Ball mouse turned into cheap tachometer...

I am trying to create a simple DIY tachometer using parts to a Radio Shack serial ball mouse I have. The idea is to use the shaft encoder
(disk with equi-spaced slots) and the (what seems to be) infrared emiter and reciever to obtain an electric pulse. That is, as the disk spins, the IR signal will repeatidly be broken causing the reciever to see a pulse of IR. My problem is the circuit building. Does anyone have any idea on how to get this project to work? Any help would be great. THanks. Lucas.
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Well, first off, what's wrong with the circuit that comes with the mouse? You can always desolder the sensors from the board but connect them to it with wires. If you want to build your own electronics, IIRC, the guts are something like this: pulsed led on one side, dual phototransistor with either common emitters or common collectors. The led is pulsed to save power since a serial port can't supply much. You won't need to do that. You'll want to put a resistor, probably 10k or so but you can experiment, in series with each phototransistor across your 5v supply. Put the center taps of those into a 74LS14 or a comparator with hysteresis to clean up the signals. Unless you're spinning it way faster than a mouse does you can probably decode the quadrature signals in software on an mcu.
chris
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snipped-for-privacy@uiuc.edu wrote:

I would just use the circuits and all that are in the mouse. Google for the PS2 protocol for serial mice and write a "SPI-ish" micro interface to talk to the mouse. In this way you get the fully X/Y movement which should do wonders for dead reconning on your robot.
DLC
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Dennis Clark TTT Enterprises
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