This document describes a digital stopwatch circuit built around a timer IC and 4-digit counter IC. The circuit uses one IC to generate clock pulses and another IC to count the pulses and display the time on 4 7-segment displays. It has switches to start, stop, and reset the stopwatch. When the start/stop switch is pressed, it either resets the counter to zero to start timing or latches the current count to stop timing and store the time.

1. CIRCUIT IDEAS
DIGITAL STOP WATCH S.C.
DWI
VED
I
C.H. VITHALANI push-on-switch S3. When S2 is momen- Thus, when switch S3 is pressed, reset
tarily pressed, the count value becomes pin 13 of IC2 is connected to ground
H
ere’s a digital stop watch built 0, transistor T1 conducts and it resets via transistor T1 and the oscillator does
around timer IC LM555 and 4-digit IC1. Counting starts when S2 is in ‘off’ not generate clock pulses. This is done
counter IC with multiplexed 7-seg- condition. to achieve synchronisation between IC1
ment output drivers (MM74C926). A low signal on the latch-enable input and IC2.
IC MM74C926 consists of a 4-digit pin 5 (LE) of IC2 latches the number in First, reset the circuit so that the dis-
counter, an internal output latch, npn the counter into the internal output latches. play shows ‘0000.’ Now open switch S2
output sourcing drivers for common- When switch S2 is pressed, pin 5 goes for the stop watch to start counting the
cathode, 7-segment display and an low and hence the count value gets stored time. If you want to stop the clock, close
internal multiplexing circuitry with four in the latch. Display-select pin 6 (DS) de- switch S2.
multiplexing outputs. The multiplexing cir- cides whether the number on the counter Rotary switch S1 is used to select the
cuit has its own free running oscillator, or the number stored in the latch is to be different time periods at the output of the
and requires no external clock. The displayed. If pin 6 is low the number in astable multivibrator (IC1). The circuit
counter advances on negative edge of the output latch is displayed, and if pin 6 works off a 5V power supply. It can be
the clock. The clock is generated by timer is high the number in the counter is dis- easily assembled on a general-purpose
IC LM555 (IC1) and applied to pin 12 played. PCB. Enclose the circuit in a metal box
of IC2. When switch S2 is pressed, the with provisions for four 7-segment dis-
A high signal on reset pin 13 of base of pnp transistor T2 is connected to plays, rotary switch S1, start/stop switch
IC2 resets the counter to zero. Reset pin ground and it starts conducting. The emit- S2 and reset switch S3 in the front panel
13 is connected to +5V through reset ter of T2 is connected to DS pin of IC2. of the box.
JULY 2004 ELECTRONICS FOR YOU