Help with PM-U25

Hi, I’m using this sensor to sense when a very thin metal object (0.72mm diameter) passes in front of it. The sensor seems to pick it up very rarely but works when replacing the thin object with an M4 bolt.

Am I trying to using some too thin and reflective for this sensor?

Oliver- Thank you for your request. I did some searching and the datasheet states any opaque material should work. However, I did also find that strong external light sources may cause false readings. “Take care that extraneous light is not directly incident on the beam receiving section”

Also, “If there is a large surge generating equipment, such as, motor, solenoid, electromagnetic valve, etc., in the vicinity of the sensor, use a surge absorber on that equipment. Further, do not run the sensor cables along power lines”

Please make sure your application avoids these scenarios. Your technical reference number is T4086865


I would suggest that the issue is likely due to the dimensions of the sensed object being too small; the optical beam that the object breaks does have some physical size associated with it, and if a sufficient portion of the beam is not blocked by the object the sensor will not detect it. Were the beam size made very very small, things like dust particles or pollen grains would cause false detections. An excerpt from the datasheet showing the minimum sensed object size for the sensor series in question is below. A 0.72mm diameter rod is close enough to the 0.8mm minimum that occasional, unreliable detection of the sort described is a likely result.

This is great. Thanks. Do you know if any of the other photointerrupter sensors are able to sense something this small please?

I’d suggest that the OPB490P11Z or something in that series might fit the bill. It’s not an attribute that’s well exposed in parametric data, so it’s the sort of thing one has to look through the datasheets for. As a general rule though, I’d say that smaller sensing distances tend to correlate with smalle beam sizes. The OBP49x appear to have .25mm slots on both apertures, so I’d expect that ~0.7mm object would be detected quite easily.