Re: Can the touch lamp theory be applied to D.C. powered appliance

Hi Enoch!

It is possible to build a battery-powered touch-sensor circuit. However, to reduce the required supply current would be challenging. If you intend to use the touch-sensor to turn your device on and off, then the touch-sensor must always be active (and slowly draining the batteries). It must only draw a few tens of microamperes, otherwise it will drain the batteries within weeks.

Because the 60-cycle voltage fields might not be present, you will have to supply your own AC voltage. To do so, build an oscillator, and use the oscillator's signal output to drive your touch-antenna. Place a large-value resistor (10 megs?) in series with the conductor leading to the touch-antenna, then build a second circuit which measures the oscillating voltage on the antenna. When a person touches the antenna, their body will act like a capacitor connected between the antenna and the rest of your circuit. This will cause the alternating voltage on the antenna to shrink (their body-capacitance tends to short out the antenna's voltage.) Use this change in alternating voltage and trigger some other circuit.

I've not built this sort of device myself, and don't recall seeing any schematics in magazines recently. I would probably start out with a CD4049 six-section CMOS logic inverter chip. This chip has a very low power-supply current. Use two sections to build a high frequency oscillator which puts out brief pulses (maybe 100 microseconds or less.) Send the pulses to the antenna-plate through a large resistor. Connect the antenna-plate to the input of another section of the 4049 chip, then adjust the pulse width of the oscillator until this "receiver" is barely detecting the pulses. If you touch the antenna-plate, the received pulses should vanish. Perhaps use the constant pulses to drive a transistor that discharges a small capacitor (simultaneously charge the capacitor from your 3V supply). Use yet another section of the CD4049 to measure the voltage across this capacitor. When you touch the antenna, the pulses stop, the voltage across the capacitor rises, and the last section of the CD4049 puts out a signal. What's next? Depends on what the touch-switch is for. Use the signal to trigger a CMOS flipflop chip, and use the flipflop output to turn on a big HEXFET power transistor?

Q: How does a Touch Lamp work?