capacitive proximity sensor using 555

[vinniewryan]

I'm building this circuit:
http://www.sciencezero.org/index.php?title=Capacitive_proximity_detector

After a few hours of trying things, I can't seem to get it to work well enough for my needs. The frequency is too high to work with in the default configuration, and if I lower it the output isn't as dramatic to proximity detection. Are there any good circuits that output a signal from maybe 1-10Khz with proximity detection from 1-4cm? It just needs to detect a finger from not too far away and output a signal that can be measured by a PIC.

Thanks! I've also tried using FET's instead of a timer, but I can't get a decent enough output with such a small antenna (about 1.5cm long, 0.5mm diameter).

Also, using the 555 circuit above, what's determining the high/low periods of the output wave? Moving around within proximity of the antenna only seems to be changing the low period, with very minimal (5%?) change in the high period.

[Soeren]

Hi,

I'm building this circuit:
http://www.sciencezero.org/index.php?title=Capacitive_proximity_detector

That circuit will not work reliably for that purpose I think. It's too low impedance and it's not really a capacitive touch, but rather a "mains hum injection touch" and it's not very predictable.

After a few hours of trying things, I can't seem to get it to work well enough for my needs. The frequency is too high to work with in the default configuration, and if I lower it the output isn't as dramatic to proximity detection. Are there any good circuits that output a signal from maybe 1-10Khz with proximity detection from 1-4cm? It just needs to detect a finger from not too far away and output a signal that can be measured by a PIC.

There's so many ways of making real capacitive "personnel detectors" and the most sensitive types relies on an HF oscillator with an "antenna" connected to the most sensitive part (which you would normally shield) and the oscillator output fed into a frequency shift detector of sorts (eg. a PLL).

I've also tried using FET's instead of a timer, but I can't get a decent enough output with such a small antenna (about 1.5cm long, 0.5mm diameter).

Oh well, a thin 15mm wire isn't much for a 40mm detection distance - for that I'd expect something like a 30mm*30mm to 40mmx40mm conductive plate (eg. un-etched PCB) connected to that mentioned wire.

A MOSFET like 2N7000 is hyper sensitive if you add a wire to the gate. In fact you'll kill it if you don't add protection, as it can detect the static charge if you rub two pieces of paper against each other at a 50..100mm distance from the sense wire.

Also, using the 555 circuit above, what's determining the high/low periods of the output wave? Moving around within proximity of the antenna only seems to be changing the low period, with very minimal (5%?) change in the high period.

A combination of stray capacitance, mains hum (which you inject inductively) and a small amount of loading - and of course the two resistors.

A better circuit, still using the 555, but this time in the monostable mode. R1, P1 and C1 determines the length of the output pulse and the trigger pulled high with a 10M resistor, act as the sense line. You'll still need to experiment with the size of the sensor plate though.

This circuit has got a digital output of course and won't do if you are trying to detect distance to the finger.

[vinniewryan]

Thanks for the info, I'll give it more time tonight. I want to try and somehow have the one circuit run 3 seperate antennas using switches (transistors?) to toggle between each of the three. It will constantly switch between the three antennas so it knows which one your finger is closest to.

I'll update later, cheers!

[vinniewryan]

So after reading over the above schematic and info, and some other research, is all of this really needed? Could I not just simply use a PIC to control charging/discharging 2 capacitors, one with a copper plate and one without, then use a high speed comparator to 'compare' which one charges up first? I can set it up so the cap with the copper plate takes longer to charge only when there is something within its field adding to its capacitence. Theoretically I should even be able to control the speed which the caps fill up with a variable resistor so I can calibrate them to work with slower comparators.

I would need to connect the comparator to an RS flip flop so it holds the info until the PIC resets it, assuming I can't run a PIC fast enough to do that work by itself. I really don't know what kind of speed I'm dealing with yet but surely this should be possible, and not too far from the capabilities of a 40Mhz PIC with interrupts. I can probably even use a PIC with a built in comparator and save myself some work.

Thoughts?

[vinniewryan]

Okay, so here's where I'm at; I've decided once more to use a 555 timer for the time being, simply because nothing else seems to be sensitive enough given the distance and area I have to work in. The PIC is reading pulses from the 555 and converting them into i/o for a second PIC to read.

Now for the hard part. Right now the trigger of the 555 is connected to a small piece of wire (antenna). When my finger is within 10mm proximity of the antenna, the duty cycle begins to change dramatically. I want to have 3 different antennas connected to the trigger, but only be connected one at a time. If the world was simple, I would simply connect 3 transistors in line from the trigger pin of the 555 each going to a seperate antenna with the bases of the transistors controlled by the MCU. This won't work for obvious reasons, so my new question is;

How would one go about electronically routing 3 different switchable antennas to one input?

[billhowl]

You have to use relay to switch the antennas.