Bounce-free switch

Switch with change-over con-200 tacts traditional debouncing circuits are not always usable or economical. Keyboard switches, for instance, seldom have change-over contacts. Furthermore, change-over switches have one extra connection, which can not always be accommodated in the construction.

The small circuit here operates with one make contact or one break contact. Which one does not matter in practice, because the Q or Q output may be chosen to invert the switch action.

The logic level at the input of the circuit is determined by pull-up resistor R1 and the position of S1. The input signal goes straight to the data input of bistable (US: flip-floP) IC 1a. where it is clocked as soon as the contact bounce has disappeared (after 0.5- 1 0 ms)

The clock is generated by IC2a, an XOR gate. Every time its input level alters, this gate generates a pulse, whose width is determined by R2 and Ci. That pulse is, however, not devoid of contact bounce, which is, therefore, filtered- out by C2 and the output resistance of IC2a. The potential across C2 is smoothed and inverted by IC2b before the pulse is applied to the clock input of the bistable. The result of all this is that the output signal is clean, albeit delayed by a few milliseconds.

Since the bounce filter uses the output resistance of IC2a, this circuit cannot be replaced by just any other type. When replacing is unavoidable, the value of C2 must be adapted to the new circumstances or a resistor connected in series with the output of IC2a.

The current drain of the circuit is 3 mA.

Bounce-free switch Schematic diagram

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