Issue 155 June 2003
E-Field Evalulation Module

CAN'T TOUCH THIS

The first thing I did after completing the assembly and verification of my new Z8 E-Field EVM was create a chart of the capacitance versus voltage. I put various capacitance values on the EVM Ref A and Ref B pads and noted the resulting voltage. Thanks to Microsoft Excel, I can show you the fruit of my labor in Figure 2.

Then, I went crazy. I used pieces of copper-clad printed circuit board material as electrodes. In my sensor feeding frenzy, I noticed that my larger PC board electrodes produced a lower ADC count than the smaller boards. So, I went to the kitchen and got a piece of aluminum foil and started making electrodes in a multitude of shapes and sizes.

I found that I could “tune” an aluminum foil electrode to a capacitance that would match a desired arbitrary ADC voltage level by simply folding it into a smaller electrode or expanding the foil into a larger electrode until I got the ADC count I wanted. Because the voltage is not a linear function of capacitance, I was able to use my Excel chart to make an intelligent determination as to the capacitance of the electrode.

I graduated to the aluminum sheeting that is used when drilling printed circuit boards to eliminate burrs in the copper. Because I couldn’t solder the aluminum, I cut slots in the aluminum sheets and used some of those push-on automotive quick connects to connect the Z8-based EVM to the aluminum electrode.

I passed everything I had on my bench through my EVM’s electric field and noted the changes in the ADC count using my modified Z8 ADC-to-DMA-to-Tera Term Pro sample code. Then, I turned to the fridge and started examining fruits and vegetables and even bottles of water filled at various levels. I thought I had really gotten out of hand until a friend suggested hanging a piece of Life Saver candy on a string to measure body-area electricity. The direction the candy turned (clockwise or counterclockwise) could determine whether or not my body was “stressing” in that area. No way, I said to myself. But, curiosity overcame me, and I tried the candy trick over a foil electrode about the size of a deck of playing cards. The Z8-based EVM picked up the Life Saver’s presence, but I won’t comment on the direction of rotation versus how my EVM felt.

You would think the candy-on-a-string-rotation thing would have been the ultimate play toy, but I wasn’t finished yet. Next, I started placing pieces of Saran Wrap, paper, and plastic between the copper and aluminum electrodes and my fingers. Touching the electrodes directly with my body overloaded the MC33794 electrode inputs and reduced the ADC readings to a point near zero. This scenario was similar to having the electrodes touch grounded or voltage-carrying objects. By placing an insulator between the electrode and my body, I could “touch” the electrode and generate a change in the electric field without overloading the electrode input. I’m sure you’re getting the idea now. Are visions of glass or plastic-covered touch panels and keypads without switches coming to your mind, too?