Hey there!
So I've got this project where I need to sense human touch on an aluminium rod (47cm long, 2cm wide, 2mm thick). I've tried using the TTP223, but I guess the capacitance of the rod is too big. I tried experimenting with different sized capacitors, but I could not get it to work reliably. So my questions: is this even a feasible approach? If so, how? Also, what other methods could I use to sense touch on at least one side of the rod?
I need to sense human touch on an aluminium rod
Do you want to exclude animal touches as well?
Basically you do what you have tried already but do it properly. I don’t know what mistakes you have made because you have not told us what you have done
https://playground.arduino.cc/Main/CapacitiveSensor/
When the send pin changes state, it will eventually change the state of the receive pin. The delay between the send pin changing and the receive pin changing is determined by an RC time constant, defined by R * C, where R is the value of the resistor and C is the capacitance at the receive pin, plus any other capacitance (e.g. human body interaction) present at the sensor (receive) pin. Adding small capacitor (20 - 400 pF) in parallel with the body capacitance, is highly desirable too, as it stabilizes the sensed readings.
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Resistor Choice
Here are some guidelines for resistors but be sure to experiment for a desired response.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). Common resistor sizes usually end at 10 megohm so you may have to solder four 10 megohm resistors end to end.
One tradeoff with larger resistors is that the sensor's increased sensitivity means that it is slower. Also if the sensor is exposed metal, it is possible that the send pin will never be able to force a change in the receive (sensor) pin, and the sensor will timeout.
Also experiment with small capacitors (100 pF - .01 uF) to ground, on the sense pin. They improve stability of the sensor.
Note that the hardware can be set up with one sPin and several resistors and rPin's for calls to various capacitive sensors. See the example sketch.Grounding and other known issues
The grounding of the Arduino board is very important in capacitive sensing. The board needs to have some connection to ground, even if this is not a low-impedance path such as a wire attached to a water pipe.Capacitive sensing has some quirks with laptops unconnected to mains power. The laptop itself tends to become sensitive and bringing a hand near the laptop will change the returned values.
Connecting the charging cord to the laptop will usually be enough to get things working correctly. Connecting the Arduino ground to an earth ground (for example, a water pipe) could be another solution.
Another solution that seems to have worked well on at least one installation, is to run a foil ground plane under the sensor foil (insulated by plastic, paper, etc.), and connected by a wire to ground. This worked really well to stabilize sensor values and also seemed to dramatically increase sensor sensitivity.
Perhaps cover the rod with clear lacquer or plastic sheet?
Touch sensing with an antenna is not a big problem, the bigger the antenna the more sensitive the circuit.
One problem is the common ground. If both the Arduino and your testing person is well grounded, you can use the CapSense code. If this doesn't work, split your rod into 2 sensitive areas and connect one to the Arduino Gnd and one to the input, e.g. a plastic rod with 2 stripes of aluminium foil glued to it. These stripes should not be too close to each other, for best sensitivity.
If your Arduino is not well grounded you can use the antenna approach, where an input pin toggles at mains frequency when somebody approaches the attached wire or rod. If the input does not stop toggling, you have to connect yourself to Arduino Gnd, so that your presence is not detected by the circuit. But that circuit can catch whatever EM noise from the ambient (wall warts, LED lamps...).
Hi,
What is the application that needs that sized cap sensor?
If it is touch, why capacitive and not resistive?
Thanks.. Tom... 