Reference a sensor DC output signal to Vcc instead of ground

[micromet]

I am working on a soil sensor to measure moisture, the dc output signal decreases as moisture increases. For example, wet soil = 1400mV and dry soil= 2400mV, all running on a 3.3V micro-controller

https://wiki.dfrobot.com/Capacitive_Soil_Moisture_Sensor_SKU_SEN0193


I would like to transform the output so voltage increase as moisture increased.

Currently I just do this in my microcontroller with software.

But I was wondering if I could use a differential amplifier or other circuit to reference the output voltage to VCC instead of ground to accomplish this goal? Then a person could just read the sensor with a voltmeter and still "make sense" of the results.

If so, please suggest a part and circuit.

Thanks in advance, I'm a soil scientist trying to learn electronics.

 

[srizbf]

possible. a sort of inverting amplifier configuration
modified to sensor output range.

 

[KlausST]

Hi,

Then a person could just read the sensor with a voltmeter and still "make sense" of the results.

Simplest solution:
The person with the voltmeter just does not measure with respect to GND but with respect to VCC.
No hardware modification necessary.

Klaus

 

[stenzer]

A differential amplifier [1] would be a further solution. By subtracting the decreasing sensor voltage from a higher voltage results in an incrasing output voltage. The output voltage range can be adjusted by choosing appropriate resistor values.

Maybe also the sensor circuit itself can be adopted. I think those kind of capacitive sensors are using a 555 timer IC.

[1] https://www.electronics-tutorials.ws/opamp/opamp_5.html

 

[BradtheRad]

Here is a more complicated method than relocating the voltmeter (see post #3). Readings are referenced to 0V ground.

A single transistor is able to invert the signal, as well as apply voltage gain in order to expand the range of measurement. Shown in lefthand circuit.

Righthand circuit is an option where led brightness alerts you that soil is dry.