 In today's session, we are going to discuss monostable multivibrator using IC-555 timer. At the end of today's session, students will be able to explain IC-555 timer operation as monostable multivibrator with circuit diagram and output signal waveforms. These are the contents of today's presentation. A monostable multivibrator, also known as one-shot multivibrator, is a pulse generator circuit in which the time duration of output pulse is decided by RC network connected externally to IC-555 timer. The output pulse width may range from microseconds to many seconds or minutes. For monostable multivibrator, one state of output is stable, while the other state is quasi-stable or unstable or temporary stable. Stable state equals a low voltage level and quasi-stable equals a high voltage level. Or it may be reverse. For auto triggering of output signal from quasi-stable state to stable state, that is, from a high state to a low state, the external capacitor C charges to a repress level, that is, a threshold level. The time taken by capacitor C to charge up to a threshold level, that is, a repress level, decides the time duration of output pulse, that is, output pulse width. The transition of output from stable state to quasi-stable state is done by external triggering signal. Now, this figure shows the circuit diagram for IC-555 as a monostable multivibrator using IC-555 and few external components like resistor RA and capacitor C are collected and additional capacitor C1 is collected at pin number 5 with respect to ground and output signal is taken from pin number 3, pin number 4 and 8 are joined together and connected to plus VCC. So, with the help of this circuit diagram, we are going to discuss the operation of monostable multivibrator. Pin number 1 is connected to ground. The external trigger signal is applied to pin number 2, that is, the trigger input of IC-555 in stable condition of output. This trigger input is kept at plus VCC voltage level to change the output from stable state, that is, low voltage level state to quasi-stable state or temporary state, that is, high voltage level output signal, a negative going pulse of narrow width, that is, less than the output pulse width and amplitude greater than a two-third VCC is applied to pin number 2, that is, the trigger input of IC-555. The output signal is taken from pin number 3 and pin number 4 is connected to plus VCC to avoid any accidental resets. Pin number 5 is grounded through 0.01 microfarad capacitor C1 to avoid any noise problem. Pin number 6, that is, the trigger input of IC-555 is shorted to pin 7. External resistor RA is connected between pin number 6 and pin number 8. At pin number 7, an external timing capacitor C is connected, while pin 8 is connected to plus VCC, as shown in figure 1. Now, let us use this figure 2, that is, the internal circuitry of IC-555 with external connections for the discussion of operation of a mono-stable multivibrator to understand the operation of a mono-stable multivibrator using IC-555. Necessary internal circuitry with external connections is shown in figure 2. Initially, when the output at pin number 3 is at low voltage level, that is, the stable state of a mono-stable multivibrator, the circuit is in stable state. The internal resistor is on and the external capacitor connected to pin number 7 is shorted to ground, so external capacitor will not charge to any voltage. Now, students should pause the video here and think over this question and then continue. How much trigger voltage is required at trigger input to change the output of mono-stable multivibrator to a quasi-stable state? A trigger voltage of less than one-third VCC is required at pin number 2, so as to switch internal lower comparator 2 to high and thereby the output of IC-555 working as a mono-stable multivibrator changes from stable state to quasi-stable state, becomes high voltage level. When negative wing pulse is applied at pin number 2, that is, the trigger input of IC-555, the voltage at trigger input falls below one-third VCC, so the output of comparator 2 goes high, so this resets internal flip-flop of IC-555 and therefore the internal transistor, that is, the discharge transistor turns off, so the output at pin number 3 goes high, so output equals high voltage level, that is, a quasi-stable state or temperate stable state of mono-stable multivibrator. This transition of output from stable state to quasi-stable state is shown in Figure 3. As a discharge transistor is off, the external connected timing capacitor C starts charging towards plus VCC through external resistor RA with the time constant RA into C. When increasing voltage across capacitor C becomes slightly greater than two-third VCC, the output of comparator 1 goes high, so this sets internal flip-flop so that its key output becomes high and key over output becomes low, so key output is used to drive the internal discharge transistor, so that internal discharge transistor becomes on and thereby external connected discharge capacitor starts discharging and the output of timer goes low, that is, the output of mono-stable multivibrator returns to stable state as shown in Figure 3. Now, this Figure 3 shows the trigger input signal, output signal of mono-stable multivibrator, output voltage waveform, and voltage waveform appearing across externally connected timing capacitor. The timing capacitor charges from zero to two-third VCC, again from two-third VCC its voltage returns to a zero-hold. As external capacitor charges towards VCC up to two-third VCC, the output of mono-stable multivibrator is in high state, that is, a temporary stable state or unstable state. When the external capacitor reaches to two-third VCC, then output of mono-stable multivibrator returns from quasi-stable state to stable state, equals low voltage level. Thus output returns back to stable state from quasi-stable state, the output of mono-stable multivibrator remains low until another trigger pulse is applied again. The capacitor C charges through resistor RA, if time constant RA into C is larger, the capacitor C takes longer time to reach to threshold voltage, that is, plus two-third VCC. Thus, RC time constant controls output pulse width. The voltage across capacitor at any time instant during charging period, charging process, is given by Vc equal to VCC into bracket 1 minus e raised to minus t upon RA into C. So, putting voltage across capacitor Vc equal to the target voltage two-third VCC, we get time taken by the external timing capacitor to charge from zero to two-third VCC, so that two-third VCC is equal to VCC into bracket 1 minus e raised to t upon RA into C. So, the pulse width Tp for which output of mono-stable multivibrator remains high, that is, quasi-stable state or temporary stable state is given by T is equal to Tp is equal to RA into C log to the base of e3. So, time duration of output pulse or time duration of output quasi-stable state Tp is equal to 1.0986 RA into C seconds, that is, approximately Tp is equal to 1.1 RA into C seconds. This is the reference. Thank you.