Is electric field of capacitive sensing circuit harmful and is it radiation?

Question

I have constructed a circuit for capacitive sensing. The code charges and discharges a conductive plate. This uses an Arduino which is connected to a computer via USB.

I have decided to measure the electromagnetic radiation of the metal plate. I have used a "Trifield Meter Model 100XE". The measurements are close to the plate. Magnetic field is low as well as RF/MW. However the Electric field is showing values of around 200-400 V/m.

I have seen most standards of radiation refer to milligauss not V/m. Is this radiation and is this harmful? The meter is supposed to measure AC electric field. My device is 5VDC. Where is AC coming from? Is the AC a result of the capacitive sensing algorithm? How can I know what frequency this field/radiation is?

I have found data about allowed values of EMF for humans, but I am not sure how to interpret them.

Is there a formula for the Electric field I am measuring?

Per the design of this sensing method, I am using it while it is connected to a computer over USB so it is "grounded". The fields exist also if I am connected to a laptop using a battery. If I connect the Arduino to a DC battery then sensitivity drops and so does the field (to about 40 V/m). If I connect the Arduino to a AC-DC power adapter (without a computer) then the field becomes higher than 1kV/m.

Answer 1

This type of capacitive sensing circuit works by changing the voltage applied to the sense pad, and calculating the time required for the pad to reach that voltage. When you are near the pad, the capacitance is changed, thus changing the charge/discharge time.

The Arduino Capsense library uses this technique.

The voltage output is only as high as the MCU operating voltage (3.3V, 5V etc) and will vary in frequency depending on whether a touch is detected or not. The capsense library has about 2s timeout period waiting for the pad to change states, then the process is repeated. Even so, the maximum frequency couldn't be more than a couple MHz. With oscillating input to a metal pad, there would be some radiated EMF. However, this would be quite weak and harmless, but explains why you see an "alternating current" field.

Connect your oscilloscope and watch the pads being charged/discharged, or set up a counter in the capacitive sensing algorithm to output the approximate period, then you can calculate the frequency.

I see no way for this to produce high voltages or harmful levels.

Answer 2

I have done a research online about safety of electric fields.

The field I was testing is generated by charging and discharging capacitors at a rate of about 100Hz-10Hz. Such fields are classified as “Extremely low frequency (ELF) fields”

Here are the resources I have found

https://www.osha.gov/SLTC/elfradiation/ - Occupational Safety and Health Administration

Extremely low frequency (ELF) fields includes alternating current (AC) fields and other electromagnetic, non-ionizing radiation from 1 Hz to 300 Hz. ELF fields at 60 Hz are produced by power lines, electrical wiring, and electrical equipment. Some epidemiological studies have suggested increased cancer risk associated with magnetic field exposures near electric power lines.

There are currently no specific OSHA standards that address extremely low frequency (ELF) fields.

Mentions magnetic fields as problematic.

http://www.who.int/peh-emf/publications/facts/fs322/en/ - World Health Organization

average residential power-frequency magnetic fields in homes are much lower - about 0.07 µT in Europe and 0.11 µT in North America. Mean values of the electric field in the home are up to several tens of volts per metre.

In 2002, IARC published a monograph classifying ELF magnetic fields as "possibly carcinogenic to humans".

Mentions magnetic fields as problematic.

http://ec.europa.eu/health/scientific_committees/opinions_layman/en/electromagnetic-fields/l-2/7-power-lines-elf.htm - EU Health and Consumer Protection

In 2002, the International Agency for Research on Cancer (IARC) classified ELF magnetic fields as “possibly carcinogenic to humans” (Group 2B). This was based on statistical studies indicating children are more likely to develop leukaemia if their exposure to extremely low frequency magnetic fields exceeds 0.3-0.4 µT, which would be relatively strong. Experimental studies on animals did not support these findings.

Furthermore, the IARC concluded, there was no evidence for a link between ELF magnetic fields and any other type of cancer.

As far as ELF electric fields are concerned, the IARC classified them as “unclassifiable as to carcinogenicity in humans”.

http://www.cancer.org/cancer/cancercauses/radiationexposureandcancer/extremely-low-frequency-radiation - American Cancer Society

In the studies that have looked at a possible link between ELF radiation from magnetic fields in the home and childhood leukemia, the results have been mixed. Still, when the findings from these studies are combined, a small increase in risk is seen for children at the highest exposure levels compared to those with the lowest exposure levels.

Studies that looked at the effect of ELF electric fields on childhood leukemia did not find a link.

Studies of other childhood cancers have generally not found any strong links to ELF electric or magnetic fields.

Answer 3

Your reported readings would appear to be incorrect by several orders of magnitude. No circuit that operates at 3-5V can be producing "200-400 V/m." You are using or reading the meter incorrectly. There is probably a larger AC static field around a mains power outlet in the wall than from any capacitive sense device.

You are subject to a larger charge from simply walking across a carpet or sliding along your car seat than you would ever see from a capacitive touch-sense circuit. It wouldn't make the top-1000 things to be concerned about.