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Can i make a touch sensitive button? Answered

im working on a project that needs a non-tactile button switch. mainly because i am rebuilding the shell and the original button design wont work anymore. there isnt enough space for a tactile switch so i need somthing that can see when i am touching the surface, but it needs to be a rather simple circuit since i dont have a very good access to ICs.
if it helps any, the button has a metallic paint that is conductive, and there is a part beside it with the same paint.
is there any kind of circuit i can build that allows me to make use of that metallic paint and use it like its a tact switch?

any help is appreciated, i do have a 555, a couple L/R audio op amps and a NAND gate.




Best Answer 6 years ago

Here are two touch circuits, and  a pointer to a grand collection of
touch circuits..................    A

ON-OFF-touch-switch-555.gif555 timer touch switch - Flickr - Photo Sharing!_1312086138386.png

i built the second one and t works flawlessly. (as soon as i noticed i had the vcc and Gnd mixed up :P)

thanks so much that is just awesome, i even had all the parts on hand too!

Very Cool !

I still reverse things myself, must be some dyslexia.

Have a look at my iPhone Saddle ible.

Keep on touching.


i am going to check out the second one, since it seems to better apply to my needs.
im assuming the "switch" part of the circuit is pin 3 and ground?

Pin 3 is the output in relation to ground pin 1.
Pin 3 goes high after a touch.
High is 5V or 12V or 15V, depends on you.
The delay is a function of R1 * C1
Increasing C1 can be up to 100uF.


ok thank you.
assuming i can get the part that this would be used for working properly, i will try this.


6 years ago

Here is how to drive a relay from the 555 output pin_3..
Resistor R2 depends on your supply voltage.....A


the relay is still sticking in the on position, its a 5v replay with a 3.5v activation minimal.

So to continue, I hope the relay does not really need 1amp at 5v
it should be 400 ma or less.

I suspect when first you tried the relay you did not have the
necessary diode ( 1N4001 or better 1N4004 etc ) and the
555 was damaged.
Test the 555 circuit only first !

Then the transistor relay
R2 needs to saturate ( fully turn on ) the NPN.. You can
push 20 ma into the base  R2_base = [ 5v-.5v-.7v ]/ 0.02 = 190
so 120 to 200 ohms..

When a transistor get hot,  it is pulling more  load then
the base can drive fully on..  Lower the R2 value down.

Good luck, avoid wiring errors...

The 555 can drive 300 ma...... what current does your relay draw ?



theres no info on that, at least not printed on the relay.

You got to buy an a $6 meter with a current scale at Harbor freight. . . . . . A

i have 2 multimeters, 1 analog and 1 digital.

Great the opto-isolator works, use that.

I'm sure at least one meter could measure current not needed now.
You did a good job measuring the relay activation voltage.


but i went and made a optocoupler for it instead, and it works well.

the transistor gets really hot too.

i decided to go the deceptively simple way and use a white LED + a photoresistor for it :s

What's the chip with your nand gates? Is it the one with 4 2-input NAND gates? The CD4011, or similar?

If so,  I suggest wiring up two of the NAND gates on that chip as set-reset-flip-flop.  Then take those two inputs, (S)et and (R)eset and connect them both high, each through a very large resistor, maybe 1 Mohm or 10 Mohm.  Then construct your switch in such a way that your thumb is on an electrode connected to ground, and your index finger touches electrodes connected to the S and R inputs, to switch the flip-flop, high or low, respectively.  Connect the output from the flip-flop to a transistor, and/or relay, to do the heavy lifting of whatever thingy it is you want to turn on and off.

Here i am assuming that the electical resistance between your thumb and index finger is about 100 Kohm, and that that's sufficient to pull those NAND inputs low, since they are pulled up only weakly, by the much larger 1 Mohm or 10 Mohm resistors, and that's the trick that makes this thing work.

ok so i have looked at it again and ive started putting it together.
and from what i understand, when you touch the electrodes for S and R, the 1 Mohm resistors are ignored and the resistance of your finger is used instead?
if tht is how it works, would it not also work to have s and r disconnected from vcc, creating 100% resistance, and then when you touch it it would pull them low, right?

i want to skip the 1 mohm resistors becasue when i tested with my multimeter, it read 2-3 Mohms of resistance on just the tip of my finger, about 3mm apart, which is how far apart the electrodes would be.

if there is any way i can pull the S and R high without connecting them to vcc? or are they high to start with?

yes its the cd4011.

so if i had the two contacts close together, i could do it with just one finger, right?

and are the S and R inputs connected to the same electrode? or two seperate ones?

a diagram of the circuit i need to build would be helpful, so long as it isnt a bother to you or anything, im just more of a visual learner is all.

Um, here, I drew you a picture of that thing I was describing. The link to the big enough to see picture is here:

Basically this is just a SR latch made from two NAND gates with it's inputs tied high through a couple of 1 M-ohm resistors. 

Here's the link to the Wikipedia page describing this circuit, I mean just so I know I'm calling this circuit by the "right" name, or at least the name everyone else calls it.