Clock & Timer Circuit DiagramsSecurity & Saftey

Bounce-free change-over switch

With the aid of two D-type bistables contained in a 74HCT74 and some external components, any push-button switch can be used as a bounce free changeover switch.

Now, first let’s discuss what is changeover switch: The rotary changeover switch is available as a single pole changeover switch, a four pole changeover switch, and also in 2 and 3 pole changeover switches for applications in industrial control, instrumentation, and machinery. The rotary changeover switch line is available in one to four poles, from 12 to 175 amps. The silver alloy contacts on the rotary cam changeover switch, which are a double break, feature a unique design that maintains high pressure for increased contact reliability and long life and are available in gold plating for low power applications.

In the diagram, IC1b provides the change-over function. Pin 8 of the bistable is linked with pin 12, which results in the logic levels at pins 8 and 9 alternately changing state when a leading edge appears at pin 11.

Circuit IC1a serves as a pulse generator and debouncing element. The push-button switch, S1, is connected between its reset input (pin 1) and ground. Normally, because of R2, there is a high level at pin 1. When S1- is pressed, IC 1a is reset.

The clock input (pin 3) is also connected to S1 via R1-C1. When the switch is operated, C1 is discharged rapidly via D1; when the switch is released, it takes a little while before C1 is recharged to a logic high level.

When Si is open, pin 9 is low, whereas pin 8 and pin 5 are high. When the switch is closed. IC la is reset immediately, resulting in pins 5 and 3 going low.

When S1 is released, the reset is removed. but it takes a little while before C1 is charged to a logic high level. Only when that level is reached• and a leading edge appears at pin 3. does pin 5 go high again. This results in ICib being clocked, whereupon its Q outputs (pins 8 and 9) change state.

Bounce-free change-over switch Schematic diagram

Bounce-free change-over switch Schematic diagram

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