This circuit can determine which event out of several happened first. The circuit provides a one-and-only-one output, from a total of 16 possible inputs. The circuit can be used in a game show to indicate which contestant responded first, or the switches can be replaced with TTL compatible inputs for applications like a model race track, to show which car won the race.
The design is based on a 74150 TTL multiplexer, a 74LS154 demultiplexer and a 4-bit binary counter (74LS193). Initially all switches are open and pin 10 of IC1 is low, enabling the gated clock oscillator made up of IC5b and IC4. As a result, the counter generates a 4-bit address that sequentially scans all the switch inputs, as well as sequentially addressing each output of IC2.
When a switch is closed, and the counter reaches the input the switch is connected to, pin 10 of IC1 goes high, sending a low via IC5a to the input of IC5b, which disables the the clock signal to IC3. As well, pin 18 of IC2 is now low, the corresponding output for the particular switch goes low. The clock remains disabled as long as the switch is closed. Closing the other switches at this time has no effect on the outputs. Once the switch is opened, scanning continues.
The main advantage of this circuit is that only a few IC's are required. The circuit can be extended with additional devices for IC1, IC2 and IC3. The speed of the clock should be as high as possible, but around 200Khz will suit most purposes.
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