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capcitive touch sensor


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hello everyone,
                        i looked up this site for a project in my curriculum and came across the capacitive touch sensor, although i did get a clear schematic i need  sufficent explanation about the working of the circuit. i would be highly grateful if any one will be willing to help me this matter. i really need this...
please help.
thank you

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hi audioguru,
                    thanks a million for the link u sent me altho it doesnt use capacitance as its main component, which is wat i require now . I totally forgot to mention the link and it is www.electronics-lab.com where i got the schematic but insufficient working... if i am not pesterin you could you please try and find out and tell me.ill be more than grateful.
thanks alot again
prasad

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The Capacitive Sensor project explains very well how it works:
"Q1, Q2 & Q3 form a high impedance super-Darlington that drives the relay, amplifying the 50Hz alternate mains-supply frequency induced in the sensor by the human body.

Note: For proper operation, circuit ground must be connected via a small value, high voltage-rating capacitor to one side of the mains supply socket. The "Live" side is the right one."

Is there anything about the explanation you don't understand?

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Hi Ksp,
Transistors connected as a darlington are current amplifiers.
Each transistor has a current gain of 100-150, so Q3 drives the 52mA relay coil and Q2's current is only 350uA. Q1 supplies only 60uA to R4 and next to nothing to Q2.The sensor supplies only 3uA.
The darlington connection also multiplies the input impedance of the transistors.

post-1706-14279142595473_thumb.png

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hey audioguru,
                        thanks alot for the explanation uve given. but then there's another thing thats been bothering me... how does the proximity of the object to the sensor trigger the relay??? what significance does da darlington have in this aspect? thanks alot for the trouble uve taken ... i really appreciate it. my knowledge in this is as u can see very low... i'd like to learn how exactly does each component make a diff...id be more dan grateful if u cud do this for me.
tc
hope to hear from u soon

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Hi KSP,
The circuit is capacitor-coupled to the live wire of the mains. Your body is grounded which is the other wire of the mains. Your finger, air in between and the sensor plate form a coupling capacitor to feed mains into the very high impedance circuit.

The 1st transistor is an emitter-follower and rectifies the mains signal and charges C1. Q2 is also an emitter-follower and Q3 is a common-emitter amplifier.

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hey audioguru,
                        if the first transistor rectifies the mains supply and den charges the first capacitor den wat is the role that the three diodes have??? once our finger touches the sensor element the mains supply is capacitively coupled to the transistor q1 which i have understood perfectly well ... and den the capacitor c1 starts charging... as u said da the second transistor is also a emitter follower and hence it is used as a current amplifier... dat is connected to the third transistor which has two diodes connected to it... i would be grateful if u cud explain da operation after that

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hi audioguru,
                    thanks a million for the explanation you ve given me... thats exactly wat ive wanted to know. i have finished building the circuit but den wenever i try to check da working it gives me a shock...wat cud be da problem??? and is dere ne alternative to dat?

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ive dun exactly wat has been said in the explanation...the circuit ground bein??? jus wanna clarify


Its shown on the schematic and in the parts list.
I've never seen a 470pF cap rated at 630V. Most are only 50V to 100V and would breakdown and maybe short with the mains voltage. If it shorts then the project is deadly!

post-1706-14279142606126_thumb.png

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