Measuring voltage using human tissue as conduction.

[yowler]

Measuring voltage using human tissue as conduction: i want to measure hand contact with an object. Idea is that a voltage that's applied on one place on the object, is being measured on another place on the object. When the hand comes in contact with both objects it will conduct the voltage that's applied on one, and will be measured on the other. Voltage < 0 therefor indicates hand contact... voltage = 0 means there's no contact.

To make this I've made a circuit, S1 representing the hand. Below the results I'm getting, reading the results on the raw data analog A0 (in bits 0-4095). Bumbs in the signal means hand contact, little bumbs very delicate grip, 'big' bumbs a tight grip. I'm concerned about the signal, I'm afraid it's not strong enough, meaning to little voltage is being measured over the hand. The noise on the signal concerns me as well.

I would like to know what I possibly can improve in the circuit to achieve a better signal, meaning a higher voltage and less noise, any suggestion? Things I might be able to improve on the circuit?



Blue is pin A0 (measured signal), red is not being used.

 

[KlausST]

Hi,

increasing of R1 to 1MOhms gives higher signal, but also gives higher noise.
probably the "noise" you see is influenced mains AC voltage.

What to do to improve what is called "signal to noise ratio"?
* Place a filter to avoid mains frequency influence.
* use a battery powered measurement tool (instead of wall adapter) so that no mains connection and no earth connection influences your signal.
* use a fixed ADC samplerate with an integer multiple of mains frequency and calculate the average of your ADC readings within one mains period (and lower your C1)
--> example: Mains frequency = 50Hz: use samplerate = 16 x 50Hz = 800Hz. take an average every 16 ADC readings. This filters out mains frequency.

The best way i see is:
Generate your own frequency as testsignal and correlate the ADC signal with this test signal.
So the mains frequency can not influence your test anymore.
It isn´t that difficult as it seems....

BTW: place a low leakage diode from GND (A) to ADC input(C) and another one from ADC input(A) to VCC (C)to protect your ADC input from ESD. (A)=anode, (C)=cathode

Hope that helps
Klaus