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Touch Switch





Although quite simple, this touch switch circuit works really well. It operates from 12 volts and is designed for use with loads that don't mind being switched on their low side, like for example a light bulb that doesn’t need to be referenced to ground. The circuit uses a capacitive touch control module featuring the TP223 IC, readily available from eBay. These little boards come in several styles but the most common seem be the Red ones which is the type used here. If you use a different type, the order of the three connections may not match those shown in my diagrams (VCC and OUT are sometimes reversed). The modules have four switching options that can be configured by bridging out a couple of solder pads, though I find it much easier to bridge the actual legs of the IC as they are spaced closer together




Schematic


The TP223 switching options are: latching (pin 6 connected to pin 5), non-latching (pin 6 not connected), positive edge triggered (pin 4 not connected) and negative edge triggered (pin 4 connected to pin 5). The configuration I use here is positive edge triggered/latching (pin 6 connected to pin 5). When pin 3 is touched the output (pin 1) rises to about 5 volts (if you’re using a 5V supply) and stays high until the next time it's touched. The main circuit operates from 12 volts but as the TP223 has a maximum voltage rating of 5.5 volts, it is fed from a 78L05 regulator. To switch higher voltage and higher current loads, the output is fed to a STP55NF06L MOSFET which is specifically designed for use in logic circuits and can be turned fully 'ON’ with 5V or less at its gate. Because there is hardly any voltage drop between drain and source, the transistor stays cool even without a heatsink




Veroboard layout


The circuit is constructed on a standard 9 x 25 hole veroboard with the touch module mounted as a sub board. A 3 way plugable header is used to connect the two boards together (socket fitted to the veroboard and plug pins fitted to the touch board). For the main connectors I used 3 way screw terminals instead of 2 way simply because I find that 2 way types twist too much when tightening the screws! The finished assembly can be housed in many ways, but I opted for a self-contained unit that can be added in line with the power feed to the device you want to switch. For the enclosure I used a 16mm single surface box with an attractive chromed blanking plate for the touchpad (Screwfix) which, being metal, also has a convenient earthing screw as a connection point. A wire is run from this point back to the touch module which I wanted to be plugable, so for this I removed a single pin from a turned pin IC socket and soldered it directly onto the touch area of the board (usually indicated by a circle) to act as a 1 pin receptacle for the wire. Because the switch now has a large touch plate, it is very sensitive and will operate even when your hand is about 3 inches away. So with no contact actually required, I guess this project should really be called 'Touchless Switch'




Internal View