PBS Switch Debouncing
This circuit will remove the transient
spikes and contact bounces from a non-latching push button switch.
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.