Why does a capacitive sensor become more sensitive as resistance increases?

[Kencephalon]

I'm currently messing around with "capacitive sensors" by trying to light an LED using an Arduino.(Project #13 of Arduino Starter Kit). My capacitive sensor that i am using is just made up of an Arduino,3 wires and a resistor.My set-up is simply 2 wires hooked to an Arduino and both ends of a resistor, with the 3rd wire connected to the resistor on one side, and nothing on the other.

When i am using a 1m ohm 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.

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?

[johnwasser]

http://playground.arduino.cc/Main/CapacitiveSensor?from=Main.CapSense

You are measuring the time it takes for the receive pin to change state. The higher the resistance the slower it changes state.

[Kencephalon]

Hello john, thank you for taking the time to answer, but i have just another quick question about capacitive sensing. From the link you provided,it states the following:

Use a 1 megohm resistor (or less maybe) for absolute touch to activate.
With a 10 megohm resistor the sensor will start to respond 4-6 inches away.
With a 40 megohm resistor the sensor will start to respond 12-24 inches away (dependent on the foil size).....

However,as you've mentioned that what i'm measuring is the time taken to change state, and i understand that more resistance = more time taken = higher value, how exactly does increasing the resistance make the sensor more sensitive, allowing it to respond without even touching it(i.e inches away)?

Am i not simply increasing the time taken to change state by increasing resistance? How would that make it more sensitive?!?

[Paul__B]

Am I not simply increasing the time taken to change state by increasing resistance? How would that make it more sensitive?!?

That is indeed what you are doing - increasing the time constant of the combination of the resistance and capacitance.

Since the time involved at such capacitances is very short, the longer you make it, the easier it is to measure it accurately.

[johnwasser]

The time constant for an RC circuit is R * C.  If you increase R then the value of C can be lower for the same time constant.  Your circuit is therefore sensitive to smaller changes in capacitance.

[JulianReis]

So just changing the time constant your able to make the capacitive sensor more sensitive. This forum has helped me out with my prototyping project.