Mono stable mode:

In this mode one state (either high or low state) is made as stable state, when interrupt is applied the circuit goes to unstable second state and immodestly returns to original stable state. For example if high state is chosen as stable state, and when interrupt is applied the device goes to low state temporally and immediately returns to stable high state and remains in this state.

When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer oscillator, the internal comparator, (comparator No1) detects this input and “sets” the state of the flip-flop, changing the output from a “LOW” state to a “HIGH” state. This action in turn turns “OFF” the discharge transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1. This action allows the timing capacitor to start to charge up through resistor, R1 until the voltage across the capacitor reaches the threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage divider network. At this point the comparators output goes “HIGH” and “resets” the flip-flop back to its original state which in turn turns “ON” the transistor and discharges the capacitor to ground through pin 7. This causes the output to change its state back to the original stable “LOW” value awaiting another trigger pulse to start the timing process over again. Then as before, the Monostable Multivibrator has only “ONE” stable state.

555 Timer in Astable Mode

555 Timer IC Circuit Diagram in Mono Stable Mode

The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully. Once triggered, the 555 Monostable will remain in this “HIGH” unstable output state until the time period set up by the R1 x C1 network has elapsed. The amount of time that the output voltage remains “HIGH” or at a logic “1” level, is given by the following time constant equation.

            t = 1.1xR1xC1 

Where, t is in seconds, R is in Ω and C in Farads.

A Monostable 555 Timer is required to produce a time delay within a circuit. If a 10uF timing capacitor is used, calculate the value of the resistor required to produce a minimum output time delay of 500ms. 500ms is the same as saying 0.5s so by rearranging the formula above, we get the calculated value for the resistor, R as:

555 timer calculation in Mono stable mode

555 Timer IC Calculations in Mono Stable mode

In case exact resistor is not available then we can use nearest value like 47k or add two or more resistors in series to get desired value.

Applications of Monostable multivibrator :

  • The monostable multivibrator is used as delay and timing circuits.
  • It is also used for temporary memories.
  • It is often used to trigger another pulse generator.
  • It is used for regenerating old and worn out pulses.

Leave a Reply