Creating a Bounce-Free Changeover Switch
By utilizing two D-type bistables from a 74HCT74 along with additional external components, it’s possible to convert any push-button switch into a changeover switch free from bouncing issues. First, let’s understand what a changeover switch entails: Rotary changeover switches are available in various configurations, such as single pole, four pole, and 2 or 3 pole versions, catering to applications in industrial control, instrumentation, and machinery. These switches, with options ranging from one to four poles and current ratings from 12 to 175 amps, feature silver alloy contacts on the rotary cam. These contacts, designed as double breaks, maintain high pressure, ensuring enhanced contact reliability and a longer operational lifespan. Gold-plated versions are also available for low-power applications.
Circuit Operation and Components
In the circuit diagram, IC1b facilitates the changeover function. Pin 8 of the bistable is connected to pin 12, causing the logic levels at pins 8 and 9 to alternate states when a leading edge is detected at pin 11. Circuit IC1a acts as a pulse generator and debouncing element. The push-button switch, S1, is positioned between its reset input (pin 1) and ground. Ordinarily, due to R2, there is a high level at pin 1. When S1 is pressed, IC 1a is reset, initiating the changeover process.
Charging and Discharging of the Capacitor
The clock input (pin 3) is linked to S1 through R1-C1. When the switch is activated, C1 discharges swiftly via D1; upon switch release, it takes a brief moment for C1 to recharge back to a logic high level.
Switch Operation and Reset Signals
With S1 open, pin 9 remains low, while pins 8 and 5 are high. When the switch is closed, IC1a experiences an immediate reset, causing pins 5 and 3 to go low.
Reset Removal and Clocking IC1b
Upon releasing S1, the reset signal vanishes, but it takes some time for C1 to reach a logic-high level again. Only when this level is attained and a leading edge is detected at pin 3. Does pin 5 return to a high state. This triggers IC1b, causing its Q outputs (pins 8 and 9) to change their states.