# Capacitance change by human touch

Hi

I’d like to build a system that requires two people making a chain by holding hands to complete a circuit and trigger a switch.

Originally I thought they would touch a very low voltage/current terminal with one hand each, then when they joined hands the circuit would complete and trigger the switch, but I have been discouraged away from this method on safety grounds. (I was only planning on attempting 5v max with a current less than 1ma - no idea if this would have even worked?)

Can anyone suggest a technique that can be used to create such a system? I’m wondering if measuring the electrical capacitance and looking for a change equal to two people would work? But I’m aware there are many different factors involved in this theory, ie. how big the people are.

Any suggestions or guidance greatly appreciated please!

TIA

I think with right circuit it could be done.
I'm pretty sure I could do it if I wanted to spend
the time. I worked on hospital bedside patient
monitors in the factory. The signal processing circuits contained circuitry that could be used to
do that but it's way beyond the level of a beginner.
If you're serious about doing you would need a firm
grasp of Ohm's Law before you would even understand the theory of operation. The fact that
it would have to be very low voltage signals is
the reason sophistcated amplifier circuits would be
required that are way beyond the scope of a
forum newbie post. I' built over a hundred op amp circuits my first few years in electronics four
years before I got the job at the hospital mfg.
One signal processing section alone contained
three or four op amps and dozens of support components. It's possible a very simple
'cheap and dirty' circuit could be made but
only by someone who has dedicate themselves
to learning Ohm's Law inside and out. Without
that , we mught as well be talking Greek.
Learn ohm's law and the draw a block diagram
and come back and try to connect the dots.
Without the theory , your only chance is playing Legos with amplifier and ADC breakout boards.
Even with those , it would probably like the
blind leading the blind without engineering support.
There's more than enough of that here but
again, I doubt you would understand it.
Try spending a few days with Google and then come back and prove you are serious about doing this. Prove you could understand any instruction we you.

1. Learn op amp theory and operation.
2. List all the standard op amp circuit configurations.
3. Demonstrate the knowledge of how to change the gain (where applicable) of all thise configurations.
3a. List a modern op amp (modern means not
20 or more years old)
4. Learn comparator theory and operation
Name the most recommended IC for that.
5. Learn the comparator threshold detector.
6. Learn the Window detector.
7. Learn optocoupler theory an operation.
Name the most common IC for this.
8. Learn diode theory application.
List the 1N914 electrical specs
9. Define 'switch' (electrical specs, like you
would give someone to order one)
10. Explain Ohm's Law in detail.
Give examples
11. Learn basic transistor theory and application.
Give an example of how you would use one. (NPN is sufficient)

Shortcut:
If the above is too much work,
Pretend you're a resistor.
Pretend your friend is a resistor.
Make a voltage divider.
Trigger the switch when the voltage divider output is half of it's input.
Hint: You may need to calibrate it first.

Maybe someone with experience with capacitive
occassion to use them.

Hi

I'd like to build a system that requires two people making a chain by holding hands to complete a circuit and trigger a switch.

Originally I thought they would touch a very low voltage/current terminal with one hand each, then when they joined hands the circuit would complete and trigger the switch, but I have been discouraged away from this method on safety grounds. (I was only planning on attempting 5v max with a current less than 1ma - no idea if this would have even worked?)

Can anyone suggest a technique that can be used to create such a system? I'm wondering if measuring the electrical capacitance of human touch and looking for a change equal to two people would work? But I'm aware there are many different factors involved in this theory, ie. how big the people are, skin thickness etc

Any suggestions or guidance greatly appreciated please!

TIA

Hi,

I worked as a telephone engineer for 23 years, on 50v DC circuits.

No problems at all, you don't even feel it.

But bear in mind that in the UK 50v AC is classed as dangerous...

5v DC @ 1mA wouldn't be a problem, unless the person has a heart pacemaker, maybe?

Some years ago I got a (9v) battery operated electronics kit to play around with our grandkids and we built an 'electronic organ'.

Basically a wire that we dragged along a pencil line on paper and the varying resistance changed the tone output.

Playing around, I had two of the kids hold one wire each and then hold hands and we got a tone out of the circuit. No damage done and those kids now have their own children.

Peter

I've merged your other cross-post @kilmo.

Cross-posting is against the rules of the forum. The reason is that duplicate posts can waste the time of the people trying to help. Someone might spend 15 minutes (or more) writing a detailed answer on this topic, without knowing that someone else already did the same in the other topic.

Repeated cross-posting will result in a suspension from the forum.

In the future, please take some time to pick the forum board that best suits the topic of your question and then only post once to that forum board. This is basic forum etiquette, as explained in the sticky "How to use this forum - please read." post you will find at the top of every forum board. It contains a lot of other useful information. Please read it.

Indeed, voltage divider.

5-12V DC (the higher voltage would give better sensitivity); 470k-2M fixed resistor between analog pin and GND, the people between analog pin and the + voltage. Add a 10k resistor on that end to protect your pin from short circuits if you go over 5V (the clamping diodes will happily take care of the excess voltage and protect your input pin). That pull-down keeps your pin grounded unless a connection is made; it doubles as protection for the people so that even if they would have near-zero resistance the currents are still at a safe level.

May work on a digital pin as well but analog is more reliable as you can take any reading >100 as "connection made". Maybe add a small cap, 1-100nF or so, in parallel with the pull-down resistor to make it more noise resistant.

470k would mean a 12V source can deliver no more than 25µA which I would think is perfectly safe even with pacemaker, much less with a typical skin resistance which is also in the order of 0.5-1M for each person. For better sensitivity you have to use a higher value resistor, but if you go too high you start picking up more and more noise, and with the filtering cap the reaction time becomes longer (probably nothing to worry about: 1MΩ * 10nF = 10 ms RC time constant, slow on a microcontroller scale but pretty much instantaneous on a human scale).

The danger comes not so much from using 5V or 12V or whatever but from where that voltage comes from. If it comes from a mains power supply then you are running a small risk of the power supply being faulty and getting a connection to mains. If you run it off batteries then you are safe. Who ever got a shock from a 9V battery?

I worked as a telephone engineer for 23 years, on 50v DC circuits.
No problems at all, you don't even feel it.

Me too. I can't feel 50VDC. I did once turn the voltage up slowly on a PSU while someone held the wires, they said they could feel it at about 35VDC.

Ringing current on the other hand.... :o