How to Stabilize Capacitive Sensing using External Power Supply?

Questions

  1. Why does connecting an external power supply cause the capacitive sensor to output erratic values?
  2. How can the 'No Touch Value' be stabilized, when connected to an external power supply?

Setup
I am creating a capacitive sensor on Arduino. A 10M ohm resistor sits across digital pin 4 (Send) and 2 (Receive). The conductive thread on a piece of cloth is my sensor, which is elevated off the table using a piece of plastic. I am using the Capacitive Sensing Library by Paul Badger.

What I understand

  • The laptop, which is connected via USB to the arduino, is part of the capacitive sensor. Touching the laptop = Touching the conductive thread

  • When replaced by aluminum foil as sensor, it yields the same erratic values.

Tests using different power supply

  1. Power supply using USB on laptop
    Stable: 'No Touch Value' ranges from 0 - 35. 'Touch Value' is around 3000.

Screen Shot 2016-03-04 at 12.00.10 PM.png

  1. Power supply using USB on laptop, which is charging on a power adaptor
    Unstable: 'No Touch Value' ranges from 0 - 3000. 'Touch Value' is around 180000.

Screen Shot 2016-03-04 at 11.16.50 AM.png

  1. Power supply to arduino using 9VDC Switch Power Adaptor, and connected using USB on laptop
    Unstable: 'No Touch Value' ranges from 0 - 2200. 'Touch Value' is around 160000.

Thank you so much for your advice and time. I appreciate it.

Screen Shot 2016-03-04 at 11.16.50 AM.png

Screen Shot 2016-03-04 at 12.00.10 PM.png

I don't understand how your circuit shall work, and what is connected to its terminals. Perhaps you constructed an antenna, that catches signals from everywhere?

DrDiettrich:
I don't understand how your circuit shall work, and what is connected to its terminals. Perhaps you constructed an antenna, that catches signals from everywhere?

I am replicating the capacitive sensor tutorial of the 'touchy-feeling' lamp and using the basic capsense library.

This tutorial also demonstrates the antenna effect, not how a capacity is measured. When you come near the antenna, or touch it directly, you extend the sensitivity of the antenna, and thus get higher noise readings.

Try the following:
Touch the Arduino Gnd with one hand, then move your other hand towards the sensor. When everything else is okay, the readings will increase with decreasing distance between you and the sensor.

Hi there
I just Tried to connect my arduino to 5V power source and it worked as I connect it to laptop.
anyway, u can use a regulator like LM7805 to get 5V from 9V.