SWITCH DEBOUNCING CIRCUIT

Switch debouncing is one of those things you generally have to live with when playing with switches and digital circuits. If you want to input a manual switch signal into a digital circuit youll need to debounce the signal so a single press doesnt appear like multiple presses. This circuit will remove the transient spikes and contact bounces from a non-latching push button switch.

Circuit Diagram

Notes

Using a 555 timer as a monostable circuit, it is easy to build a good switch debouncer circuit. There are many circuits for SPDT debouncing, but not many for a normally open, push-to-make press button switch (PBS). The 555 monostable gives an output pulse of around 20 ms with component values shown. The formula for determining the output pulse is:

Tout = 1.1 R1 C1

With the values in my circuit this equates to :

Tout = 1.1*1.8*106*10*10-9 = 0.0198 sec or 19.8 ms

The 555 circuit can be re-triggered if the input is held low longer than the output pulse. To prevent this happening, I have included a further timing circuit comprised of the 1Meg resistor and 47n capacitor. Normally, the 47n capacitor is discharged via the 1 Meg resistor. When the switch is pressed the capacitor quickly charges and provides a brief negative pulse to the 555 input. When the capacitor is fully charged, the potential across the voltage divider formed by the 10k and 1Meg resistors is insufficient to retrigger the monostable. Releasing the switch quickly discharges the capacitor. The output of a 555 monostable is suitable for connecting to TTL and CMOS logic circuits.