I was actually testing the optoreflector in circuit and estimating “resistance” based on the value of the series resistor on the collector. However, I think I tried some resistors that I had misfiled and was grabbing things too fast to double-check their values, so I wasn’t using what I thought I was using, which led me far astray.

Further, I hadn’t looked for a datasheet and assumed the optoreflector would function at a distance of an inch. In fact, now that I’ve found a datasheet, I see the nominal distance is about .1″, which is far too short for my application, and further explains why I was getting ludicrous results in my testing.

So out of curiosity I just hooked the optoreflector back up and tested with a bunch of resistors. My bench power supply was producing 4.93V and R_D was 220Ω. The phototransistor’s series resistor was on the collector (not emitter), hence R_C.

You can see that the best bang for the buck comes around 100KΩ — a large enough voltage swing that I’d be comfortable feeding that into a Schmitt-trigger input with no amplification or buffering (assuming the input impedance was high enough, which I think it is).

Of course there’s still the problem that it’s optimized to work at .1″ and really doesn’t work at all at the 1″ that I needed for this application. But I’m very glad to know how to use these right, for future reference.

Thanks again!

For future reference, those photointerruptors would probably be ok. Depending on your ohmmeter, they probably won’t show any resistance change. As they’re transistors they need a bias voltage applied between the collector and emitter. In a normal bipolar transistor the current that flows from collector to emitter is controlled by the current flowing from base to emitter. In a phototransistor the base current is created by incident light.

So connect the emitter to ground, and the collector to your power rail via a bias resistor. The voltage on the collector is your output signal. Depends on your device, but you probably want a collector current around 1mA. The voltage across the transistor will be between 1 and 2V, so with, say, a 5V supply the bias resistor will be around 3-4k. Check the datasheets for some more accurate numbers, and experiment with values to tune the sensitivity. A very similar circuit may be used for a photodiode; just make sure it’s reverse-biased.

Your ohmmeter probably only puts tens of mV across the transistor, so it’s nowhere near conducting and measures high resistance. Even if it did put a couple of volts across the transistor, without knowing the circuitry of the ohmmeter the results won’t help you design a circuit.

Good luck,
Jeff