# Measuring a varying capacitance using the Wien bridge

I am going to make a capacitive soil moisture sensor. The principal that I'm going to use is the change of capacitance of the soil with moisture. What I'm going to do is make a capacitor that would be buried under the soil, and connect it to an oscillator. Then I'm going to get the output frequency from the oscillator and use it to calibrate and measure the moisture level of soil.

If I am to use a Wien bridge oscillator, will it give me a considerable change in frequency if I replace the one of the capacitors (C1 or C2) with my sensor capacitor? Refer image below.

• Compute the derivative of the output frequency with respect to a change in capacitance. Your computation should be: $$\frac{\frac{\text{d} f}{f}}{\frac{\text{d} C}{C}} = \frac{\text{d} f}{\text{d} C}\cdot\frac{C}{f}$$ That will tell you what % change in frequency will result from what % change in capacitance. This is just math. See the analysis section on Wein Bridge wikipedia for equations. Take complex derivative. Plug values. Done. – jonk May 12 '18 at 22:44

## 1 Answer

Wien bridge oscillator depends on two capacitors changing. The Schmitt inverter Oscillator only depends on one shunt capacitor and a feedback resistor. If you want to measure a wide dynamic range then I would suggest the Schmitt trigger with an R much lower than soil R. Then consider dual oscillators for different depths to indicate depth of moisture.

What is your R and C expected range with water and fertilizer affecting R and C independently?

What is your current or power or uWh budget?