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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.
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. |