MAINS-OPERATED touch switches for light dimmers and table lamps
are not new but up till now they have all involved a metal plate or exposed
metal work which you need to touch to operate. This Proximity Switch circuit is
different – there is no exposed metal plate; you just wave your hand near a
concealed plate and hand capacitance does the rest, turning the circuit on and
In practice the circuit is built into a small plastic case in
series with the mains cord to the lamp. Alternatively, if the lamp base has
space inside and is not made of metal, you could build the Proximity Switch
right inside it.
Before we go any further, this circuit design is not suit-able
for permanent installation as a light switch in your house wiring. This is
because it is a 4-wire circuit and a light switch is normally a 2-wire
So how do you produce a Proximity Switch for 240VAC lights? It
has been made possible by a new IC which is designed to work with a proximity
sensor. The sensor is located behind the light switch pad, fully protected by a
dielectric (ie, insulating) barrier. Thus the unit offers increased safety.
The sensor works on a principle of "charge transfer sensing",
which has its origins back in the 1700s, when investigations where first being
made into electricity. A study of this was made by Mr William Watson in England
and also by the renowned Benjamin Franklin.
All parts in this circuit (including the sensor plate) operate
at mains potential (ie, 240VAC) and must be fully isolated from the user.
Do not operate this device unless it is fully enclosed in a
plastic case. Similarly, do not work on the circuit or touch ANY part (sensor
plate included) while it is plugged into a mains outlet.
We recommend that only experienced people build this design.