Touch Switches Collection

Touch operated switches are an attractive project for DIY electronics but they are not so common in commercial products. The reason for this is that, although there are many different ways of implementing a touch switch (leakage, hum pickup, capacitance, albedo) all of them rely on body parameters which vary a lot with the person's body, age and environment so that a circuit which gives reliable operation with one development engineer may cause problems in use with, for instance, old folks living in arid areas. Nevertheless touch switches can give excellent results.
Touch Switches Collection - schematic

Touch Switches Collection - img1

This one is the equivalent to those banks of push buttons where you push any switch and it releases one already pushed in. I've only shown two stages but you can have 5 - or ten if you wish. Consider the state with the first stage on. Current flows from the 0v line through the 1K resistor, the LED and the bottom two transistors. The centre line will then be at about -10v. Now touch input 2. The top transistor will pass current into the bottom transistor: the two bottom transistors will turn each other hard and suddenly on. But the 100n capacitor has no voltage across it so it takes a pulse of current as it charges up. Instantaneously he centre line drops to -12v and the LED which was on extinguishes. Since the current that was flowing through this LED also flowed through the first pair, these turn off. The first 'button' has been released. Quite a nice, simple circuit for a multi button touch switch. This is a timer or power switch. Touch the contacts and the switch turns on. After a preset delay it turns off again. Its output is from a pass transistor so you can use it to operate another circuit during its timeout period. Touching the contacts causes a current to flow through the 4M7 into Tr3's base turning it on. Its collector current turns on Tr2. The 10µ capacitor is not charged, so it charges up, holding Tr3 on. Meanwhile Tr2's emitter current turns on Tr1, applying power to the load. While the 10µ is charging...

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