This project can be used for a quiz with up to 4 contestants (or teams). Each contestant has a trigger pushswitch and LED. When a trigger switch is pressed it lights the corresponding LED, sounds the bleeper
and prevents the other trigger switches from working - therefore showing which contestant was the first to
press their switch. A reset push-switch (operated by the quizmaster) cancels the bleeper and switches off
the LED so the circuit is ready for the next question.
Take great care to arrange the parts correctly on the compact stripboard layout. The LEDs are shown
mounted directly on the stripboard but you may prefer to mount them on a box using short wires. The
trigger switches need long cables of about 2 metres so they can be held by, or placed near, the contestants.
The circuit consists of four 555 timer bistables which are triggered or reset when their inputs are low.
Their reset inputs are connected together and operated by a single reset push-switch. The trigger switches
are connected to the bistable trigger (pin 2) through a 0.1µF capacitor so that only the initial press triggers
the bistable; continuing to hold the switch closed will have no effect. Connecting the switch directly to
the bistable would prevent the quizmaster from resetting the circuit until the trigger switch was released
and trials showed that many contestants kept the switch pressed until asked to give their answer! When
triggered the bistable output (pin 3) lights an LED and makes the ‘trigger line’ high - this prevents any
other bistable being triggered and sounds the bleeper. A diode is used to link the output to the trigger line.
Parts Required
• resistors: 470 ×4, 1k ×2, 10k ×8 • 555 timer ICs (such as NE555) ×4
• capacitors: 0.1µF ×4, 1µF radial • 8-pin DIL sockets for ICs ×4
• diodes: 1N4148 ×4 • on/off switch
• battery clip for 9V PP3 • push-switch ×5
• bleeper suitable for 3 to 6V • LEDs: 1 each red, green, yellow, blue, all 5mm
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