1031Views11Replies

Author Options:

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

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.

11 Replies

user
steveastroukBest Answer (author)2009-09-07

It'll take a couple more pieces than an op-amp, and it'll depend on the op-amp type. Steve

Select as Best AnswerUndo Best Answer

user
amishra123 (author)2009-09-07

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!

Select as Best AnswerUndo Best Answer

user
steveastrouk (author)amishra1232009-09-07

Err. That's not a touch switch. I presume he means where NOTHING moves as he touches a pad.

Select as Best AnswerUndo Best Answer

user
seandogue (author)steveastrouk2009-09-07

Agreed.

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.

Select as Best AnswerUndo Best Answer

user
steveastrouk (author)seandogue2009-09-08

...if you wanted it to oscillate of course, it wouldn't.....

Select as Best AnswerUndo Best Answer

user
seandogue (author)steveastrouk2009-09-08

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...

Select as Best AnswerUndo Best Answer

user
orksecurity (author)steveastrouk2009-09-07

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.

Select as Best AnswerUndo Best Answer

user
arhodes18 (author)steveastrouk2009-09-07

yes, i misunderstood that the first time i read it... but i believe there is way to do it similarly to how they said...

Select as Best AnswerUndo Best Answer

user
arhodes18 (author)amishra1232009-09-07

wow, that was very helpful... I'll have to check it out, thanks alot!!

Select as Best AnswerUndo Best Answer

user
seandogue (author)2009-09-07

using *only an opamp? not really with any reliable operation.

Select as Best AnswerUndo Best Answer