The parts came in but I still can't make the MK120 receiver work as detector for LED light. From looking at the schematics carefully I think the receiver is tuned to the sender. What I want is quite simple: just detect on/off state and convert that to connecting or not connecting +5V and a counter input. Maybe some variant of the Infrared remote control extender circuit can do the work. The LM324 opamp in the Velleman design can be fed with +5V so maybe it is simple to go from a small current (light on a photodiode) to +5V. But this means it is not going to be an 'out of te box' working circuit. Oh well, my electronics knowledge is slowly coming back from way deep...