This is repository of PCB Exposure Box [expbox] which uses PIC16F886 for it's brain and shows the time on display made up out of four seven-segment led modules.
Because display is made up out of 4 digits, I have decided to limit maximum exposure time, that can be set, to 1 hour (59m59s). Even 1 hour is more than enough, because exposure process takes no more than maximum 10 to 15 minutes.
After the countdown comes to zero, finish of exposure process will be signalized with 1 second of high frequency tone.
Microcontroller used in this project is PIC16F886 produced by Microchip company. This MCU has three timers and each timer is assigned to a separated task. Timer0 is responsible for multiplexing four seven-segment led modules. Timer1 is shared between tasks for precise time measuring and buttons debouncing. And last, but not least, Timer2 is in charge of driving piezo speaker with Pulse Wide Modulated signal.
The controller uses an internal oscillator and it's set to maximum frequency of 8 MHz.