Monday, 14 November 2011

Charlie-plexing 90 LED array for word clock

Despite somehow printing the PCB for our word clock project at about 90% (so the 0.1" pitch connector won't fit properly!) we decided that we'd put too much work into the first version just to scrap it - drilling over 200 holes by hand was over an hour's work on it's own!. We do now have a "proper" sized board, but we figured that we could use this messed up one as a proof-of-concept prototype.

So we got 100 white 3mm LEDs off eBay and set to work.

We started out with the top row of 10 LEDs - soldering the legs but NOT cutting them to length. Then after the second row of ten LEDs had been soldered in, we could bend the legs on the top row across to solder to the LEDs on the second row. This is how we're making our connections between each horizontal "row" on the PCB.

Care should be taken to make sure that all "ground" legs (i.e. the shorter, cathode leg) are clipped short enough so that the legs being bent DON'T make contact with them (effectively shorting one or more LEDs to ground).

Here's the top side of the board after the first two rows of LEDs were installed

After installing a couple of rows of LEDs, getting the snips in between all the flying legs was a bit of a problem, so we cut the cathodes of all the LEDs to length before putting them onto the board.

This made installing the LEDs much easier - less than 30 minutes later and we had a fully populated board:

The reverse side showing how the LED legs were bent in different directions, to create connections across the horizontal rails on the PCB

(note the connector is still in place but the final job will be to remove this and solder the ends of the wires from a 10-way multi-core cable straight onto the board)

The final LED matrix with current limiting resistors in place on each "column" of the matrix.

Now we're ready to hook it up to a microcontroller and start flashing those lights! Sadly, it's a little late to be starting such a thing now, so it'll have to wait until the morning......