QUOTE=MrDEB]The emitter will be in the track and the dector in a bridge that spans the width of the track.
Can I use the 3 pin dector as a simple switch (present setup uses photo diodes or phototransistors??) with 2 wires. The light source is a simple light bulb located in the bridge over the track.
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I think we need to standardize nomenclature. By emitter, I meant light source. In that context, the above statement is confusing with respect to the detector being in the bridge. If I wanted to use a single light source, I would put it in the bridge and have the detectors in the track, so you could know which lane won.
There are units made for such purposes as detecting paper feeds in printers (e.g., Vishay TCRT5000). They combine both the emitter and detector, are non-modulated, and are relatively inexpenive. One thought would be to put one in each lane, then use simple logic gates (the 2n2222's refered to below) or IC logic chips to determine the winner.
See:
https://pdf1.alldatasheet.com/datasheet-pdf/view/26406/VISAY/TCRT5000.html
When my son was doing pine cars, I made simple timing gates using non-modulated IR emitter and detector, a few 2n2222 transistors, and a couple of hacked electronic stopwatches from a local discount chain. Basically, I pulled a lead from the start/stop switches. We easily measured times to 0.01 sec and used his test information to figure out the best place to put the weight. He built a one-lane test track.
Good luck. John