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MCP6L01T circuit not oscillating on a Capacitive Moisture Sensor PCB

tbstar

New Member
Hi,
After much research, trial and error, I finished designing a low power Capacitive Moisture Sensor with an Op Amp to boost the signal from the probe. Before finalizing the PCB, I prototyped the oscillating circuitry (Op Amp on breakout board + Probe + Resistors) on a breadboard and got decent results. Once I fabricated the PCB, the Op Amp did not produce any oscillation and yet the output is over 1V. Naturally I thought I made a mistake so I redesigned the PCB and manufactured a second iteration which did not help. So I started troubleshooting the issue following these methods:

- Testing on BreadBoard ( Op Amp breakout + External Resistors): Success
- Hooking the PCB probe to the working breadboard model: Success
- Hooking the PCB to the Op Amp breakout board + PCB/SMD resistors: Failure
- Probing each lead on the PCB for voltage drops, continuity and joins: OK
- Populated the Op Amp and supporting resistors only: Failure
- Op Amp pin assignment between the PCB and Bread Board: Match

A 3V power supply has been used throughout the testing. I am running out of options, something tells me the circuitry on the PCB somehow creates the issue even thought I followed the same schema the PCB is based on to build a successful breadboard circuitry with same Op Amp and same resistor values.

I've attached the schema and PCB screenshots in the hope someone might find what's preventing the Op Amp from oscillating as in the breadboard.

Thanks you in advance.
 

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rjenkinsgb

Well-Known Member
Most Helpful Member
Try adding a small cap across the probe circuit so you get an in-range measurable output with the probes open.
I suspect your breadboard was providing some stray capacitance to give an effect like this?

Unless the probes are insulated already, I'd also add a capacitor in series with one or both probe connections, to avoid any DC resistance effects. That should be somewhat larger than the highest capacitance the probes can ever see, to minimise the effects on the probe readings.
 

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