Electronic – arduino – Why are the values I’m getting through a capacitive sensor decreasing as the resistance gets lower

arduinocapacitancecapacitorcapsense

I'm currently messing around with "capacitive sensors" by trying to light an LED using an Arduino. My capacitive sensor that I am using is just made up of an Arduino, three wires and a resistor. My setup is simply two wires hooked to an Arduino and both ends of a resistor, with the third wire connected to the resistor on one side, and nothing on the other.

When I am using a 1 megohm resistor, I am getting values in the range of 2700 ~ 2800 when I grasp the exposed end of the wire with my fingers. However, when I try switching the resistor to one with lower resistance (10,000 ohm), I am only getting values of 60 when I grasp the exposed wire, and I get values of 0 when I use a 220 ohm wire.

An illustrative example:

.............
|           |--------- <-- wire
| ARDUINO   |        |
|           |      Resistor
|           |        |
|           |--------- +++++++++++ <---- Exposed wire which I grab
.............

Should I not be getting a greater value by using a resistor with less resistance as that would allow more current to flow through? Yet, the inverse is happening where the values outputted from my Arduino decrease as I lower the resistance and increase as I increase the resistance.

Why is this so?

Best Answer

Capacitive sensing works by using the RC network created by the resistor (R) and the human body (C) as a timing circuit and measuring how long it takes the analog signal to reach a digital threshold (\$\tau = {k RC}\$) when the capacitance is charged or discharged. When the resistance decreases the timing speeds up, and lower values are measured.

Related Solutions

Electronic – arduino – Reading very low capacitances with Arduino

You can get an approximate measure of the capacitance with just 2 microcontroller pins, 1 resistor and 1 known capacitor. The circuit looks something like this:

cap sensing circuit

C2 is the unknown capacitance you're trying to measure. C1 is a reference capacitor of known value, and about 50-1000 times the value of C2. R1 isn't too critical, 1k Ohm or so.

The idea is to charge C2 with a known voltage (5 or 3.3 V, whatever V+ is), and then transfer its charge (q = C*V) into C1. Each time we transfer the charge from C2 to C1, C1's voltage increases by a tiny amount proportional to C2. The number of times we have to transfer charge to make C1's voltage exceed some threshold (for us, the logical "1" threshold of pin A) is then inversely proportional to the value of C2.

The trick is to take advantage of the high-impedance state of the microcontroller pins. If we kept the bottom side of C1 grounded while charging C2, then we would also end up fully charging C1. Instead, we let the bottom side of C1 float by configuring pin B as high-impedance. Now, whatever the top side of C1 does, the bottom side does too, always keeping the same voltage across C1.

The measurement algorithm goes like this (in pseudo-C):

// Step 0: discharge C1 to prepare for a new measurement
PIN_A = 0;
PIN_B = 0;
delay(some_time); // long enough to discharge C1

bool under_threshold = true; // has the voltage across C1 exceeded the threshold?
int count = 0;
while (under_threshold)
{
    // Step 1: Charge C2
    PIN_B = Z; // Z means high-impedance
    PIN_A = 1;
    delay(some_time); // long enough to charge C2

    // Step 2: Transfer charge from C2 to C1
    PIN_A = Z;
    PIN_B = 0;
    delay(some_time); // long enough for C2 to discharge into C1

    // Step 3: Check if the threshold is exceeded
    if (PIN_A || (count > COUNT_MAX))
    {
        under_threshold = false;
    }
}
return count;

You can see a demonstration of the circuit at https://www.circuitlab.com/circuit/uq2zs6/cap-sensing/

Trouble Detecting pF capacitance with Arduino

In the capacitance meter tutorial it calculates the time taken to charge to 63.2% - it counts in milliseconds from what I can see.

500pF x 1Mohm is a CR time of 0.5 milli seconds - too low for the resolution

You need a substantially bigger resistor to get decent resolution

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