ivan@rzsin.sin.ch ("Ivan D. Reid") (03/03/89)
I was interested in the messages last week about mice "dying", because one of our rodents started exhibiting the same behaviour (only moving horizon- tally & not vertically) the day before. The messages suggested to me that the cause is the loss of pulses on one of the four outputs (two for each axis), and blocking the light-path to one of the phototransistors on the good axis duplicated the behaviour on this channel too. A little bit of investigation showed that each phototransistor is connected between +5V and a resistor to ground; the signal developed across the resistor is then fed to one section of an LM339 quad-comparator. The reference for the comparator was 0.20V, and the output should switch between 0 and 5V as the input passes through 0.2V. A multimeter showed that one signal from the faulty axis was only just coming up to 0.2 while the other three were reaching up to 0.3 to 0.4. Obviously the circuit characteristics had changed enough with time that the output pulses were no longer being reliably generated. The quick fix was just to increase the sensing resistor from 470 to 820 ohms, since for small signals the phototransistor should develop a given current flow; passing this through a higher resistance gives a proportionately higher voltage. I hope this info might be of use to anyone with a dying mouse and a little bit of electronics equipment (hint: SolderWick or a SolderPuller are highly recommended when unsoldering the resistor, as the circuit board is easily damaged). idr