Description
A digital stop watch built around timer
IC LM555 and 4-digit counter IC MM74C926 with multiplexed 7-segment LED
display. MM74C926 consists of a 4-digit counter, an internal output
latch, npn output sourcing drivers for common cathode, 7-segment display
and an internal multiplexing circuitry with four multiplexing outputs.
The counter advances on negative edge of the clock. The clock is
generated by timer IC LM555. The circuit works off a 5V power supply. It
can be easily assembled on a general-purpose PCB. Enclose the circuit
in a metal box with provisions for four 7-segment displays, rotary
switch S1, start/stop switch S2 and reset switch S3
Testing
First, reset the circuit by pressing S3
so that the display shows ‘0000.’ Now open switch S2 for the stop watch
to start counting the time. If you want to stop the clock, close S2.
Rotary switch S1 is used to select the different time periods at the
output of the astable multivibrator (IC1).
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