Saturday, 22 October 2011

Monostable Multivibrator- Debouncing

What is de-bouncing?

Consider a simple push button connected to 5V supply. When the push button is pressed and released the signal across the output terminal of the button and the ground is expected to be like a signal given below:

But the actual signal looks like this:

As we can see from the above figure, bouncing of signal occurs during the transient time when mechanical switches are used, due to their surface roughness. This bouncing of wave should be avoided for proper functioning of the circuit in which the switch is used. This is known as switch de-bouncing. 

How is it done?

As mentioned in the previous post Monostable Multivibrator- 555 timer, a monostable multivibrator (MSMV) can be used for debouncing. We here by,  use 555 timer for that purpose.

Components Required:

1. IC 555
2. Resistor 10Kohm
3. Capacitors 10uF, 0.01uF
4. 5V supply


We need to design the timing resistors and capacitor values depending on the need. The time for which the MSMV remains in unstable state is given by the equation:

For R=10Kohm and C=10uF, we get a time delay of approximately 110ms. 

Circuit Diagram:

A Simple Application:

Let us now use the 555 timer in MSMV mode in the following test circuit. 

About the test circuit:

In the above circuit we use IC4029 to count the number of times the SPDT switch SW1 is pulled to ground(0V) and 5V. The number of counts is displayed in binary form using 4 LED's which count up-to 15 and then resets. Since the switch used is a mechanical device, waveform bouncing occurs which is rectified by the 555timer in MSMV mode. The wire from the output terminal of switch is connected to the trigger(pin2) of the 555timer and the output(pin3) is connected to the clock(pin15) of the counter.

A time delay of 110ms is obtained by placing suitable timing resistor and capacitor. 

Sample Input and Output Waveforms:



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