Electronic Design

First-Come, First-Serve Circuit Chooses Winners

This circuit is a first-come, first-serve selector (see the figure). Its genesis comes from a teacher who asked me to create a "Jeopardy" style quiz show apparatus that offers a fun way to test students.

Three contestants each have a simple single-pole, single-throw switch to activate when they know the answer. The first one who thinks he or she has the answer locks his or her light on and wins! The contestant switches are SW2, SW3, and SW4. SW1 is used by the moderator to reset and hold the circuit reset until ready to run the quiz.

The circuit takes into account that in an RS flip-flop, when the preset and the clear are both low, the outputs Q and not-Q will be "1." Then when the first preset or clear goes high, the flip-flop will assume the "0-1" or "1-0" state, indicating whether the preset or clear went high first.

U1a, U1b, and U2a debounce the switches. U6a, U3a, and U3b are the RS arbitration flip-flops that determine which contestant is first. U4a, U4b, and U4c decode the arbitration flip-flop outputs to light the winning light or LED. U5a determines when any one switch is pressed, and U5b and U5c generate a delay so that the arbitration flip-flops settle out before the LEDs are illuminated.

The number of arbitration flip-flops needed for a given number of contestants is the combination of the number of contestants taken two at a time. Thus:

F = \[C × (C — 1)\]/2

where F is the number of arbitration flip-flops required, and C is the number of contestants. Therefore, three contestants require three arbitration flip-flops. And, four contestants require six flip-flops.

For the contestant switches, I used a 35-mm film can. The switch was mounted in the bottom of the can so that the person can hold it and push the switch.

R8 is a protection resistor in case the switch wires are shorted to ground. The capacitors reduce the noise on the debouncing flip-flops. With the appropriate drivers and voltage, a lamp could replace the LEDs.

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