Encountered the Ululatron before?
Here is some background info on it that I found in a forum:
The principle behind building it is very simple, although it can be time consuming. It is based around getting 555-based LED flasher circuits to trigger key hits on the keyboard. A 555 flasher circuit is very simple to build (see the Tiny Tremolo thread for an example – although that one is CMOS, which we don’t need for this project). I can post the specific layout of the circuit I used later. I experimented with some parts to get the most play out of the pots I was using, but it’s a pretty standard LED flasher.
1. Open up the keyboard you want to use. Inside you will find a long strip of contacts where the keys, when struck, make a connection that in turn makes the keyboard make a sound. Get rid of the keys and the rubber pads–you won’t need them. On the contact strip you will find that each key makes contact between two points (these will either be small copper pads or squiggles). Use a knife or something to scrape off the coating on the contact points to expose the copper underneath. Now, do some experimenting by touching a piece of wire in between various contact points. In my keyboard (and you can see this reflected in the design) the contact points were divided up into six groups. Each group of six connected to a common contact point. It looked something like this:
C C# D D# E F F# G G# etc.
——————————
1 1 1 1 1 1 2 2 2 etc.
If you see what I mean. Each keyboard will be slightly different, but in every one you will find that a group of notes connect to a common. You can also figure this out simply by following where the copper traces go. Figure out how many groups of notes your keyboard has – this will be how many timer circuits you will need to build. My keyboard had 6 groups on a 32 key keyboard – 5 groups of 6 notes with two notes left over on their own circuit.2. This is the tedious part: Solder a wire to every contact point on the keyboard contact strip. For the common nodes you only have to solder one wire to one of them–this will be the pole connection on your rotary switch.
3. Build the timer circuits. Make sure to connect two LEDs to each circuit – one will drive the photoresistor in a DIY vactrol and one will be a rate indicator.
4. Build the vactrols. All this is is a photoresistor (LDR) that a blinking LED shines on. You will need to encolse the pair in a light-tight package. Don’t use electrical or duct tape – these come unsticky and your vactrol will pull apart. You could do duct tape covered in hot glue, or one brilliant solution I saw was modelling clay (the kind that doesn’t dry out)
5. Get as many single pole rotary switches as you need (one for each group of notes). Solder each of the wires coming from the individual key connections to a throw point on the switch. Solder the common node wire from the keyboard to one side of the photoresistor. Solder the center pole of the rotary switch to the other side of it. Now, you see that the blinking LED will connect or break the connection between the common node and the note wire by raising or lowering the resistance of the photoresistor (you need to use pretty high value photoresistors – mine were 500M Ohms at complete dark, which I think is pretty standard and more than enough to completely resist the small charge that signals the keyboard keys. Repeat the proces for all the groups of keys your keyboard has.
That’s it. Now the pots on your timer circuits will control the rate at which the vactrols signal the key hits. You use the rotary switch to dial in the notes you want. You’ll probably want to label where each note is on the rotary switch on your enclosure.
Disclaimer: There are some weird keyboard designs out there. I can say that if you have a Yamaha PSS-50 this project is easy as pie. I have seen some weird configurations inside keyboards though. I do think, however, that they pretty much all work on the same principle. Some keyboards will be better than others for this particular mod.
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