This evening I was experimenting with a simple LM339 comparator circuit and a photo transistor. Everything appeared to be working fine but when I attempted to take some voltage readings with my Fluke 177 they were not what I expected. The inputs to the comparator were reading too high and acted as if there was voltage leakage from the inputs of the LM339. These input are very high impedance and there should be no leakage. The chip seemed to be working as it was sensing the change in voltage from the photo transistor. I set up several experiments to see why my voltage readings were so strange. When I connected the oscilloscope to the bread board its earth ground seemed to stabilize things a little and my readings from the scope looked almost normal, that is until I looked at the output of the comparator. There staring at me from the little round green screen was the clue to where my problem was originating. My fluorescent lighting in the shop was turning on and off 120 time a second. This on and off, while not detectable by my eyes was very detectable by the photo transistor. Since my test circuit was in effect oscillating at 120Hz the Fluke was reading the voltages as pulsating DC and calculating the effective voltage. The lesson that I learned tonight is that whenever I am working with photo sensitive sensors I have to be aware of the effect of the 120Hz strobe lights that I am using for illumination in my shop. An additional recommendation to self is to more quickly turn to the oscilloscope for it diagnostic ability.

John