#electronics diy

See, I've got a couple of cute little displays with no accompanying data:

The number on the side isn't an identifiable part number, and the markings on the other side — "07817N" — is more likely a date code or batch number than anything else. With this many segments and 24 pins, it's likely either a common-cathode or common-anode device, and not a matrix, so that's somewhere to start; I popped one into a breadboard and started probing with a little LED tester and some jumper wires. The first few pins I tested turned up some red light, so that was very encouraging.

It turned out to be a common cathode device, mostly.

A single-digit common cathode display works like this: you ground the single wire for the whole display — the common cathode of all the LEDs — and supply voltage to each segment's anode. So to make a three on one of these, you put the voltage into the anodes for segments a, b, c, d, and g, and to turn the whole thing off, you stop grounding the cathode.

A multidigit common cathode display works the same way, but all the identically named segment displays are tied together. You usually need a microcontroller to manage the timing, but you have it ground the cathode for the digit and light up the segments, then stop, unground it, and do the same for the next digit. You can't light them with different layouts at the same time, but thanks to human persistence of vision, you don't have to. You just loop through really fast and it looks like it stays lit. On this one, pins 2, 5, 8, and 10 are the cathodes for digits 1-4 respectively, and the pins for segments a-g are 17, 19, 20, 3, 11, 18, and 22. The decimal points for digits 1-3 have their anode at pin 9, while the one for digit 4 (the bottom of the final three dots on the display) is on pin 16 for some reason.

But while the device I have operates like that for the digits, the dots are handled a little differently. The first three are an independent common cathode display, with individual segment pins that aren't connected to anything else (CC on 24, with a on 23, b on 4, and c on 1), and the top two of the last three are the same way (CC on 15, top on 13, middle on 14). The top dots of the middle colons are independent LEDs — the first has its cathode at 12 and anode at 21, and the second has its cathode at 7 and anode at 6.

This one was a Velleman MK119 Roulette Wheel — 37 LEDs in a circle, with an animation and spund effect that picks one at a button press; 18 of the LEDs are red, 18 are green, and one (labeled 0) is yellow.

I found the kit at a thrift store for about $4, and decided to take out all the pieces to sort them and check if any were missing, which led me straight into impatience and building the whole thing over the next hour or two. (I should also check the clock more often.)

Here's my assembled unit, on the paper I'd taped the parts down to; below is a manufacturer photo of all the pieces.

so, we've been trying to build an analog synthesizer (as an electronics newbie). our oscilloscope just arrived, so can finally check how this vco performs, which went surprisingly well (although we still gotta hook a speaker up to it). works MUCH better than the simulated version, too. we relied a little too heavily on that to judge how this would behave...

this doodle was produced late at night, in a burst of frustration/procrastination.