When I started working on some touch switch projects many years ago, I searched my circuit archives for any existing technologies and circuit designs.
Most of the switch designs I could find at the time were classified as “AC hum” switches. These switch circuits used a sensitive high impedance circuit
to detect the small AC power line signals, picked up by the human body from nearby appliances. AC hum switches rely on the fact that the “hot” side of
the power line is always referenced to earth ground. The human finger touching a metal button transferred the power line signal to the sensing circuit and triggered the switch. After much testing, I concluded that these AC hum switch approaches were very unreliable. Often in some remote
locations, there were no nearby power lines or appliances powered by the power line, to generate the needed electric field to operate the switch circuit properly.
In other cases, the switch circuits were so sensitive that activation of some line powered devices near the switch, caused it to operate without any human contact.
During my research, I did conclude that although an electric power line field was not always nearby, making
AC hum switches useless, there almost always was an electrical path, often invisible, to an earth ground. Water pipes, heating ducts and cement
structures all formed earth ground paths. In almost all cases, I was able to get a capacitance operated touch switch circuit to work reliably, thanks
to nearby objects that were connected to an earth ground that I could use as a reference. A key to the success of these circuits was to measure the
capacitance change relative to earth ground using a frequency much higher than standard power lines. Most of the circuits shown in this discussion will
use techniques that detect the change in capacitance between a metal plate or button and earth ground.