Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Touch switch problems

Status
Not open for further replies.

baxterdmutt

Member
Hello,
I am trying to build the touch switch linked below. I want a touch switch that needs only one touch pad and this seemed ideal.
I just can't get it to work. It is always on.
I was trying to build it with just an npn transistor in place of the darlington. I'm just trying to turn on an LED so don't need a lot of power running through it. I thought that might be my problem so I improvised a darlington with 2 npn transistors. That didn't work either.
HELP!
Any ideas??? Should it work?
**broken link removed**
 
The circuit in the link doesn't have a Darlington??

Edit:
You could try this form of touch switch (which requires points a and b to be bridged by a finger). R2 provides some protection against ESD.
TouchSwitch.gif
To use components you may already have, use R1 = 2 x 1meg, R3 = 1meg, C1 = 470nF.
 
Last edited:
The circuit in the link doesn't have a Darlington??
The schematic calls for a BD679 as the transistor. The datasheet for a the BD679 says "Monolithic Darlington configuration with
integrated anti parallel collector-emitter diode" That sounds like a Darlington to me!

I don't have two contact points available to me on my project. I want to be able to turn on the LED on an LCD by touching the frame. At least that was my hope.
 
The circuit in the link doesn't have a Darlington??

Edit:
You could try this form of touch switch (which requires points a and b to be bridged by a finger). R2 provides some protection against ESD.
View attachment 81768
To use components you may already have, use R1 = 2 x 1meg, R3 = 1meg, C1 = 470nF.
Maybe it's me, but this one doesn't work for me either.
 
The schematic calls for a BD679 as the transistor
The symbol is confusingly not that of a Darlington, however :confused:.
this one doesn't work for me either.
That's a pity. Skin resistance is a major factor with that contact-bridging type. Also, did you have the inputs of the 2 'spare' gates in the 4011 package grounded? CMOS circuits with floating inputs can misbehave.
 
Also, did you have the inputs of the 2 'spare' gates in the 4011 package grounded? CMOS circuits with floating inputs can misbehave.
I did not know that. I will try again with all the unused gates grounded.
Thanks. Maybe that is the problem.:wideyed:
 
I will try again with all the unused gates grounded.
Whoa! Unused inputs must be tied to ground (or the +ve rail). Leave unused gate outputs unconnected.
 
Whoa! Unused inputs must be tied to ground (or the +ve rail). Leave unused gate outputs unconnected.
Sorry. That was poorly worded on my part. I understood what you meant the first time. Thanks again.
Before I try the one I posted again, does it even look to you like it should work. No use in trying again if it's complete junk.
 
Before I try the one I posted again, does it even look to you like it should work.
It seems temperamental. In simulation the assembly of gates oscillates at high frequency.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top