ksp1234 Posted January 3, 2006 Report Share Posted January 3, 2006 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 3, 2006 Report Share Posted January 3, 2006 Hi Ksp,You didn't provide a link to the project so I guess it is this Touch Switch one:http://www.electronics-lab.com/projects/sensors/002/index.htmlIt doesn't have a schematic, just a pcb layout drawing.The text explains about how its flip-flop works pretty well. Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 4, 2006 Author Report Share Posted January 4, 2006 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 againprasad Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 4, 2006 Report Share Posted January 4, 2006 You still didn't include a link to the project you are talking about. On the projects index page, right-click on its English version and left-click Copy Shortcut. Then left-click in your reply and right-click Paste. Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 5, 2006 Author Report Share Posted January 5, 2006 hi audioguru, http://www.electronics-lab.com/projects/sensors/013/index.html, this is the link to the project site. Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 5, 2006 Report Share Posted January 5, 2006 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? Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 6, 2006 Author Report Share Posted January 6, 2006 The operation of the super darlington pair that enables it to drive the relay is what that baffles me... thats precisely what id like to know ... the operation of that part of the circuit... Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 6, 2006 Report Share Posted January 6, 2006 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. Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 9, 2006 Author Report Share Posted January 9, 2006 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. tchope to hear from u soon Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 9, 2006 Report Share Posted January 9, 2006 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. Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 10, 2006 Author Report Share Posted January 10, 2006 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 D2 and D3 don't do anything. The circuit will work without them.D1 arrests the few hundred volts flyback voltage spike that occurs when the current in the relay coil is stopped. Q3 would be destroyed without it. Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 10, 2006 Author Report Share Posted January 10, 2006 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? Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 wenever i try to check da working it gives me a shock...wat cud be da problem??? and is dere ne alternative to dat?Dangerous! What low value capacitor and what is its high voltage rating did you connect the live mains to the project's circuit ground? Quote Link to comment Share on other sites More sharing options...
ksp1234 Posted January 11, 2006 Author Report Share Posted January 11, 2006 ive dun exactly wat has been said in the explanation...the circuit ground bein??? jus wanna clarify Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 11, 2006 Report Share Posted January 11, 2006 ive dun exactly wat has been said in the explanation...the circuit ground bein??? jus wanna clarifyIts 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! Quote Link to comment Share on other sites More sharing options...
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