Improving my UV-box

For a long time now I have had to boxes to expose my PCB's for etching. These two are made as simple as can be. Two identical wooden boxes with a vero board full og 3mm UV LED's in the bottom and plexiglass on top.

It's a very simple construction that does the job, but now I want to make something more robust and a timer which can shut down the exposure when the PCB is ready.

What I have done so far is just "eagling" up the schematic. It bothers me to hell that I had to make a jumper, but I just have to swollow my pride on this one.

Board

 

Schematic

 The resistors are about 60-100 ohms, I'll just have to adjust that when I am happy with the intensity of the UV's, as long as I don't exceed 2.8V per diode. From experience I know that I should keep the voltage at a minimum, since the LED's from Ebay is of fairly bad quality. As one can see from the simulation, a 100ohm resistor should do the job fairly well.

Simulation of three LED's in LTSpice IV
The green line is the voltage over the LED's,
The blue line is the current through each LED

If each LED draws 36mA and has a forward voltage of ~2.8V, the voltage over the resistor has to be
U(R) = 12 - (2.8V * 3) = 8.4V. With 36mA in the circuit that leaves us with
R = (12-2.8) / 36mA = 3.6 / 0.036 = 100ohm.

The next thing I will do is to make two of these boards on the CNC-machine I have access to at work, and then start building a chassis and a timer circuit .


- Wilhelmsen