Here\u2019s a digital stop watch built
\naround timer IC LM555 and 4-digit
\ncounter IC with multiplexed 7-segment
\noutput drivers (MM74C926).
\nIC MM74C926 consists of a 4-digit
\ncounter, an internal output latch, npn
\noutput sourcing drivers for commoncathode,
\n7-segment display and an
\ninternal multiplexing circuitry with four
\nmultiplexing outputs. The multiplexing circuit
\nhas its own free running oscillator,
\nand requires no external clock. The
\ncounter advances on negative edge of
\nthe clock. The clock is generated by timer
\nIC LM555 (IC1) and applied to pin 12
\nof IC2.
\nA high signal on reset pin 13 of
\nIC2 resets the counter to zero. Reset pin
\n13 is connected to +5V through reset
\nC.H. VITHALANI push-on-switch S3. When S2 is momentarily
\npressed, the count value becomes
\n0, transistor T1 conducts and it resets
\nIC1. Counting starts when S2 is in \u2018off\u2019
\ncondition.
\nA low signal on the latch-enable input
\npin 5 (LE) of IC2 latches the number in
\nthe counter into the internal output latches.
\nWhen switch S2 is pressed, pin 5 goes
\nlow and hence the count value gets stored
\nin the latch. Display-select pin 6 (DS) decides
\nwhether the number on the counter
\nor the number stored in the latch is to be
\ndisplayed. If pin 6 is low the number in
\nthe output latch is displayed, and if pin 6
\nis high the number in the counter is displayed.
\nWhen switch S2 is pressed, the
\nbase of pnp transistor T2 is connected to
\nground and it starts conducting. The emitter
\nof T2 is connected to DS pin of IC2.
\nThus, when switch S3 is pressed, reset
\npin 13 of IC2 is connected to ground
\nvia transistor T1 and the oscillator does
\nnot generate clock pulses. This is done
\nto achieve synchronisation between IC1
\nand IC2.
\nFirst, reset the circuit so that the display
\nshows \u20180000.\u2019 Now open switch S2
\nfor the stop watch to start counting the
\ntime. If you want to stop the clock, close
\nswitch S2.
\nRotary switch S1 is used to select the
\ndifferent time periods at the output of the
\nastable multivibrator (IC1). The circuit
\nworks off a 5V power supply. It can be
\neasily assembled on a general-purpose
\nPCB. Enclose the circuit in a metal box
\nwith provisions for four 7-segment displays,
\nrotary switch S1, start\/stop switch
\nS2 and reset switch S3 in the front panel
\nof the box.<\/p>\n
\n","protected":false},"excerpt":{"rendered":"
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