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  #1  
Old 04-21-2010, 12:46 PM
hazim hazim is offline
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Default Touch on/off switch circuit

Hello,

I'm trying to make a touch on/off switch circuit with one plate; i.e. when I touch the plate it switches on, when I touch it again then it switches off. I built the attached circuit for a friend from several months, it worked fine with 12V power source, but with 9V or less it didn't. I want to use the circuit with 5V source, so I tried to increase the 10M resistor but it didn't worked. I think with 12V the current through the 10M resistor and the human body to the earth decreases the voltage on pin 2 and triggers the IC.. with 5V the 555 triggers only if I touched the ground (0V) with my second hand at the same time I touch the plate.
Anyway, my aim is to turn a circuit on and off by touching a plate as I described. You my suggest another way, other than the attached circuit if it consents my purpose. Any ideas are appreciated.

Thanks,
Hazim

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Old 04-21-2010, 01:19 PM
Bosparra Bosparra is offline
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Have you tried lowering the value of the 10M? With the decreased voltage comes decreased current for the same resistor value.
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Old 04-21-2010, 01:29 PM
rjenkins rjenkins is offline
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That circuit requires contact with the sensor to pull the trigger input below it's operating threshold.

It won't be reliable with a single contact, it requires definite earth (0V) contact (albeit via a high resistance) to operate.

You would be better off with a capacitive style switch that does not rely on a dc circuit.

Example of a 5V one here:
http://www.discovercircuits.com/DJ-Circuits/5vmom1.htm
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Old 04-21-2010, 01:39 PM
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Bill_Marsden Bill_Marsden is offline
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How about this circuit?

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Old 04-21-2010, 02:29 PM
hazim hazim is offline
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Yes I tried lowering the value of 10M, as I expected, it went from bad to worse. A capacitive style switch should work fine, but the circuit will be bigger and more complex, I think I can find a simpler way using a timer or maybe an op-amp/comparator.
The circuit Bill provided seems to be ok, but it uses a momentary push button switch instead of a touch contact
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Old 04-21-2010, 02:34 PM
kingdano kingdano is offline
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perhaps you could try his circuit using your touch-contact switch - and see if it works?
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Old 04-21-2010, 05:32 PM
hazim hazim is offline
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kingdano, the touch-contact switch isn't working properly.
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Old 04-22-2010, 05:47 AM
John P John P is offline
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I think the right way to do this is to use an oscillator where the frequency depends on the value of a small capacitor connected to ground. Touch the wire connected to the capacitor, and you'll add your own capacitance in parallel with the existing cap, and it'll change the oscillator frequency, which can be detected and used to switch the circuit. But I can't deny that it's a step up in complication.

Wikipedia says:
"The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΏ resistor."
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Old 04-22-2010, 06:37 AM
rjenkins rjenkins is offline
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The simple capacitive touch switches use an oscillator fed via a high resistance to the touch plate.

The touch plate also connects to a simple rectifier / comparator.

When the plate is touched, the level of the AC signal is reduced due to body capacitance.
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Old 04-22-2010, 03:49 PM
John P John P is offline
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OK, there you go. Use an oscillator. I don't think you need a rectifier exactly, more like a pulse and filter arrangement, where each cycle puts a bit of charge on a capacitor, and it drains off through a resistor. The faster the pulses come in, the higher the voltage will go, and then you use a comparator to test it.
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