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Switch Timer V2.0

Timer V2.0 Redesign Timer V2.0 Redesign

After some modifications on my UV exposure box (scanner) for better UV expose, i desided that a better pcb must me designed for switch timer. The old one had over drilled holes and it was designed and built on my very fist steps. Also the high voltage side from the low voltage wasn't seperated as it needed to be safe. 

So i redesigned it in a more compact and easier to use pcb. The firmware has been also updated and now you can program the timmer by using the two buttons. The time is calculated by timer interrupt triggering using a 32.768KHz RTC Crystal with better accuracy. The display update also has been changed from static to dynamic.

The board is homeprinted at my exposure box.

Here is the schematic in pieces.

For 5V supply used for AVR, 7-segments etc, has been used a full bridge rectifier, a big decoupling capacitor and LM7805 :

The two seven segments are connected in parallel and are updated using mutliplexing. Current protection resistors are used for each segment. Q2 & Q3 are switching each display.

Here is the relay drived by Q1 witch switches on/off the relay. On the one side of the relay is connected the AC Mains and on the other the lamps.

The AVR ATmega8. A buzzer has been used to indicate when timer is on/off and when a button is pressed. Here you can also see the RTC crystal 32.768 used to trigger the timing counter.




Switch Timer

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


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.