Countdown timers can be constructedusing discrete digitalICs including up/down countersand/or 555 timers. If you wishto incorporate various facilities likesetting the count, start, stop, reset anddisplay facilities, these circuits wouldrequire too many ICs.Here is a simple design basedon 20-pin Atmel AT89C2051 microcontrollerthat performs countdownoperation for up to 99 minutes withtwo 7-segment displays showing theactual time left. During the activityperiod, a relay is latched and a flashingLED indicates countdown timing’sprogress.Four tactile, push-to-on switches used to start/stop and to set theinitial value for countdown operation.The timing value can also be changedwhile the counting is still in progress.Auto-repeat key logic also works,i.e., if you hold ‘Up’ or ‘Down’ keycontinuously, the timing as shown on7-segment LED displays changes at afaster rate. The program code in hex isonly 800 bytes long, while AT89C2051microcontroller can take up to 2 kB ofcode.This program can be easily modifiedto suit users’ requirements. Thehex code should be ‘burnt’ into thechip using any universal programmer


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