Switch De-Bouncer Circuit

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

debounce circuit Notes
Using a 555 timer as a mono-stable circuit, it is easy to build a good switch de-bouncer circuit. There are many circuits for SPDT de-bouncing, but not many for a normally open, push-to-make press button switch (PBS). The 555 mono-stable gives an output pulse of around 20 msec 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 x 1.8x106 x 10x10-9 = 0.0198 sec or 19.8ms
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 1 Meg 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 1 Meg resistors is insufficient to re-trigger the mono-stable. Releasing the switch quickly discharges the capacitor. The output of a 555 mono-stable is suitable for connecting to TTL and CMOS logic circuits.

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