Wednesday, February 22, 2012

Rotating LED Thingy

Idea and Design
My son William wanted to build an LED light chaser. So I hunted around for a circuit, and dug around in my spare parts.



I decided to build something based on this circuit.
The only differences are, I used a 4069 hex inverter instead of a 4049, because I have a bunch of them, and I changed out the 100 kohm resistor. I replaced the resistor with a 10 kohm fixed resistor in series with a 50 kohm potentiometer. That gave me a nice slow speed at one end and a very peppy speed at the other end.
BTW, the parts list for this project listed on the web page above specifies 150 ohm current limiting resistor, which is what I used, not the 470 ohm resistors shown in the schematic.  Also not clear on the schematic is that the ends of all those current limiting resistors need to to to ground.

The Gathering
As it happened, I had a circular prototyping board from Radio Shack. I did have to go out and buy a 4017, plus I picked up five pair of different color LEDs, and a bunch of 150 ohm resistors. Radio Shack didn't actually carry a 4017 chip, but they had their own LED chaser kit for sale that used the same chip. So I picked it up and stole the chip. I'll order a replacement and build the kit or give it to someone at some point. 
As I was laying out the circuit on a breadboard, I remembered I had the wooden base of a snow globe that had broken lying around. Then I found a plastic dome that almost fit. I would have to route the hole in the base a bit bigger. A toggle switch for power and a potentiometer and a few other parts and I was ready to go.

The Build
Here's the thingy all breadboarded out. I used a small multi-turn pot for the breadboard. It was replaced with a panel mount pot for the final build.
(click image for close-up of the breadboard)

As I mentioned, I had to route out the edge of the base to fit the dome. That was a bit tricky, but I managed fairly well and didn't even endanger myself. I can't recall what the dome was from. Some broken toy or something. It's fairly thick plastic, though. I'm glad I saved those two pieces, and that they found each other in this project.

The base also needed some other work. I drilled out holes for the power switch on one side and the potentiometer directly across. Some careful measuring and drilling were required. The sides of the base are pretty thick, so I had to countersink the mounting holes using Forstner bits.
Quick tip: When making countersunk holes for a switch or potentiometer or whatever, you need three sized bits. First a very small bit to drill a pilot hole. Next the largest bit to make room for the fastening nut on the outside. That cut doesn't pass all the way in, but stops at an appropriate depth. Lastly, a middle sized bit just slightly larger than the diameter of the mounting shaft is used to drill all the way through. With this method, you always have a small pilot hole to keep everything centered.
I mention this, because I forgot to do that when I drilled the holes for the power switch. So my last cut was a little rough without a proper pilot. Compare the cut for the power switch on the left, with the cut for the potentiometer on the right. You can also see some of my sloppy routing along the top of the left picture. It's not perfect, but it look OK.

Last cut on the base, I countersunk two more holes to hold the mounting screws.

I used brass tubing to make some standoffs, and some 6-32 threaded rod and brass nuts to mount the circuit board.

Despite the crazy nest of wires above and below the board, and the solder blob jungle underneath, the project came off rather well.

Here's the finished product. If you click on it you'll get a higher resolution view. I still need to find a nice knob for this project.  It looks very Victorian to me. I won't say it looks steampunk, because integrated circuits are really, really not steampunk.

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