When doing more than one thing at the same time it is very easy to lose track of time. Exposing PCBs to UV light is delicate work and overexposing will result in, well, you know what. Housing for the exposure system shouldn’t be a problem because those old broken PC scanners come very cheap.
-two buttons are used to set the countdown time in 10 seconds step
-the third button is for counter start/stop
-timer counts to zero and at the end it turns off the timer, and beeps on the piezzo buzzer
-the first two buttons are disabled during countdown procedure
-previous set time is saved in EEPROM so it is required to set the time only once. EEPROM will save the settings after power-down and upon power-up it will display previously saved time
-microcontroller will go to sleep after two minutes of no activity, and the current consumption is reduced to less than 5mA
-by pressing on start/stop button, it will wake up