Home
Touch Switch





Although quite simple, this touch switch circuit works flawlessly. It operates from 12 volts and is designed for use with loads that don't mind being switched on their low side, like a light bulb for example, which is not referenced to ground. In fact, I use mine with a 12V LED table lamp. The circuit uses the TP223 touch control IC which I was only able to find in a surface mount package, so it had to be soldered onto an SMD to 8 pin DIL converter board before it could be mounted on stripboard as used here (and for most of my projects!). The TP223 chip can be purchased from eBay as part of a prebuilt touch module pretty much ready to go but where would be the fun in that? Although I had to wait a few weeks, I only paid 99p for 10 chips from China (eBay again) but I think it was worth it




Schematic


The IC can be configured in several ways: latching (pin 6 connected to pin 5), non-latching (pin 6 not connected), positive edge triggered (pin 4 not connected) or negative edge triggered (pin 4 connected to pin 5). The configuration I use here is positive edge triggered/latching. The output (pin 1) rises to about 5 volts when pin 3 is touched 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. This device is specifically designed for use in logic circuits and can be turned fully 'ON’ with 5V or less at its gate, which means because there is hardly any voltage drop across it, it stays cool even without a heatsink




Veroboard layout


The module can be housed in many ways but I opted for a separate self contained unit that can be added inline with the power feed to whatever device you want to switch. For the enclosure I used a 16mm single surface box with a nice chromed blanking plate for the touchpad (Screwfix). Metal blanking plates are relatively expensive but I would guess that covering the inside of a plastic blanking plate with tinfoil would work just as well though I haven't tested this. The datasheet for the IC suggests that a capacitor can be added from the touchpad down to ground to decrease the sensitivity but this was omitted because I want as much sensitivity as possible. Without the cap you don't even need to make contact with the touchpad as the switch will operate even when your hand is about 3 inches away. If placed on the floor it becomes a foot switch where all you have to do is wave your foot somewhere nearby to activate it… no contact required. It should really be called a wave switch rather than a touch switch!




Internal View