Op-Amp circuit begins to work only when oscilloscope probe connected

Thread Starter

bmoney77

Joined Mar 24, 2023
1
Hi,

First time posting a question on here.

Circuit:
Screen Shot 2023-03-24 at 4.15.41 PM.png

The 3k in series with the switch in the above circuit represents a metal probe that gets connected to ground when water is present (switch open = no water; switch closed = water).

Due to the metal probe starting to corrode I need to use a bi-polar square wave (approx. -2.5V to 2.5V) to drive the resistor divider/metal probe. I achieved this by adding a differentiator circuit on the output of a 555. The output of the resistor divider is fed into a rectifier circuit to have a DC signal again. That DC signal is then fed into a unity-gain buffer to be able to read the value on an ADC, and also fed into a comparator to read the signal on an I/O pin. Op-Amp used was LMV324.

Problem: Op-Amp circuit begins to work only when an oscilloscope probe (set to 10x) is connected to the input of the op-amp. With a probe on the input I can at least see values on the ADC and get an accurate reading on the I/O pin.

I assume if I were to use a 10M resistor to ground on the input signal of the op-amp it would fix this issue, but can someone explain why this is happening. I am relatively new to using op-amps.

Note: Everything up to the input of the op-amp has been tested and works as expected!

Thank you
 

Audioguru again

Joined Oct 21, 2019
6,674
The output of the old 555 is +3.6V and +0.1V.
The output of the 100uF coupling capacitor is plus and minus 1.8V.
The output of the diode floats to a little higher than +1.1V (PNP input transistors on the opamp) but your scope probe will drop the voltage a little.
The other inputs of the opamps are at +1V, divide down their voltage much lower.
 
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