Newbe Help with this schematic

[stspringer]

Hello All,
Newbie here.
I am not clear on this schematic, on how the touch metal, hooks up. Is it just 1 wire going from the 100k resistor to the touch metal? Please advise.

Thank You
stspringer

 

[Bluejets]

That's it.

 

[stspringer]

That's it.

Thanks. I was confused because most touch switches show 2 wires. Why is this one different and why a 100k resistor isn't that pretty high in ohms?

 

[Harald Kapp]

Your body works as an antenna. The tiny signal is fed to the base of the transistor Q1. As there are 3 amplifying stages, a very small base current sufices.
The 100 kΩ on the other hand protects the transistor from high currents in case of e.g. an ESD discharge.

 

[stspringer]

Your body works as an antenna. The tiny signal is fed to the base of the transistor Q1. As there are 3 amplifying stages, a very small base current sufices.
The 100 kΩ on the other hand protects the transistor from high currents in case of e.g. an ESD discharge.

Thank you, very nice

 

[stspringer]

Your body works as an antenna. The tiny signal is fed to the base of the transistor Q1. As there are 3 amplifying stages, a very small base current sufices.
The 100 kΩ on the other hand protects the transistor from high currents in case of e.g. an ESD discharge.

I just tried it with metal and my finger and it doesn't work. It only works if I jump the gate and the collector of Q1

 

[Harald Kapp]

It only works if I jump the gate and the collector of Q1

The "gate " is called base in a bipolar transistor and yes, of course it works then because the base gets a positive current from R1 and R2.

I just tried it with metal and my finger and it doesn't work.

Did you try barefooted? I'm not joking.Standing on well isolated soles may reduce the amount of noise available below the required minimum.

A more sophisticated single contact sensor is shown e.g. here.
A more reliable method would use a sensor with 2 plates which have to be touched (bridges) by a finger. You can use the circuit you found and add a second plate which is connected to +9V. Or use this even more simple circuit.

 

[Minder]

Would Mosfets work better? They operate on almost zero current and relay on voltage to switch.
The 2n7000 is cheap.
Maybe clamp the gate with a zener.
M.

 

[stspringer]

The "gate " is called base in a bipolar transistor and yes, of course it works then because the base gets a positive current from R1 and R2.


Did you try barefooted? I'm not joking.Standing on well isolated soles may reduce the amount of noise available below the required minimum.

A more sophisticated single contact sensor is shown e.g. here.
A more reliable method would use a sensor with 2 plates which have to be touched (bridges) by a finger. You can use the circuit you found and add a second plate which is connected to +9V. Or use this even more simple circuit.

Thanks I will check out your sites. Thank you

 

[stspringer]

Would Mosfets work better? They operate on almost zero current and relay on voltage to switch.
The 2n7000 is cheap.
Maybe clamp the gate with a zener.
M.

Hello,
I am a newbie don't forget you are speaking greek to me.

 

[Bluejets]

Hello,
I am a newbie don't forget you are speaking greek to me.

Nah...He's from Canada so more likely "French".

You might want to take a peek at Talking electronics web page.
It has many such circuits and a host of other projects good for beginners as well as others.
Books, circuits, explanations...all good.

For your present project scroll down the left side to "touch switch"

Edit:- just take note that they show some circuits which do not work and the reasoning behind why.

Cheers Jorgo

http://www.talkingelectronics.com/te_interactive_index.html

 

[Cannonball]

This is a poorly designed circuit. It could be made to work but by the time it takes to redesign it. It wouldn't even look the same and not a circuit for a newbie.http://www.talkingelectronics.com/te_interactive_index.html would be the place to find a circuit that would work.

Don't be discouraged.

 

[stspringer]

Thanks to all