Quiz Circuit
Circuit :  Andy Collinson
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Description
I've had a few requests for a quiz circuit, so here is a 4 input design which can easily be modified.

Notes
This design uses four IC's and has four input circuits and four independent outputs and a single master reset switch. The outputs here are LED's but may be modified to drive lamps or buzzers. Only one output LED can be lit at any time. The first person to press their input switch, A,B,C,D will light the corresponding output LED, disabling the other inputs.

The circuit uses all CMOS IC's part numbers shown on the diagram. The supply voltage may be anything between 3 and 15 volts. Alternatively, it may be built using equivalent TTL IC's and powered on 5 volts. The main component in this circuit is a bistable latch, here it is based on the dual 4013 D-type flip flop.

Circuit Operation
Pressing the reset switch will clear all flip flops and extinguish any lit LED's. Under this condition the Q outputs will all be low (logic 0) and NOT Q outputs will be high (logic 1). All four NOT Q outputs are fed to a 4 input AND gate, the 4082 whose output will also be high. The output of the 4082 is wired to one input of each 2 input AND gate (4081). Switch inputs A,B,C,D are all non latching push button switches, the first person to press their switch will cause the corresponding AND gate (4081) to go high and trigger the preset input of the 4013 D-type flip flop. This will latch and light the appropriate LED. Also the triggered flip flop will have its NOT Q output, set at low, this changes the 4082 output to low and prevents any further triggering of the other flip flops.Switch contact de-bouncing is not required as the first press will latch one of the bistables. Pressing the reset switch, restores the circuit to its former state. I would recommend using heavy duty push button switches, as in use they are likely to be under some stress.

Quiz Circuit on Breadboard
I am pleased to display one readers quiz circuit on breadboard. Special thanks to Andrew Duguid for this fine image.

Modification to Drive Extra LED's or Lamps
CMOS IC's have a limited output current. To drive high current loads like a lamp, or maybe extra LED's, then modify each output as shown in the diagram below:

The transistor acts as a switch to boost the output current. In common emitter the transistor inverts the output; to get around this the LED polarity is reversed. Any general purpose transistor will work for loads up to 100mA.

This circuit is available to download in the Circuit Maker section for anyone wishing to experiment further.

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