555-Monostable

The circuit diagram of the 555 monostable circuit is given in figure 2. The value of R and C determine the length of time that it's output is in the high state. It can be calculated using the equation below...

...where T is the time period in seconds, and R and C are the component values in Ohms (Ω) and Farads (F).

Calculations

1. Firstly, decide the time period T that you require. This can be very small (milliseconds) or large (minutes), but it must be expressed in seconds. {I choose T = 10 seconds}
2. Next, guess a value for the capacitor C, expressed in Farads. For starters, try 100μF. {I choose C = 100μF}
3. Put the values of T and C into the equation below and calculator resistor R...
   

If the resistor value you calculated is smaller than 1kΩ or larger than 1MΩ, you should re-do the calculation with a different value for capacitor C until you get a resistor value within the acceptable range.

Varying the Time Period

If you will need to adjust the time period of the monostable circuit in use, you can use a linear variable resistor for R, as shown in figure 3.

Because the resistance of a variable resistor goes down to around 0Ω at one end of its range, a 1kΩ resistor is placed in series with it so that the value of R will never fall below 1kΩ. As the shaft of the variable resistor is turned from its lowest setting to its highest, T will become longer.

If your chosen variable resistor has three connections, it is a potentiometer, and you should connect to the centre connection and either of the end connections.

Further Information

Resistors - Includes information about variable resistors

The Trigger Input

As you can see from figure 1, the 555's Trigger input must be taken low to trigger the monostable. This is achieved in figure 2 by placing a button in series with a resistor across the power supply. Normally, the 10kΩ resistor keeps the Trigger input high, at the voltage Vs, and the monostable is in its steady state. When the button is pushed, the Trigger input is directly connected to 0V and the time period T starts.

The Reset Input

If you want to make the monostable output go low before the time period has elapsed, simply take the 555's Reset input briefly low. This can be achieved with a push button in exactly the same way as with the Trigger input.