[EDIT: Seems that this post was featured on Hack a Day So excited about that!! Many thanks to the Hack a Day Staff]

[UPDATE 02/10: Writing the second part of the post is taking me more time than I originally planned, this because I’m moving into a new house. I’m sorry for the delay, I hope to publish the next part during the second week of October. ]

[UPDATE 05/11: I have finally written the LAST POST about the UV exp box]

WHY

First objective of all this trip was learning to make a decent pcb from scratch. Honestly there was’nt just one, but multiple objectives:

• Learn to use Kicad (Kicad is an open source EDA  suite – check wikipedia, some tutorials here);
• Design a fully working pcb with a microcontroller on it;
• Learn to impress and etch a board at home;
• Finally, make a uv exposure box with a programmable timer and display, for making other boards.

THE INITIAL IDEA or Let there be more light

To expose presensitized pcbs you need an uv light source. Common commercial solution is based on neon – For example - .

I choosed to use some uv leds bought from china. They are cheap, I bought 200 of them for 13 euros. With all this leds I tought to make a uv led array with current limiting resistors, driven from 12V power supply. To select wich resistor value and how many resistor to use we have to do some math; If you won’t bother with math, you can use this online tool available here.

For those that want to see annoing numbers, we go ahead with the math for a single resistor in series with a led:

VAcrossResistor = VSupply - VAcrossLed
ResistorValue = VAcrossResistor / IacrossLed #Ohm value
PowerDissipatedByResistor = VAcrossResistor * IacrossLed

So, suppose we got a 12V supply, a led with a forward voltage of 3.1 V and 20 mA (20 mA are 0.02 A) of forward current:

VAcrossResistor = 12V - 3.1V = 8.9V
ResistorValue = 8.9V / 0.02A = 445 Ohm
PowerDissipatedByResistor = 8.9V * 0.02A = 0.178 W = 178 mW

That was ok, but if you wanna drive an array of 135 leds using 135 445Ω resistors, you result in a lot of energy wasted. Almost 24000 mW of power are turned into heat (Do the math: 178mW * 135 = 24030 mW) . We must improve our efficiency, trying using less resistors and putting them in series with leds.

Now I’ll try to perform the needed calculations, just to see if is it possible to put three leds in series using a single current limiting resistor. If the calculation ends positively, we can use the same obtained configuration and repeat it 45 times, calculating in the end the total consumption of my array.  (This kind of configuration is almost ok if you are pretty sure that all 3 leds have same voltages & current drop. A lot of other efficient ways to driving led exists, this is a very basic way)

Let’s begin:

VAcrossResistor = 12V - (3.1 + 3.1 + 3.1) = 2.7V
ResistorValue = 2.7 / 0.02 = 135 Ohm
PowerDissipatedByResistor = 2.7V * 0.02 = 0.054 W = 54 mW

So we are wasting only 54 mW of power. For 45 groups of leds, this become:

54 mW * 45 = 2430 mW 

2430 mW of wasted power, that is almost 10 times less than using a single resistor per led.

How much is the power drawn by the array, totally? A single led drains:

3.1V * 0.02A = 0.062 W = 62 mW

135 leds burns about (62 mW * 135) 8370 mW of power. Now we sum led power and resistor power:

8370 mW + 2430 mW = 10800 mW

That are 10.8 Watts. A 135Ω resistor is probably uncommon and hard to find (maybe impossible, who knows!) so we can go ahead and use a more common 150Ω resistor, without exceeding our specs:

V = 150Ohm * 0.02A = 3V   #Voltage drop across resistor
P = 3V * 0.02A = 60mW     #Power wasted for each resistor
60mW * 45 = 2700 mW       #Total power wasted by resistors
2700mW + 8370mW = 11070mW #Total power required for the led array

Total current consumption is:

I = Veq / Req
Req = 1/( [1/(150Ohm * 4 = 600Ohm) * 45 ] = 13.33 Ohm # 4 = 3leds + 1Res
I = 12V / 13.33Ohm = 900 mA = 0.9A

Done!

We have now the specs for choosing a power supply:

• DC 12V
• Maximum current 1 or 1.5 A
• Maximum power almost 15 Watts

I found this on a local fair.

Leds are soldered on a pre-drilled proto board. That was a painful and tedious process, maybe could be useful if you want to learn how to solder.

Some suggestions:

• Keeping in place the leds can be difficult. I soldered all the leds without the aid of a panavise tool, the results are ok but I wasted lot of time trying to keep all the leds properly aligned. A panavise or a third hand tool is HIGHLY recommended.
• Double check cathode and anode positions before soldering, ending with wrong soldered led after 1 hour of work sucks. Really.

Boxed light

The box his a modded ikea photo frame. More precisely, is made from three ikea photo, and a sheet of plexiglas cutted to fit. The upper side of the box was made using two frames glued together, the third frame is linked to the glued two frames using a hinge bought on local hardware store. The upper frame was used to contain the array of leds, the timer board and a display with pushbuttons. Push buttons was made cutting three screws with a dremel, using nuts to keep they in place.

It’s all for this post, stay tuned for the next part. (I’ll try to put it online for the next week)

(If you note something wrong on the led resistor calculation, drop me a line!)

(Drop me a line also for everything else. Feedback is appreciated a lot.) (Forgive me for my messy usage of english language.)