Re: [CAD_CAM_EDM_DRO] Re: Optical limit switches

Andrey,

Finding definite specs can be difficult, but as my physics teacher once
said, "You can replace nearly any transistor with nearly any other
transistor". Of course, that was in the days before FETs, IGBT, SCR,
etc. That rule does, however, generally apply to opto interrupters.

Most consumer product salvage optical interrupters have an infrared LED
for the source. It will drop about 1.5 volts (could be 1.2 - 1.9
volts) in the forward direction, and resist perhaps 4-5 volts in the
reverse direction. (Apply much more than that, and they will fail
permanently.) If you look carefully at the case of the interrupter, you
may see a small diode symbol imprinted in the case. That will identify
the emitter side. The pins may also be marked with K (or C) and A on
one side for the cathode and anode of the emitter LED, and C and E on
the other, for the phototransistor output.

You can test the emitter side with three volts of battery power (two
flashlight batteries) through a 1,000 ohm resistor. If you find a pair
of pins that passes about 1.5 milliamps of current with this setup, then
you have identified the emitter side of the unit. If you have a digital
still or video camera, either can "see" the infrared light thus
produced, and that will confirm that the emitter is working. For
reasons I still haven't figured out, the cameras indicate the IR output
as white light. I would have expected that it would show as red, but it
doesn't with any of the units I have ever looked at.

Once you have verified that the emitter is working, you can figure out
the output end. Most of these are NPN phototransistors. If you apply
three volts through a 10,000 ohm resistor across the output pins, you
should see the voltage across the output pins fall to about 0.6 volts
when the emitter is turned on, and rise to three volts with the emitter
off, or the beam interrupted. If there is no change, then you have the
polarity wrong. (Actually, it might work even in reverse, so try both
possible orientations, and use the one that results in the lowest
forward voltage drop when the beam is on.)

Once you have identified the pins, select dropping resistors to pass 5 -
10 milliamps through your emitter side, and 1 or 2 milliamps through the
detector side, depending on the supply voltages available in your
system. If you need help figuring those out, let me know.

You can also get nice deep throat interrupters from used photocopy
machines, laser printers and dead VCRs as well.

All the best

(a different) Dave

>Dave,
>The mouse and floppy drive idea is very handy, thanks. Any idea of where to
>find specs for such reclaimed devices? It's great to have free stuff but
>only as long as I can get it working. My main concern is that I have a
>device and a clear method of attaining its functionality.
>
>Andrey
>
>
>