Simple DIY tachometer made from old serial ball mouse

Once you've got the mouse apart, look at the component values around the optical interrupter. The LED current is probably something on the order of 20 mA, since that's fairly standard for LEDs, but it would be worthwhile to see what resistor they're using in the mouse, and use the same value to drive the LED. The output device is just a phototransistor or photodiode - ground its emitter (or whatever is supposed to be the negative pole - I don't remember how to use PHDs), run a 4.7K or so resistor from the collector to VCC, and you should get a pulse train.

It's a trivially simple circuit, to us self-appointed "experts" - what I'm wondering is how you're going to manage the mechanical part.

Good Luck! Rich

Thanks for the advice, there is one problem, the phototransistor/diode has three leads to it. Im not sure what it is exactly. Do you have any idea? I mean, i have played around with the IR phototransistor/diodes sold at radio shack that have two leads (look very similar to a LED), but this is different. Anyway, if anyone knows what exactly im dealing with, I guess thats what I need to know. thanks a lot, Lucas.

Sounds like one lead is ground, and the other two are the + of the LED and PHT. Do some experiments - use an ohmmeter to see which is the diode: only two of the leads should behave that way - that's the LED and ground; the other lead is the collector of the PHT. If you read two diodes, one of them should be "upside down" - that would be the photodiode if that's what's in your device.

This is just a semi-educated guess, of course - is there any way to trace out the circuit it came out of?

Good Luck! Rich

Google is your friend:

stick it back in the mouse, power it up, and make some voltage measurements.

Thanks rich for the great link, that pretty much answered most of my questions.