Monostable Multivibrator using 555 Timer IC

Monostable Multivibrator (Single Pulse Generator Circuit) using 555 Timer IC:

If you understand the monostable and astable multibrator operation of the 555 timer IC, it will be helpful for you while troubleshooting the 555 timer circuits. It is recommended to read the Pinout details of 555 timer IC before proceeding further.

555 Monostable Pulse Generator (Single Pulse generator)

Monostable_Multivibrator

  • A standard 555 timer operating as a monostable pulse generator is shown in the above figure. We suggest to read the internal arragements of the 555 timer IC to understand the circuit operation
  • The duration of the output in high state is called as monostable timing period.
  • When a triggering pulse(falling edge) is applied to the Trigger Input (Pin-2), the monostable pulse period is started. It is shown in the below figure.
    555_monostable_configuration_waveform
  • When the triggering pulse goes below 1/3 of VCC, the 555 timer internal IC2 output reaches high state. The Bistable goes to the set state. The Bistable inverted output becomes low.
  • TR1 is in the OFF state and the output(pin-3) becomes high state.
  • Then the capacitor C charges through the series resistor R till the voltage at the threshold input reaches 2/3 of VCC.
  • At this point, the upper comparator(IC1) output changes it’s state. Then the Bistable resets.
  • The inverted output of bistable becomes high state, TR1 is driven into conduction and the final output goes low.
  • The 555 timer IC remains in inactive state til another trigger pulse is received at TR1. 

The output waveform has the following properties:

  1. Output High state Duration: ton = 1.1 C R
  2. Recommended triggering pulse duration: trr<(ton/4)
    Here
    ton and trr are in seconds
    R is in ohms.
    C is in Farads.
  • The output time period of this circuit can be easily changed by simply changing the values of the timing resistor R and/or timing capacitor C. 
  • Doubling the R value will double the timing period.
  • Doubling the C value will double the timing period.

The performance of the timer will be unpredictable if we select a wide range of R and C values. The recommended range of capacitor and resistor values are given below:
C = 470pF to 470μF
R = 1kΩ to 3.3MΩ

The required C and R values can be found out from the above formula for any particular monostable timing period. The output period can be easily adjusted by making R a preset resistor with a value of about twice that of the calculated value.

To understand fully about the monostable multivibrator let us design a single pulse generator.

Design Requirement:1
Design a timer circuit that will produce a 10 ms pulse when a negative going trigger pulse is applied to it.

Solution
From the circuit shown in above figures, the value of monostable timing period can be calculated from the formula:
ton = 1.1 C R
We have to choose an appropriate value for C. The capacitor range is given in above paragraph. As we need a modest time period we have to select midrange value for C. It will ensure that resistance value is neither too small nor too large. A value of 100 nF should be appropriate. Making R the subject of the formula and substituting for C = 100 nF gives:
R = tON/1.1C = 91kΩ

Design Requirement:2
Design a timer circuit which will produce +5V output for 60s when START button is pressed. The time period is to be aborted when STOP button is pressed.

Solution
Consider both START and STOP buttons are Normally Open (NO) type buttons.

With the help of the formula:
tON = 1.1 C R
the value of monostable timing period can be calculated.

  1. We have to select an appropriate Capacitor value(from the range mentioned above).
  2. Since we require a fairly long time period we have to select large capacitor value as we need long time period(60S).
  3. By doing this, R value will not be too high.
  4. Capacitance of 100μF will be suitable. 
  5. To find the appropriate R value, substitute the C value in the formula.

tON = 1.1 C R

R = tON /(1.1 C)  = 545kΩ ≈ 560kΩ

  • Connect the START button across pin-2 and ground. 
  • Connect the STOP button across pin-4 and ground.
  • To ensure that the input is at high state when the switch is off, pull-up resistors are connected in each input lines.

Generally in circuits, 10KΩ resistor is used as pull-up resistor. The resultant design circuit is given below.
555_monostable_Multivibrator_Design_CalculationRead More about 555 Timer IC:

555 Timer Based Simple Projects
Astable Pulse Generator based on 555 Timer IC
Fixed & Variable Pulse Generator Circuit Design Project
How the 555 Timer IC Operates Internally?
Simple 555 timer based LED Project

Thanks for Reading… Please try with this circuit with various pulse duration.

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