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Identifying and using a rotary encoder, help!

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Hey everybody!

I'm trying to just understand and use a simple rotary encoder, so I can use them for bigger projects eventually. So right now I opened up a working ps/2 mouse I had and took out one of the IR LED/sensor pairs.

But I'm having trouble using them. I can't find the datasheets because they don't have P/N's on them. The LED isn't hard to use. I've found its forward voltage drop by using the meter's diode test, and in a simple battery-diode-resistor circuit. It is about 1.35V.

But the sensor is proving to be trickier. From looking at the circuit board in the mouse, I know that V+ and GND go straight to two of its terminals, and the third pin goes to one of the pins of the chip on the board.

At first I just tried running about 20mA through the LED with it the same distance from the sensor as it was on the board. I had the sensor hooked up with the appropriate pins to 4V (I know it gets 5 from the mouse but my batteries are weak) and measured the third pin with the voltmeter. No change if I blocked the LED from it or not.

I tested it straight on the board too, since there are two more LED/sensor pairs. Nothing there...can anyone help me?
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