Hi Everyone,
I'm trying to put together an electric field detector. To accomplish this I looked at a bunch of different plans on the Internet and decided on 2 different designs. Each design used a different op amp and each op amp seemed to pick up on different emanation types. I wanted to combine the two designs and came up with the attached schematic.
My question has to do with the center portion. After many tests, I added the diodes. I wanted to make sure that one output wouldn't adversely affect the other. When I added the diodes to the circuit, the LED bar graph being driven by the LM3914 driver, would fully light up making the detector unusable. If I removed either diode from the path and replaced it with a wire, the whole thing would begin to operate normally again. I replaced both diodes and decided to take some measurement with a multimeter. As soon as I touched the voltmeter with one lead on ground and the other at the junction of the two diodes, the detector began to work properly again. When I removed the meter, the light bar maxed out again.
Because of this behavior, I stuck the 10MΩ resistor at the junction, and everything worked great. I figured a good meter probably has a pretty high impedance, so why not introduce a high impedance and see if that solves the problem. Here's the issue. I have no idea why this works. Admittedly I am missing some knowledge about op amps and also how this LM3914 works. It is the first time I ever used either of these particular amps or an LED driver.
Any knowledge you could share was to why I experienced this behavior would be great appreciated. Also, if you think the schematic could be helped in any other way, I'm all ears.
Thanks for your help,
Skinny