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wikimhackthreadEdit: add to elaborate specifically on the 5v and ground.
As you may have read,
most mice will have 5v on the middle pin of the optical receiver.
(on your pic, looks like they're marked Q1 and Q2.) To get ground, just grab it from the ground pin from
one of the microswitches (on the back of the board, you should see which ones are common (joined) from the
microswitches (ground) - and which are isolated).
If you have a multimeter, you can easily check for ground and verify the pin on the optic receiver is 5v.
Some mice do not work for hacks (or not easily) but your appears to be of a kind that is hack friendly.