Optical level sensor help

I have been having issues with these optical level sensors failing after about two months of use 725-1288-ND.
Currently the red “Vs” wire is connected to 5v, the green “OUTPUT” wire is connected to an arduino pin configured as an INPUT_PULLUP (internal 50k pullup resistor to 5v), and the blue “0v” wire is connected to ground. There is also an LED with a current limiting resistor (25mA at 5v) connected to the “OUTPUT” wire to indicate when the sensor is active. Do these sensors need to be current limited to within their supply current spec since they use an infrared emitter?


The open-collector output of the sensor is rated for 100mA, so the 25mA LED load + 50K pull-up shouldn’t be coming anywhere close to the part’s rated current capability. Current control for the built-in IR emitter is built into the part, so as long as one stays within the supply voltage spec, there shouldn’t be any significant issue there either.

How many such failures have been experienced? What are the failure symptoms? (how much supply current is being drawn? What’s the output doing in response to changes in input condition?) Are there any noticeable system event occurring in conjunction with these failures, does one notice immediately when failure happens or is it the sort of thing that only gets noticed after a while?

Some things one might look for in terms of an electrical cause for failure might be static discharge or unexpected power supply events. (what else is in the system? what routes for introduction of static discharge might be accessible?) There might be a mechanical explanation if there’s shock or vibration happening, or perhaps there’s some sort of optical effect taking place as a result of chemical action or surface abrasion. Many possibilities exist…

about 3 failures in the past year, the sensor output has been stuck high (not triggered) and one has been stuck low (triggered), regardless of whether its in water or not. They are not mounted in a high vibration environment. Currently no static mitigation, however we have some sensors that are three years old now and haven’t had any issues even without static mitigation.

If it is a push-pull type, would that cause issues with a pull-up resistor?

The datasheet indicates that it’s actually an open-collector type, suggesting an error in the product listing which has been noted and reported. So no, that wouldn’t seem to be an issue.

I could imagine stuck-high and stuck-low faults both being attributable either to an emitter-or detector-side failure. Any chance the failed devices are still around, so that you might measure supply current draw? That might offer a clue.

Another question; does the sensed fluid level change slowly or rapidly? It’s conceivable to me that the output could have been implemented as a raw phototransistor, leading to the possibility of it operating in linear mode and thus burning off more power than it should if the sensed level changes slowly.

The failed sensors are gone. The water level changes rapidly. in the datasheet it says “Output High Vout = Vs-1V max”. If the output voltage of the sensor is at most 4 volts when powered off of 5 volts, would a 5 volt pull up resistor damage it?

That line in the datasheet indicates that the voltage drop across the output transistor is at most 1v when the load current is at the maximum-rated value of 100mA. The external pull-up should not cause any issue at all.