PBS Switch Debouncing Circuit

Design :Andy Collinson
Email: anc

Description:
This circuit will remove the transient spikes and contact bounces from a non-latching push button switch.

debounce circuit


Notes:
I do not claim originality here for a 555 monostable circuit, but in this application, it finds use as a good switch debouncer. 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 msec with component values shown. 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 monstable is suitable for connecting to TTL and CMOS logic circuits.

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