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The circuits below light a 20 watt lamp when the contacts are touched and the skin resistance is about 2 Megs or less. The circuit on the left uses a power MOSFET which turns on when the voltage between the source and gate is around 6 volts. The gate of the MOSFET draws no current so the voltage on the gate will be half the supply voltage or 6 volts when the resistance across the touch contacts is equal to the fixed resistance (2 Megs) between the source and gate.
The circuit on the right uses three bipolar transistors to accomplish the same result with the touch contact referenced to the negative or ground end of the supply. Since the base of a bipolar transistor draws current and the current gain is usually less than 200, three transistors are needed to raise the microamp current level through the touch contacts to a couple amps needed by the light. For additional current, the lamp could be replaced with a 12 volt relay and diode across the coil.
the shematics u gived r really useful
can u give a link or something where we can find more detailed informations.
the example of the night lamp :
the night lamp have three level lightening and when u touch it the 4 th time it turns off . i think i'll have one and try to open it and see how does it work.
a really fine solution is HALIOS (High Ambient Light Independent Optical System)
Have a look for further information there:
h**p://www.elmos.de
Look for E909.01
'The device is capable of indicating objects entering the detection area of the sensor. Furthermore switching is recognized when an object
touches the surface of the sensor. Both functions are available with the static output signals PROX and TOUCH.'
In the MOSFET circuit, when you touch the contacts, the 0.1 µF is charged to Vcc rapidly, then the MOSFET works. After a a period determined by the 2 MΩ resistor, and the 0.1 µF capacitor, the 0.1 µF capacitor is discharged through the 2 MΩ resistor.
The same idea is implemented in the 2nd circuit, where the 0.1 µF capacitor is usually charged to Vcc through the 2 MΩ resistor, when you touch the plates, you dicharge the 0.1 µF capacitor, and the PNP darlington pair works, driving the 2N3055 transistor.
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