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I'm trying to find a sensor that is similar to the ones used in floppy drives to determine the track 0.

It's an optical sensor with about 3-4mm of space between the emitter and the receiver so that it can detect if the beam is interrupted.

All parts I can find are pretty large elements but I am looking for something small and I don't know what name they may have

An example of such sensor is on the photo here, the black part:

Track 0 sensor on a floppy drive

Additional question: I have a box moving on rails and it has 5 positions; the range is about 1m, precision about 0.5cm. Are these sensors the best way to go?

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    \$\begingroup\$ You'll want to search for "photointerrupter". There are different types, but the one shown in the picture is fairly common. \$\endgroup\$ – DerStrom8 Apr 4 '17 at 11:12
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    \$\begingroup\$ thomas, unrelated, but a heads up, in case you forgot to pay your registrar: tried to click on the link in your profile, and wdindustries.net is not a registered domain \$\endgroup\$ – Marcus Müller Apr 4 '17 at 11:56
  • \$\begingroup\$ @Markus, yes, it's an old company we created; it made pc/xbox game ports, but it's defunct now; I had no idea it was still attached to the profile. thanks! \$\endgroup\$ – Thomas Apr 4 '17 at 13:35
  • \$\begingroup\$ For your actual use case, look into Digital Readouts used for milling machines, lathes etc. Some can be had quite inexpensively and if anything would be overkill for your application. \$\endgroup\$ – UnconditionallyReinstateMonica Apr 4 '17 at 21:59
  • \$\begingroup\$ a forked light barrier? \$\endgroup\$ – dlatikay Apr 5 '17 at 7:20
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To add to what Olin said, the sensors are also known as "photointerrupters" when searching some of the distributors.

There are at least two major types of these sensors: Logic output and transistor output. Logic being the type that drives it's own output signal, and the transistor type acting more like a switch.

photointerrupter sensor

As for:

I have a box moving on rails and it has 5 positions; the range is about 1m, precision about 0.5cm. Are these sensors the best way to go?

I don't see how you're going to use these sensors to detect a box's position if you want a "small" sensor. The beams are between two appendages about 1cm apart. So unless your box will fit in a 1cm space, you may want a different approach.

Note that you can use the same concept over a larger area, you just need separated emitters and "receivers". The kind used in the floppy disk drives combine the two into a small package. Instead, you'll probably want an emitter on one side of the box, and the receiver on the other.

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    \$\begingroup\$ I would have a small tab running outside of the box, like the floppy has on its head assembly \$\endgroup\$ – Thomas Apr 4 '17 at 13:34
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Try "opto-interrupter" or "optical interrupter".

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