De-bounce Circuit for Push Switch                                                                                            Back        Home

This circuit is used to eliminate contact bounce when using a push switch with a digital circuit.

What is Contact Bounce?    Contact bounce occurs with all types of switches. As the switch contacts meet and then separate again there is a brief point at which the contacts are just at the point of separation. This can cause a moment of uncertainty where the contacts may or may not be passing current. The whole episode lasts for only a few milliseconds at most but due to the speed of electronic circuits, each splutter from the switch contacts is seen by the circuit as a legitimate push of the switch. The result is that the circuit sees several switch operations instead of just one.

The circuit uses a few simple components to remove the contact bounce. When the push switch is open, the voltage at point A is held low through R1 and R2 and C1 is discharged. Pressing the switch causes C1 to charge rapidly through D1, bypassing R1. This causes point A to go high fairly rapidly.

When the switch is released, the voltage at point A can't fall quickly as it is held up by the charge stored in C1. The capacitor can only discharge through R1 and R2 in series as D1 is now reverse biased. The switch contacts have time to bounce and separate cleanly before the voltage on C1 falls significantly.

The output at point A should be connected to a suitable logic gate to 'square up' the edges of the output.

 Push switch de-bounce circuit

 Push On, Push Off Switch Circuit                                                                                         Back        Home

A further development of the above circuit is to connect the output to a J-K flip-flop. This provides a circuit where the output alternates between on and off at each successive push of the switch. The circuit uses half of a 4027 flip-flop IC - the IC contains two separate but identical flip-flops which share common power supply connections. each flip-flop has two outputs which complement each other. ie. one output is always the inverse of the other.

The circuit shown is triggered on the leading edge of the switch pulse. This means that the outputs change as the switch is pushed. The circuit may be modified to operate as the switch is released by swapping the positions of the switch and R2.

This circuit is used as part of the electronic switching in our range of guitar pedals.

 push on, push off switch circuit using J-K flip-flop