Two years ago i modified a scanner replacing its mechanism and all the internal electronics with UV lamps converting it to an UV exposure box for PCB prototyping. By the need of making my life easier and not waiting for the pcb to be exposured i designed an AVR timer to automatic switch off the lamps after 1 minute of exposure which was much enough for the PCBs. Here is the circuit

The circuit

This was one of my first completed projects from design to production. As you can see the scheme isn't that good and there have been by passed some capacitors at the power supply. But the circuit works with no problem.

B1 is an AC voltage rectifier which in compination with the C1 (filtering AC) converts the AC to DC voltage. The LM7805 comes to regulate the voltage at 5Volt for ATMEGA8 power suppling. The AVR drives two 7-segments D1 and D2 for timer digits displaying, a buzzer SG1 to beep when lamps are switched off and a relay K1 driven by a NPN transistor Q1 to switch on and off the UV lamps. Also there is a diode D3 connected in parallel with the relay's inductor to prevent reverse voltages burning the transistor. Switch S1 is used for AVR restart and switch S2 is used for timer reset.

Testing on breadboard

Timer-bread board testing

The finally PCB printed with the UV exposure box

Timer-PCB

The timer

The timer

 

 

The code is written in C and the delay is produced by a delay routine and not by a timer. This because when i had designed the timer i didn't have enough experience with timers so it was much easier and familiar for me to use a delay routine. The device is accurate enough for uses like the one i designed it for. The code is small, very simple and understandable.

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2 thoughts on “Switch Timer

  1. i know that is it wil be hard for u but can u make it with 99 min control system and on time in seconds i want to control a motot that shuold be off for 99 min and on for 3 sec i will be looking forward 

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