Is there any way to make a touch sensitive switch using an OP Amp?

I just saw an i'ble about making a touch sensitive switch. The problem was that it used 3 transistors, and I wondered if there was any way to make one using only an OP Amp. Thanks.

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It'll take a couple more pieces than an op-amp, and it'll depend on the op-amp type. Steve
amishra1238 years ago
yeah probably if you use a standard 741 then you could use it as a comparator and and then use two pieces of foil or something as contacts so when they are pressed down they touch each other it goes high. just google op amp as a comparator it will explain everything ;D hope i helped!
Err. That's not a touch switch. I presume he means where NOTHING moves as he touches a pad.

At the basic level, (just an opamp) you could touch the inverting and non inverting inputs and maybe get it to switch, since the open loop gain is high enough on most to register the minuscule potential difference in your finger tip and amplify it by the open loop gain to a slew positive or negative, but I'd suspect it would simply oscillate.
...if you wanted it to oscillate of course, it wouldn't.....
Or only part of the time...just to give you a false sense of security before it doesn't work the way you wanted it to when you've embedded in into the system in a way that makes it tough to get at...
Yep, that too :-(
Usual approach is to have a tuned circuit which is thrown out of tune when an additional capacitance (the human) comes in contact with the touch surface.

If you insist on trying to do it with an op-amp, you *MIGHT* be able to instead do something with using the human as an antenna that picks up 60-cycle hum, and amplify that enough to use it to throw a relay (or whatever). Not very reliable since how much hum gets picked up will depend on the environment, but I've done it accidentally; "if it happens it must be possible."

Another solution might be to have two parallel non-touching contacts, which the finger bridges across. In this case we're using the human as a resistor rather than a capacitor. Again, this is a comparitor-like approach.

I think I'd go looking for an existing circuit design, rather than trying to reinvent this wheel.
arhodes18 (author)  steveastrouk8 years ago
yes, i misunderstood that the first time i read it... but i believe there is way to do it similarly to how they said...
arhodes18 (author)  amishra1238 years ago
wow, that was very helpful... I'll have to check it out, thanks alot!!
seandogue8 years ago
using *only an opamp? not really with any reliable operation.