The Gladly Touchable Electric Wire

Sometimes the projects of an electronics hobbyist actually find very practical and even by their own family well recognized applications :

My dear wife asked me to provide the many LED spots at the workplace under the kitchen wall cabinets with a “better” switch. You (and me? 🙂 ) should also be able to switch the light “good and easy” even during baking and cooking with floury or sticky fingers.


Alexa and Siri have a kitchen ban with us. This quickly gave rise to the idea of building a relay-based switch that should have “only” a touch sensor running along the entire edge of the cabinet in the form of a thin wire.
In the large box with sensors and actuators for the microcontroller world, I found a suitable 5V relay with 250V/16A contacts and also a TTP223 based touch sensor board.

In contrast to all current trends: It doesn’t always have to be an Arduino or ESP32 microcontroller that meaningfully connects such peripheral components, as my schematic circuit shows.

Schematic diagram of the touch sensor switch


However, I still had to invest a few thoughts and a bit of time in the design and test of the sensor. The touch sensor chip TTP223 works with a fingertip sized sensor plate whose capacitance against ground affects an internal 1MHz-oscillator. However, the sensor area – respectively its capacitance value – must not become too large.


Instead of the old sensor plate, I then experimented very successfully with sensor wires of different lengths and diameter, including a variable telescopic antenna.

My short over all result: Thin insulated wires brought the best results in my kitchen environment.

Test with a telescopic antenna as variable sensor wire


Finally, a two-meter-long thin insulated wire (0.5mm) well optically hidden along the entire edge of the kitchen cabinet’s down side did perfectly fulfill my wife’s wish, which then earned me a very nice compliment 🙂


I changed the original circuit of the TTP223 board at Pin6. As this pin is connected to Vdd now, output Pin1 goes into toggle mode. Every touch of the wire now turns the relay state and holds it then. Pin4 remains on Vss, which ensures that the relay remains switched off initially when the operating voltage is applied.


For experimenting with your own sensor plates or wires, you should know that the TTP223 dynamically adapts very well to changed capacitance values of the sensor at startup and also during operation. With an additional capacitor (C1) of a maximum of 50pF between Pin3 and Vss/ground, the sensitivity of the circuit can also be reduced in a targeted manner.

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After laboriously rebuilt my old blue touch sensor board, the long-awaited ordered 10 pieces of red TTP223 mini sensor boards arrived for 6€. These are even smaller and are even easier to customize as described above. The additional sensor wire can be soldered directly on the “touch” plate. The board is so small that it could also be mounted separately from the relay board directly next to the sensor wire. A uncritical dimensioned even up to some meters long and thin three-pole cable then connects the two boards.

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