Thursday, February 23, 2012

Desktop Microscopy and Charlieplexing

One of the things I used to have to do with all my custom boards is take them into the lab at work. There they have a really nice optical microscope. This one looks at things instead of through things, so it is appropriate for electronics (funny that, being in an electronics lab). It doesn't have an eyepiece as much as it has a facepiece. The end you look through has a screen (pure optical, no electronics or cameras involved) about 5 inches wide. It gives you a stereo view with a single objective lens, because each of your eyes is looking through a different optical path in the same set of lenses.

I have been searching for something like that and finally restricted myself to something I could actually afford, a USB digital microscope. I ended up getting this microscope.

Here's what I got it for - finding solder bridges

Really short depth of field. Do you want the top of the chip in focus...

...or its leads?
The subject of this particular set of pictures is Project Precision. For about five years now I have had a design in the back of my mind for a digital handed clock with an hour hand, minute hand, second hand and third hand (1 third = 1/60 second. Yes, really. Isaac Newton used thirds in the Principia.). I had a design using all 7400 series logic, a PIC and less 7400 logic, but finally came across the concept of charlieplexing. With 16 pins, I can control 240 LEDs. Only one at a time can be on, but I only need four on at a time, so I can strobe through those quickly enough.

The thing is USB powered with no battery. It uses an ATmega328 as the core and an FT232 for USB-UART conversion. This basically makes it an Arduino. It has a port for a GPS receiver as well, and a couple of multiplexers to connect the ATmega, FT232, and GPS in any combination.

What I'm most proud of is the layering. I needed 60 LEDs for each hand, and with four hands, I needed the front and back of two boards. I figured out a way to make these boards identical, so when I got three copies, I had enough, and didn't have to order three copies of two different boards.

Charlieplexing involves pairs of LEDs, pointing in opposite directions. If you set one end of this combo to VCC and the other to ground, one of the pair will light. If you do it the other way, the other will light. What I have done is put half of each pair on one board, and the other half on the other board. All the LEDs on the board with the logic will point outward, while the LEDs on the other board will point inward. It's almost fate that 16 signals can control 240 LEDs, the exact number needed for four hands. 16 was the exact number of signals left on the ATmega after I accounted for the UART, clock, multiplexer controllers, etc.

Now for the tedious task of picking and placing 240 diodes... Fortunately I only have to do them 60 at a time.

No, I will not make one for you, not for any price. Well maybe, but it would have to be a lot. Better is to make one yourself.

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