> watch a "flashing box" on the monitor Sorry, I missed that in your OP, I hopped straight into IR I've just tried a couple of quick experiments with a phototransistor Using the DOS prompt cursor Cursor not present (ie black screen) -> 1V + 0.5V 60Hz Cursor present -> 1.8V + 0.5V 60Hz A solid cursor in GWBASIC and a white block in Paint made over 1.5V difference Get the biggest area with the highest on/off contrast and cut off ambient light with a rubber seal around the phototransistor. Filter out the 60Hz. Then clip/boost the V change into logic voltages I have no idea what baud rate you'll be able to get up to. If you can standardise the physical interface you should get repeatable and probably reasonable results Perhaps look up "light pen" - these were a common accessory for computers like the Commodore 64, SPectrum, ZX81 etc. I have a circuit for one but it's not handy. I can dig it out if you really need it -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body