Schematic for Touch-Switch.


Well-Known Member
Borrowed the book Workbench Guide to Electronic Projects you can Build in your Spare Time, and found this schematic for making a touch switch. I know SDH over at BH had asked about this for a PS1 and people told him he'd have to dissect a touch-activated lamp in order to have this work. However, that's not the case. You can build one yourself.

WARNING: These images are HUGE. 56k users beware!

C = Capacitor, R = Resistor, IC = IC, etc.

They're huge because when I tried resizing them to acceptable dimensions, the schematic became unreadable. Also, sorry about the bad/faded scans. You should still be able to read it just fine, though.

Book said:
As your finger touches or comes close to the touchplate, 60 HZ power line hum is induced into the MOSFET gate IC1A. (See figure 5-4A). The hum is cleaned up by the gate and used to trigger flip-flop IC2A. A second flip-flop (IC2B) is necessary to keep the output signal up or down when your finger is removed from the touchplate. IC1B drives LED L1 which is physically positioned to shine on photoresistor R5. When L1 light strikes the photoresistor, its resistance decreases causing gate current to flow in the triac. The triac fires switchig [sic] on current to socket S1 and powering whatever is plugged into it.

In the power suppy, line voltage is transformed to 6.3 VAC by T2. The bridge rectifier changes the AC to DC which is then filtered by C2 and C3. IC3, a 5-volt regulator, keeps the output at a constant 5-volt level.
bacteria said:
Good find. Hard for people like me to follow, but looks good!
To build this, you don't actually need to understand how it works. You just need to know how to read a schematic and you're set. I just threw the "how it works" bit in there because it felt incomplete.
That's cool, but I would use a proper octocoupler and relay. Actually, for the voltages we deal with you could probably get away with only an octocoupler and a transistor.