Tag Archive for 'etch'

ubbcom gets an upgrade

So a while back, I made a usb-to-serial converter board using FTDI’s most beloved chip: the FT232RL. This is the same chip that Sparkfun uses in their breakout boards. I just didn’t want to pay Sparkfun prices for something I could etch myself.

Well, I’ve given ubbcom a little upgrade.

First, I added the REN and RTS pins. The former is good for hooking the board up to a RS-485 level convert chip, since it can control the tri-state direction. The second I use along with the other handshake signals to bitbang program the ATtinys for bcard.

Additionally, I decided to actually bite the bullet and buy USB connectors. PCB approximations of USB connectors just don’t work. Really. They’re useless. So, I bought some connectors off of Digi-Key and made a footprint. I managed to actually orient the footprint the wrong way around the first time I tried this. That’s like the second time I’ve done this to USB connectors now. Nothing like epoxy to save the day there:

I chose to go with USB A connectors because I don’t trust mini connectors, they’re cheaper than micro connectors, and because I have USB extension cables, which work great. But yeah, now that I have a couple of these guys, it’s easy to communicate with chips, bitbang program them, etc.

DIY Etching of QSOP Traces

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.