Body contacts...

[Gordonjcp]

... have naff all to do with static electricity.

Your skin is somewhat conductive.  This is why you get a shock if you stick your finger in a mains socket.  Don't try this, just take my word for it.

If you take an ohmmeter and grab hold of the probes, you'll see that you can measure this resistance.  Depending on the size of the contact area (quite small in the case of the meter probes) you might see it go from anything from a few hundred thousand ohms, to about 1000 ohms.

Now, if you touch two points in the circuit, you are applying this extra resistance, just like if you had an actual resistor in there.  Depending on how hard you press your finger onto the contact points, you can vary the resistance.  Get an ohmmeter and try it - the harder you press, the lower the resistance.

The other thing that happens is that your body picks up electric fields from wiring, radio transmitters and all sorts of other things around you.  You act as a big aerial.  You're conductive, remember?  This means that when you "buzz" the input of an amplifier you couple some of this electricity into its input.  Mostly, what you'll be picking up is 50Hz mains hum from the wiring around you (or 60Hz, if you're in the US.  40Hz dropping to nothing, if your generator just ran out of diesel.) which is then amplified along with all the other noise and comes out as that familiar buzz.

So now you know.

[noiseybeast]

What zoinky insists is that body contacts have nothing to do with resistance but rather with static.  That's where the other guy was getting confused.

[jamiewoody]

What zoinky insists is that body contacts have nothing to do with resistance but rather with static.  That's where the other guy was getting confused.

ditto, that is what i was thinking, almost...

i had it explained (falsely maybe( that the capacitance increases when you touch a part of the circuit. my theory behind the cap thing was "less noise=low capacitance". ie. a "low capacitance" guitar cable would also be a low noise cable.

anyhoo...

[Gordonjcp]

No, it's not capacitance either, although that *does* have an effect on RF circuits.  This is how theremins work.  You are a big wet earthed conductive thing.  When you put your hand near (not touching, but near) an electronic circuit, you become one "plate" of a capacitor to ground.  If you look at this circuit here http://www.physics.gla.ac.uk/~kskeldon/PubSci/exhibits/E9/cir1.gif (from http://www.physics.gla.ac.uk/~kskeldon/PubSci/exhibits/E9/ ) you'll see that the pitch oscillator (top left) has a coil with a capacitor across it.  There's actually two, so some of the signal can be tapped off for feedback.  Notice how the upper capacitor (33pF) is connected to the pitch antenna, and ground?  When you wave your hand near the antenna, you apply a bit more capacitance to ground and lower the frequency of the oscillator.

"Aha! So why does the pitch go *up* when you put your hand near the pitch antenna?" you all say - well, you'll notice that on the right hand side there's an identical oscillator without an aerial, but with a wee trimmer to get it precisely matched to the pitch oscillator.  Both of these feed into a diode ring mixer (a ring modulator, really).  From the mixer you get the sum and difference of both oscillator frequencies, and both original ones.  The original frequencies and the sum are all too high to hear, but the difference is within the audio range - think about one oscillator at 1.001MHz mixed with one at 1.000MHz and you get a 1kHz output.

Bang it through a frequency-to-voltage converter as for the volume circuit, and you've got a control voltage that can drive a VCA.  Or a VCF.  Or anything else you care to bodge on...

[jamiewoody]

thanks for explaining that. so, a "theramin" with photo cells is closer to a real theramin than i thought. using photocells, one is changing resistance, if they are used in application as a variable resistor.

so it is...resistance, not capacitance.

[Gordonjcp]

Yeah, pretty much.  I think that's how the Roland D-Beam things work, too.  I'm not sure since I haven't had the opportunity to pull one apart - yet...

[Circuitbenders]

doesn't the D-beam work using reflected infra red?

[Bogus Noise]

Yeah, I seem to remember those being infra-red too.