I'm designing a capacitive proximity sensor for detection of human presence. My requirement is that I should be able to detect presence by monitoring change in capacitance at the highest possible distance for my plate size and configuration (I'm looking at a target range of 100-120 mm). I have a DC source of 24V. Additionally, I would also like to calculate approximate distance of the target from the sensor once the target is within range. Most importantly, the detection distance should be consistent and should have low susceptibility to noise. Looking at different approaches for measuring capacitance, I've found the following to be most commonly mentioned:
- RC decay circuit
- Oscillator circuit using a 555 timer
- Op-amp integrator circuit
Which one of these best suits my application? Or is there any other approach that I've missed? What are the advantages and disadvantages of the above methods?