July 01, 2007

LED Bench Illumination

Background

I got sick of the area under my workbench shelf from being dark, so I started to look at lighting options for this ~25cm high space that is eternally shielded from my workbench light.

Light bulbs and CF bulbs were excluded from the start, as they're way too bulky for this space. I needed something slim and low profile. A CCFL tube was employed for a while, but didnt quite have the brightness and diffused illumination I wanted. But then whilst eBay browsing I struck gold - white LEDs!

Hooray for the white LED

LEDs are now viable lighting devices, and exceed the efficiency of most other lighting sources, only narrowly being beaten out by CF lamps (excluded by their bulk). LEDs are low powered, extremely small, and give off virtually no heat. And they're cheap. A hundred bright white LEDs can be bought from Asian sellers for around ten bucks shipped. Forget buying them in the stores, they're a couple of bucks a pop. Whereas for around 10 cents each, you cant go wrong.

I quickly went and got myself a bag of 100 LEDs and around a week later they arrived in the post. I found a wide angle (120deg) type, as thie will give much more even illumination than typical 15deg types.

Putting it together

I worked out that I'd need a matrix of 16x6 LEDs, using up a total of 96 LEDs. This gives around 3.5cm spacing between each LED with the bench size I've got. The easiest way to power this was by configuring them as strings of 6 LEDs in series, then 16 of these strings in parallel. That means powering them with around 20V @ 400mA, which I could easily do, using an old plug pack lying around.

I began by drawing the matrix on the underside of the bench, and marking out where I'd need each LED. I then drilled 6.5mm holes halfway into the board (not all the way through). The photo below shows this in progress.

Next, I cut grooves in the board for the wires - I wanted this to be low profile with just the LEDs visible. Exposed wiring is messy and dangerous on an electronics bench. My trusty Dremel clone did the job amicably, although the friction made a lot of smoke, but didn't permanently discolour the board. Photos below shows this in action, as well as the final result after all the grooves were cut:

Inserting the LEDs was the next step, and pretty time consuming. Each LED had its wires bent almost 180 degrees, then pushed into the hole, looking out. The wires were then bent to meet its neighbour's legs. Care was taken to ensure all LEDs were the correct polarity. The photos below show this:

Soldering the LEDs was a bit tricky. I clipped the legs to be just short enough to overlap the neighbouring LED, then carefully soldered them flat against each other. This had to be such that the legs could be recessed into the grooves I cut. Finally I got this done, and ran main bus wires along the top and bottom row so that each chain of 6 was in parallel.

It was time for first light. I grabbed my bench PSU and wound the voltage nice and low. Hooked up the LEDs and gradually rose the voltage, and they all lit! Eventually I rose it till the current hit 380mA, and I had a massive array of white LEDs. (MAWL). See below for what this looked like. I mistakenly drilled 7 holes in each row, seen here too, but these were filled later.

Sealing the LEDs was the next task, done using a tube of two-part plastic compound. Similar to Araldite, the two parts are mixed together, and after about 20 minutes sets to a rock hard compound (as opposed to soft epoxy). Using a blade as a spatula, I smeared it into the grooves, ensuring the wires were well recessed, then let it set. This is what it looked like while curing:

Once cured, I used a knife to scrape away the excess, leaving just filled grooves and otherwise smooth laminated board. Next I used some white hobby paint to finish off the job. The result was a MAWL, with somewhat hastily obscured grooves holding the wires, eventually leading out to the edge of the board for the power supply.

Let there be light...

With the shelf done, , it was time to put it back in place and check it out. As you can see from the photos below, it does a marvellous job! The ambient lighting (ie not covered by the LEDs) is a standard 60W bulb, about 60cm above the bench, so as you can see, these things are really bright, and totally outshine it! They also give virtually no shadow, vital if I'm holding something underneath, and the mass of white points is pretty mesmerising too.

Powering the LEDs is currently done with a bench PSU, but I'm just about to add on a proper supply - basically a 24VDC wall wart, with an LM317T based constant-current circuit. Eventually I might also do a PWM based dimmer, but that's for a little later.

Posted by Ben at July 1, 2007 03:11 AM
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