Pololu Robotics & Electronics
My account Comments or questions? About Pololu Contact Ordering information Distributors

Pololu Forum

QTR-MD-01A voltage jump

I have 4 of these connected to the high impedance analog inputs of a teensy 4.0 microcontroller powered by a 3.3 volt regulator. The voltage seems steady and I have the LED’s dimmed to 50%. The sensors exhibit a jump in voltage as I move a white card slowly toward them with some significant hysteresis. So as the voltage moves down smoothly and gets to about 1.6 volts, it jumps to .8 volts, then smoothly down from there. In the opposite direction as the voltage rises smoothly to 1.4 volts it jumps to 2 volts moving smoothly up from there. Any idea what this is about?

Thanks much.


That behavior seems unexpected. Do all 4 sensors skip at the same voltages? How are you measuring the voltage (e.g. the Teensy or a multimeter)? Could you post a video of your test procedure?


Thanks for getting back to me on this. All four sensors appear to skip at the same voltage. I checked one on the oscilloscope and it seems to be an honest jump (as opposed to an artifact of the teensy). Otherwise I’m just plotting the output of the teensy A/D converter. The power supply voltages seem solid. Could it be that I’m operating this at 3.3 V? Here are some pictures (hopefully)

scope.pdf (736.2 KB)

I can’t figure it out if it’s really just a phototransistor, but maybe I don’t understand phototransistor’s that well.

It is hard to tell much from your graph since the left side scale is cut off and from the bottom scale it seems really zoomed in. The scope capture shows a jump of around 50-100mV. It might be helpful if you post the full (not zoomed) versions of those pictures. The best thing though would be a video that shows your test procedure along side a running printout of the analog reading with a multimeter.