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 betw1een 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
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.