Electronics > Microcontrollers

3-pin optical incremental encoder IC with digital output.


Stumbled upon rather interesting part in broken Logitech mouse. Maybe someone knows the part number. It's in 3-pin TH package (just like standard dual phototransitor), also marked on PCB as LQxx, but seems like it's an IC with binary output. Two pins provides the power, the third is IO pin. MCU provides 2 strong clock pulses and IC responds with weak pull-up or pull-down. If wheel is idle both bits are 0. First bit is 1 if scroll wheel rotates back, second bit is 1 if wheel rotates forward. Although I figured out the protocol, still would like to see the documentation.

I known this is an old thread.  I'm curious about this- did you ever figure it out? Can you post any other details, maybe a sketch of the circuit.  I have worked with a lot of optical interrupters, etc. and never seen anything like this. 

Mouse guys are very clever- usually in order to save power.  Keeping a phototransistor biased up with a pull up and having full swings would draw milliamps- bad. Not such a problem in the old days of hardwired PS2 mice but I was curious about how these wireless mice could run a year on an AA cell.  I ripped apart several mice and found a variety of neat techniques.  My favorite was the simplest and also the lowest power.  The hung a photodiode on a port pin, reverse biased.  They would drive the pin high to charge up the photodiode capacitance,  then go into tristate (input) mode and the capacitance of the diode would hold up the pin because of its 20 or so pF and the pin's low leakage as long as there was no light.  They would then briefly light the interrupter LED to test the state.  They would poll the sensor to see if light had been on it in the last interval- it would discharge the cap.  The leak rate was something like 1 mS so they did this all very quickly each time they wanted to read out the state.  Its very clever and very low power.  It has a bunch of kind of hidden virtues.  It has a bi-stable and integrating action, they set it and the light would reset it.  Each read was done in just fractions of a millisecond and the only power was the brief LED light pulse and the energy required to recharge the photodiode capacitance (miniscule) .   Overall, genius.  I built a mouse gadget for a Motorola design contest long ago (6805K1) and used these techniques- amazingly low power.  I don't if its patented but if I had thought of it I would have!

Take care.


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