Last week, I finally found some current sinking LED drivers that would work well with the LEDs that I chose for the wall lamps. The TLC5926 has pretty much exactly the capability that I need to power these LED arrays. Because the LEDs have a common anode instead of a common cathode, it is necessary to use constant current sinks instead of sources because they sit between the LED and ground.
Unfortunately, the only ones I could find in stock were in TI’s ultra-small QSOP package (other manufacturers use the term “SSOP”). After they arrived, I realized that it would be pretty difficult to prototype with them. Eventually, I planned to order PCBs from ExpressPCB (more on that later), but I wanted a quick way to access one of the drivers I had purchased for testing. I started by trying some 30-gauge wrapping wire, but neither of my attempts yielded anything decent.
I then began reading about DIY PCB etching. I’ve wanted to try this for a long time now (I even had a bottle of ferric chloride just waiting to be used). Unfortunately, I don’t have a laser printer. We do, however, have an old, half-broken laser copier, which decorates copied documents with ghost images and random streaks. I couldn’t find any information on manually etching QSOP traces (they’re so small that I was almost positive either the inkjet printer or the copier wouldn’t have the resolution required to print the traces out). So, I tried it out anyways. I managed to print a simple breakout design and transfer it onto the board using glossy photo paper, heating the board with an iron (on its highest temperature) for about 30 seconds, putting the transfer paper onto the board, and then just holding the iron in place for about 5 minutes. After transferring the toner, part of the paper came off very easily, but the other piece needed to be dissolved away in water.
Next, it was time for the ferric chloride. It’s pretty nasty stuff, so we did all of the work outside. After about 45 minutes of swishing the board around (we should have masked more of the PCB to speed up the process), the copper had dissolved, and I washed the masked traces off with some acetone.
Finally, it was just a matter of soldering the device onto the board. To do this, I soldered all of the pins together and then used a wick to wipe away all the bridges, leaving nicely soldered joints. If you saw the pictures above, you probably noticed that the traces aren’t the right size! This is a result of the old copier we have; it somehow slightly changed the paper size. Fortunately, it doesn’t matter for this chip, which only uses the first four pins on each side individually, while the rest of the pins are all combined together to act as one big current sink.
Surprisingly, it’s quite possible to etch QSOP-sized traces using the “standard” method of DIY PCB etching.
I’m currently in the process of testing these drivers. In the meantime, I’ve already made an order with ExpressPCB for the actual driver boards. I’ll post those, along with schematics for the entire wall lamp soon.