June 5, 1997

Debouncing networks make reliable selector

W Dijkstra, Waalre, The Netherlands

The expandable selector in Figure 1 consists of six switches, a HEX D-type flip-flop and two comparators with hysteresis and debouncing networks. The debouncing networks ensure that only one output of IC₂ is high at any time, and you can expand this circuit using more switches and D-type flip-flops.

All outputs of IC₂ are low after power-on because the R₃-C₃ network delivers a short, low-level pulse at the clear input of IC₂. The trigger point of comparator IC_1A at P₂ is approximately 0.07×V+, and the trigger point of comparator IC_1B at P₃ is approximately 0.12×V+.

When you push one switch, the magnitude of the voltage at P₁ is 0.09×V+, so only the output of IC_1A goes low, producing a positive pulse at IC₂'s clock input. In this case, only one D input is high, so after the clock pulses, only one output is high.

When you simultaneously push two or more switches, the magnitude of the voltage at P₁ is at least 0. 17×V+, so both comparator outputs go low. However, the output of IC_1B goes low before IC_1A's output because the R₁-C₁ debouncing network's time-constant is shorter than the R₂-C₂ network's time-constant. The resulting low level at the clear input of IC₂ causes all outputs to go low. Thus, it is impossible that more than one output of IC₂ is high. (DI #2028)

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