#537 Schmitt Switch Debouncer

Testing a simple Schmitt Trigger debounce circuit using a 74HC14.

Notes

The hysteresis of a Schmitt Trigger used in conjunction with a suitable RC filter provides effective switch debouncing:

• the RC time constant for charge/discharge filters most of the switch bounce
• Schmitt Trigger hysteresis can help with ensuring positive on and off transitions

The circuit demonstrated here is based on Experiment #125 from ARRL’s Hands-On Radio Experiments Volume 3.

Construction

I’ve put double-inversion in the circuit; the result is that the LED is normally on, and turns off while the pushbutton is depressed.

Time constants are purposely large:

• on, a function of R2=10kΩ, C1=1µF: 10ms
• off, a function of R1=100kΩ, C1=1µF: 100ms

R2 needs to be much less than R1, else pressing the button will not pull down the voltage sufficient to switch the inverter.

Behaviour

The on-off cycle is captured in the following scope trace where

• CH1 (Yellow) - switch output
• CH2 (Blue) - inverter input
• CH3 (Red) - output to LED

Credits and References

• ARRL’s Hands-On Radio Experiments Volume 3 Experiment #125
• 74HC14 Datasheet