T = 0.7RC. Recommended ranges of values for R and C are 4 –40 KΩ  and 10 pf to 1000µF respectively.

The IC provides complementary outputs. That is, we have a stable LOW or HIGH state and the corresponding quasi-stable HIGH or LOW state available on Q and Qbar outputs.

Figure 2 shows the use of 74123, a retriggerable monostable multivibrator. Like 74121, this IC, too, provides features for triggering on either LOW-to-HIGH or HIGH-to-LOW edges of the trigger pulses.

The output pulse width depends on external R and C. It can be computed from T = 0.28RC × [1 + (0.7/R)], where R and C are respectively in kiloohms and picofarads and T is in nanoseconds. This formula is valid for C > 1000 pF. The recommended range of values for R is 5 – 50 KΩ.

Figures 2(a) and (b) give application circuits for HIGH-to-LOW and LOW-to-HIGH triggering respectively. It may be mentioned here that there can be other triggering circuit options for both LOW-to-HIGH and HIGH-to-LOW edge triggering of monoshot.

Timer IC 555 Based Multivibrators

IC timer 555 is one of the most commonly used general-purpose linear integrated circuits. The simplicity with which monostable and astable multivibrator circuits can be configured around this IC is one of the main reasons for its wide use.

Figure 3 shows the internal schematic of timer IC 555. It comprises two opamp comparators, a flip-flop, a discharge transistor, three identical resistors and an output stage. The resistors set the reference voltage levels at the noninverting input of the lower comparator and the inverting input of the upper comparator at (+V_CC/3) and (+2V_CC/3).

The outputs of the two comparators feed the SET and RESET inputs of the flip-flop and thus decide the logic status of its output and subsequently the final output.

The flip-flop complementary outputs feed the output stage and the base of the discharge transistor. This ensures that when the output is HIGH the discharge transistor is OFF, and when the output is LOW the discharge transistor is ON.

Different terminals of timer 555 are designated as ground (terminal 1), trigger (terminal 2), output (terminal 3), reset (terminal 4), control (terminal 5), threshold (terminal 6), discharge (terminal 7) and +V_CC (terminal 8). With this background, we will now describe the astable and monostable circuits configured around timer 555.

Astable Multivibrator Using Timer IC 555

Figure 4(a) shows the basic 555 timer based astable multivibrator circuit. Initially, capacitor C is fully discharged, which forces the output to go to the HIGH state. An open discharge transistor allows the capacitor C to charge from +V_CC through R1 and R2.

When the voltage across C exceeds +2VCC/3, the output goes to the LOW state and the discharge transistor is switched ON at the same time.Capacitor C begins to discharge through R2 and the discharge transistor inside the IC. When the voltage across C falls below +VCC/3, the output goes back to the HIGH state. The charge and discharge cycles repeat and the circuit behaves like a free-running multivibrator.

Terminal 4 of the IC is the RESET terminal. Usually, it is connected to +V_CC. If the voltage at this terminal is driven below 0.4 V, the output is forced to the LOW state, overriding command pulses at terminal 2 of the IC.

The HIGH-state and LOW-state time periods are governed by the charge (+V_CC/3 to +2V_CC/3) and discharge (+2V_CC/3 to +V_CC/3) timings. These are given by the equations

HIGH-state time period T_HIGH = 069(R₁ +R₂)C  

LOW-state time period T_LOW = 069R₂C

The relevant waveforms are shown in Fig. 4(b). The time period T and frequency f of the output waveform are respectively given by the equations

Time period T = 0.69(R₁ +2R₂)C

Frequency F = 1/ [0.69(R₁ +2R₂)C]

Remember that, when the astable multivibrator is powered, the first-cycle HIGH-state time period is about 30 % longer, as the capacitor is initially discharged and it charges from 0 (rather than +V_CC/3) to +2V_CC/3.

In the case of the astable multivibrator circuit in Fig. 4(a), the HIGH-state time period is always greater than the LOW-state time period.

Figures 4(c) and (d) show two modified circuits where the HIGH-state and LOW-state time periods can be chosen independently. For the astable multivibrator circuits in Fig. 4(c) and (d), the two time periods are given by the equations

HIGH-state time period = 0.69R₁C

LOW-state time period = 0.69R₂C

For R₁ = R₂ = R

T = 1.38RC and f = 1/1.38RC

Monostable Multivibrator Using Timer IC 555

Figure 5(a) shows the basic monostable multivibrator circuit configured around timer 555. A trigger pulse is applied to terminal 2 of the IC, which should initially be kept at +V_CC. A HIGH at terminal 2 forces the output to the LOW state.

A HIGH-to-LOW trigger pulse at terminal 2 holds the output in the HIGH state and simultaneously allows the capacitor to charge from +V_CC through R. Remember that a LOW level of the trigger pulse needs to go at least below +V_CC/3.

When the capacitor voltage exceeds +2VCC/3, the output goes back to the LOW state. We will need to apply another trigger pulse to terminal 2 to make the output go to the HIGH state again.

Every time the timer is appropriately triggered, the output goes to the HIGH state and stays there for the time it takes the capacitor to charge from 0 to +2V_CC/3.

This time period, which equals the monoshot output pulse width, is given by the equation T = 1.1RC. Figure 5(b) shows the relevant waveforms for the circuit of Fig. 5(a).

It is often desirable to trigger a monostable multivibrator either on the trailing (HIGH-to-LOW) or leading (LOW-to-HIGH) edges of the trigger waveform. In order to achieve that, we will need an external circuit between the trigger waveform input and terminal 2 of timer 555. The external circuit ensures that terminal 2 of the IC gets the required trigger pulse corresponding to the desired edge of the trigger waveform.

Figure 6(a) shows the monoshot configuration that can be triggered on the trailing edges of the trigger waveform. R1–C1 constitutes a differentiator circuit. One of the terminals of resistor R1 is tied to +V_CC, with the result that the amplitudes of differentiated pulses are +V_CC to +2V_CC and +V_CC to ground, corresponding to the leading and trailing edges of the trigger waveform respectively.

Diode D clamps the positive-going differentiated pulses to about +0.7 V. The net result is that the trigger terminal of timer 555 gets the required trigger pulses corresponding to HIGH-to-LOW edges of the trigger waveform. Figure 6(b) shows the relevant waveforms.

Figure 7(a) shows the monoshot configuration that can be triggered on the leading edges of the trigger waveform. The R1–C1 combination constitutes the differentiator producing positive and negative pulses corresponding to LOW-to-HIGH and HIGH-to-LOW transitions of the trigger waveform.

Negative pulses are clamped by the diode, and the positive pulses are applied to the base of a transistor switch. The collector terminal of the transistor feeds the required trigger pulses to terminal 2 of the IC. Figure 7(b) shows the relevant waveforms.

For the circuits shown in Figs 6 and 7 to function properly, the values of R1 and C1 for the differentiator should be chosen carefully. Firstly, the differentiator time constant should be much smaller than the HIGH time of the trigger waveform for proper differentiation. Secondly, the differentiated pulse width should be less than the expected HIGH time of the monoshot output.

1. Logic Gates
2. Significance & Types of Logic Family
3. Characteristics Parameters of Logic Families
4. TTL Logic Family
5. ECL Logic Family
6. CMOS Logic Family
7. Interfacing of Logic Families
8. Microcontroller Architecture
9. Components of Microcontroller
10. Interfacing Devices with Microcontroller
11. IC Based Multivibrator Circuits
12. Astable, Monostable & Bistable Multivibrator
13. Logic Analyser
14. Types of Oscilloscope
15. Frequency Synthesizers
16. Frequency Counter