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I am trying to create a circuit that will be able to detect a touch (with fingertip) on a metal/aluminium surface that it is connected to. Currently I have this circuit (I am sorry for bad drawing...):enter image description here

My assumptions are probably wrong, but this is what I was thinking of. Due to human skin having some capacitance, when one would touch the plate, there would be some current flow due to change in capacitance. High pass RC filter would get rid off DC offset and the signal would get amplified by LM358. This would then trigger 555 timer in monostable mode.

Any help is greately appreciated!

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do you need to hook that thing to a microcontroller? –  Vladimir Cravero Jun 25 at 16:52

2 Answers 2

up vote 5 down vote accepted

If you're really after a capacitive touch sensor, this likely won't work at all because the aluminum plate is a short to ground. You're better off making an oscillator (ring or 555 based oscillator's are my favorites) that has a frequency based on R and C where C is two separate contacts where the touch sensor goes. A persons finger near the two contacts greatly changes the dielectric between the two contacts and you end up with a very different capacitance. With a different capacitance, you now have two different frequencies that your oscillator will ring at. It's actually a range of frequencies depending on how close the finger is, but that will be turned digital at the end.

Then you send the oscillator output into a high pass filter (small capacitor in series), set the DC operating point and then use a comparator with a chosen voltage level to determine whether the person's finger touched it or not.

There's always digital versions if you wanted to travel into uController land. Here's an app note by TI on capacitive touch sensors. A lot of the front-end concepts are the same regardless of whether you want to go digital immediately or just keep it analog until the end as I described above.

schematic

simulate this circuit – Schematic created using CircuitLab

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I am trying to create this, because I saw this system working at a friends house. Where he would connect a circuit to a aluminium plate and when he would touch the plate the LEDs would light up. Although I like your idea, it isn't what I am trying to do. –  Golaž Jun 25 at 15:39
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@user1806687 You could have the touch sensor be nothing more than an aluminum plate (not connected to ground). It wouldn't have quite the sensitivity of what I describe, but it would still work just fine. –  horta Jun 25 at 15:42
    
Using only aluminium with no connection to ground works, thank you. But why/how does it work? I dont really understand, isnt the whole thing just floating? –  Golaž Jun 25 at 16:10
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The other popular way for a capacitive touch sensor is "CTMU" - Charge Time Measurement Unit. Charge the capacitor (plate || ground) with a constant current for a pre-set time. The voltage after that time will be different for different capacitances. –  Majenko Jun 25 at 16:23
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@user1806687 The reason it still works is because there's still a capacitive connection to ground, it's just much weaker (because the distance is longer). A human body still acts like a reasonably large capacitor compared to the parasitic capacitances. –  horta Jun 25 at 17:16

You can create a very simple touch switch using only the 555 timer by connecting the aluminum plate directly to the trigger:

555 Touch Switch

The downside to this version is that if your plate is too big, it can act as an antenna and the output will be very unstable. Additional circuitry can be used to filter out the noise if desired.

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This works (I've built it) img841.imageshack.us/img841/6306/18094412.png –  user34920 Jun 25 at 19:28

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