A long time back, well over a year I think, I ordered five ISD1820 module boards from Ali Express or eBay or something, and when they arrived I stuck the envelope, unopened, in a drawer. It was there until a couple days ago. I finally pulled it out and opened it.
After thinking about it I decided for messing around purposes, it’d really be easier to just pull the chips (they’re socketed DIPs) and use them on a breadboard than to deal with the module boards. I recall at the time trying to find a datasheet or other information on the ISD1820 and not finding much. I tried again and still didn’t find much, but I did find this at the Bastl Instruments site which is very helpful.
The ISD1820 is a voice recorder chip. It’s not an audiophile’s dream. It’s intended for things like answering machines (does anyone have answering machines any more?) and that kind of thing. It’ll record and play back 8 to 20 seconds of audio at bandwidths of 3.4 to 1.3 kHz. Complete with noise, silence, and clicking. But it is cheap, easy to use, and fun.
(At around the same time as I got these, I also bought a module board built around an ISD1760 chip which is much bigger, actually documented, better featured, and, maybe, more legitimate — I get the suspicion the ISD1820 is purely an unauthorized imitation of the real ISD chips, but I’m not sure of that. The ISD1760 would be nicer for synth module hacking… if it worked. I can get it to record and play back but playback is accompanied by very loud noise. No idea why. It’s a socketed chip too. Maybe I should pull it, put it on a breadboard, and see if it’s the chip or the board that’s the problem.)
So I did breadboard a basic circuit. There’s a pin on the chip for an LED that lights while recording; it also flashes at the end of playback, for some reason. But that means you can connect it to the pin that triggers the start of playback and then it’ll loop.
The chip’s meant to drive a speaker directly but a capacitor and resistor suffice to make an output jack work. Bastl also shows how to send an external signal in, but I just hooked up an electret mic for this. And I did try out Bastl’s voltage divider based approach to making the chip’s oscillator speed (hence pitch and duration) voltage controllable. It kind of works, but it’s a bit sketchy. I’ve been simulating what I think would be a better way, but I haven’t tested it yet — and some parts are starting to come in for higher priority stuff, so it may be a while. Another year? Maybe not. I do have some ideas for a rather gonzo synth module using two of these chips (or, who knows, maybe ISD1760s if I can get one working), but I absolutely am not going to get embroiled in that while I have more sane stuff to work on.
I pulled the ISD1760 IC and tried it on the breadboard. Less noisy, still noisy. Yeah, I’d think I’d rather play with the ISD1820, at least to start with.