Switch Bounce / Debounce Tutorial

This tutorial is to demonstrate all five methods for debouncing switches using single pole-single throw, or single pole-double throw switches. The guide includes the two methods for single pole-double throw (SPDT) switches which require no output delay and the two best methods for interfacing with the more simple single pole-single throw (SPST) switches.

If you can spare two pins, this is the way to go. Using a SPDT switch, Pin 1 Low indicates the switch is inactive, and Pin 2 Low indicates it is activated.

The cross-coupled nand gate method is ideal for projects where two processor pins are not available for use with one switch.

Using the internal cross-coupled nand gating of a D-Type Flip flop provides an excellent Method 3 Debounce identical to Method 2. Note the Q output is normally high and goes low when the switch is activated.

Whenever a single pole-single throw (SPST) switch is used for input into high-speed digital devices, the only thing you can do is to wait out the bounce interval with either hardware or software delays (see Waveform 2 below).

There are numerous examples online for software debounce routines for every computer language known to mankind. They all amount to simply waiting out the estimated bounce period.

Note the immediate Q output. Methods 1 and 2 change the output state with no delay both on actuation and release.

Note the delayed Q output for these methods. The Hardware Debounce circuit of Method 4 and Software Debounce of Method 5 require delaying action until the bounce time is through.