Countdown Timer Based on PIC16F84

The project will be used in PCB exposure unit for the purpose of providing a countdown timer from 1 second to 99 minutes and 59 seconds.

The lighting for the Ultra Violet exposure is being controlled using this circuit for the photosensitive PCB material. An audible alarm is also provided by the project at the end of countdown time and switches the UV lights by means of a relay. The circuit is built around an 18-pin microcontroller PIC16F84 that contains 64 bytes of static RAM memory, 1K byte of flash memory for program code, and 64 bytes of EEPROM memory to be used for storing up to 15 different countdown times.

For the power supply unit, the 2×9VAC from 3VA transformer is rectified by diode D5 & D6 and smoothed by C11 & C12 to produce around 12VDC voltage that will be used to feed voltage regulators IC5 & IC6. The +5V voltage for the logic is delivered by 7805 regulator IC5 while the +8V voltage that feeds the common anode displays is delivered by 7808 regulator IC6.
The scanning of push buttons and activation of relay and buzzer is handled by PIC16F84.

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