Boards showed up on Friday. The boards contain the next generation of driver cards and four copies of the incandescent driver boards. I got a chance to build up one of the cards and immediately noticed that I put all the MOSFETs in backwards. Grrrrr. I originally planned on using one MOSFET, but opted for one that could sink more current instead. The new one is in the same package, but it is pinned out in reverse. I’m glad that I picked the new packaged because the current is higher than I expected.
The first built board is always a learning experience. You learn which components to solder first, and where the gotchas are. I rushed the board a little bit too much, and didn’t leave enough space between boards. Because of the tightness between the boards, I needed to use the Dremel tool to cut between the boards instead of the tile saw. When I use the Dremel, I can’t set up a fence to get the cuts perfectly straight. I’m disappointed by that because I feel I could have spent a little bit more time, and made sure there was enough space so I could have used the tile saw.
When I did the initial soldering of the FETs on the card, I used too much solder. I ended up needing to wick off some of the extra solder, but corrected the mistake on the second group of four MOSFETs. Luckily, since I put all of the FETs in backwards, this was now simply my practice board. The second board that I soldered went much better and looks very clean.
I measured the current through multiple 44 and 47 bulbs. These were old bulbs that were already installed in the machine. At 5V, they measured anything from 200 mA to 350 mA. That gives them a range of resistances from 14 to 25 Ohms. The new FETs (BS170) can sink 500 mA continuously so that should be plenty of headroom.
I would really like to switch over to LED bulbs, but I can’t find any succinct information about the light output at 5V. All of the LED bulbs have internal current limiting resistors, but without that info, and the spec on the LED itself, it is tough to say what it will look like. An LED has a steep voltage to light output curve at low voltages, then flattens out at higher voltages. The current limiting resistor determines the location on that graph, and it may or may not be a huge difference in the amount of light. (At higher voltages/currents, most of the extra current is converted to excess heat.) I’m going to ask some people around here if I can borrow a bulb or two for a week to see if I can make some measurements.
The big plans for this weekend are to try and get the incandescent driver board actually driving lights. I’m planning on using a computer parallel port to test the boards. I just finished the voltage converter board. A printer parallel port outputs 3.3V but must be converted to 5V since the STPIC driver needs 4.25V for a logic high. The output of the printer parallel port will drive the gate of N channel MOSFET which will convert the 3.3V level to 5V. The output will be inverted since a high level on the parallel port will turn on the MOSFET which will ground the pullup to 5V.
I will probably end up doing a simple python script to bit bang the SPI interface using the parallel port. That gives me a simple way to test the boards after I solder them to make sure they are working properly.
Ended up building a simple test fixture for testing the boards. It contains 8 LEDs to test the 8 drivers on the incandescent driver board. Take that, add an old PC power supply that I have laying around, and I should be set to go pretty soon to test the boards.
I had big plans of finishing the testing this weekend on the driver boards, but it now seems like I won’t get to it this weekend. Probably sometime during the week. The board is populated, the test fixtures are created. I just figured out that a USB to parallel port won’t work, grrrr. I started working on the Python test script, but I probably won’t have the time tonight to test, do a video, etc.
So about a month ago, somebody contacted me to get some boards so he could run an old playfield. I was still looking for another alpha site to make sure somebody else tested the hardware and gave me feedback on the documentation, etc. Since I tend not to use real names for people, I’m going to call him Cactus Jack. (The name is completely based on the fact that he has a cactus at the front of his house. Looks like a nice cactus.) He has been working hard, and has tossed up a couple of videos and a little bit of a blog entry. Of course, now he is waiting for me to finish up the incandescent driver boards.
The videos and blog are located at http://www.kerform.com/.