One of most critical parts of PCB fabrication is exposing of photo-resistive layer with ultraviolet light. You cannot underexpose and overexpose if you want to get clean and crisp PCB image. The best solution is to use automatic exposure controller.

Controller is based on PIC16F628A microcontroller. It has an LCD to show remaining time. Relay switch which is used to control UV LEDs. Generally there could be other sources of light if needed. Time value is entered and started with four push buttons. When timer runs buzzer generates a sound requesting your attention. Software for PIC microcontroller can be downloaded form here. Project is documented in non English language, but any on-line translator does the job.