No IC touch sensor for.....whatever you want!

This Design uses one wire that you touch and turns the load on or off. What is cool about this is you can actually use it as a proximity detector, it is sensitive enough to detect something from about a foot away. I did bread board tests and built one that detected me a turned a buzzer on from ten feet! The nice part about this design is that there aren't alot of parts and they are not expensive.

This is the same technology used in those ovens that have the glass panel over the controls, but they use ICs. The principal is the same however, they both detect (I think) the small electrical signals that people give off and amplifies this to a useful level. If this is wrong correct me, but all I know is that this works.

(This design works great for LEDs. I am making a touch LED flashlight that runs off rechargeable batteries, but I am not sure what batteries to get yet.)

Step 1 Parts

Parts:
-4 x 2N3906 PNP transistors or similar (note below about transistors)
-1 x 1N914 diodes
-1 x 10v 1000uF capacitor (keep the voltage below 10v / Caps rated voltage or it will  
                                                explode!)
-1 x Barrel jack
-1 x 9v 300ma adapter
-1 x 120 Ohm resistor (Brown, red, brown)
-1 x Ceramic capacitor
-some shield wire (I don't use it because I don't have any but if you do use it!)
-some copper clad is nice to have but not needed
-ugly perf board
-Breadboard/protoboard are handy as well

Tools:
-solder
-soldering iron
-tin snips for copper clad and ugly perf board



*Note: The transistors I used are random transistors similar to 2N3906 but I live in Canada and can only get a package of transistors "similar" to 2N3906 so protoboards are good to test this out on. Mine are bi polar and let some electricity through one way all the time wether they are off or turned around let no electricity through when off. Test the different combinations to get the way that works best. 2N3906 should work fine in this circuit though I have not tried it.

Comments

[ooda55]

Hi there
how can i wire this into the input of a PIC?
pic 12f629 to be exact

thanks

[Wesley666]

Honestly, I have no idea. I avoid IC's because I prefer building an entire circuit from scratch. That, and I self teach myself everything I know, and haven't gotten into IC's much...Sorry. :(

[Vividh]

@SMOKEDASPHALT-in DELHI !..that's damn cheap for us... for a touch sensor...!!!

[Vividh]

Here , in india,it can be build in under 2 $(canadian )

[shubham garg]

I agree too...:)

[Smokedasphalt]

2 Canadian Dollars is almost 80 Indian Rupees!! That is a little too costly for this project!! The transistors cost Rs 2 each, the 1,000uF Cap is Rs 10, the diode is Rs0.50, and the resistor is also Rs0.50. I already had a 9v adapter with me. so, the project cost me Rs19 only! Thats like HALF a Dollar!! :) Hey, Vividh! where in India do you live? I'm in Pune, Maharashtra!

[Wesley666]

wow that is cheap. In Canada I get stuck with a few really crappy suppliers that are within driving distance and they have really bad prices, so I can see that it could be built for less. I think I might start ordering off the internet.

[ausman08]

Would a 3v battery work on a smaller led?

[Wesley666]

It should still work. If it doesn't use a 9v battery.

[acmefixer]

This thing has so much current gain that if you get something that transmits RF, like a cell phone, near it, it will most likely trigger the circuit. One way to drastically reduce this is to put a 1000 pF capacitor from the base of the input transistor to ground, and put a 220k resistor between the touch pad and the base of this transistor. This will filter out most of the RF.

[Wesley666]

Ahhh, RF...Ya, I am self taught in everything I know, and RF I have found to be a huge pain...I wanted to make a RF transmitter that a normal radio would pick up for a school project, but I had no idea where to begin, so I used one off the internet. I thought about RF a tad when I made this but because I lack knowledge I decided to forget about it, and I wasn't sure if it would even affect this circuit. I would love it if you could explain a little bit about RF for me or send me a good link on the basics, I have been looking into RF circuits lately but haven't found anything useful on basics of RF circuitry, and could you explain about why/how the 1000pf and 220k resistor you suggested would help? Much appreciated! :D

[acmefixer]

See my blog here.http://watsonseblog.blogspot.com/2010/12/2010-dec-27.html

[Wesley666]

Oooo! Very cool blog! Definitely putting that one under my favorites! :D And that link to the book sounds like it should be really useful, unfortunately my Ipad won't load it right now, the Wifi isn't great at this particular spot, will have to wait till I get home to my desktop. Looking forward to it though! Gonna have alot of reading between that book and your blog! Thank you!

[Ben The Builder]

I'm pretty sure capacitive touch panels work on the principal that your skin is mildly conductive, while we do theoretically send out electricity from the neurons in your brain firing it'd require a ton of amplification to begin to detect, however the reason you get shocked so easily when you accidentally touch the outlet in your house is because your skin is significantly more conductive than air and thus the electricity can ground through you

[acmefixer]

Funny thing you mentioned a ton of amplification. I multiplied the gains using a conservatively estimated current gain of 100 for each transistor, in other words, 100 to the fourth power. The conservative estimated gain from input to output is _at_least_ a hundred million! Using the more common gain of 250 for each transistor, I get almost four Billion!!!

[Wesley666]

I didn't mean the small signals that our brain gives off, I meant the small charges that we build up that leads to a static shock. If you take a plastic rod, rub it, then touch it, you are mostly made of water and we ground fairly well so we gain some negative charge from the rod, and that is what this I think is detecting. I tried this while rub my feet on the carpet and it works better as you pull your feet across the carpet but in between each shuffling foot, you can see it dim a bit. Therefore, I don't think this would work if you were continually grounded.

[jacksky]

hi,
I am interested to know more about the experiment ini.can I know with more details, what are the types of components needed to make this touch sensor .. thanks, hope to answer.

[Wesley666]

Step 1 is the parts list. All are available from Radioshack.

[koney]

HiYa!
This maybe can be used in another project to make it even cooler : Table-for-Electronic-Dreams (http://www.instructables.com/id/Table-for-Electronic-Dreams/)
What do you think, would it work?

[Wesley666]

Ya, I think it should work. If you touch the under side of the board it still turns on, even though its separated by an insulator, so mounting a tiny piece to the underside of some acrylic it should still work. Testing it first would still be a good idea. There is a comment below some where that talks about using a different transistor, but you only need one or 2, which would be handy.

[rivel_rage1]

what is the size of your ceramic cap?

[Wesley666]

Its 220 pf.

[misteravocado5]

Sorry for my lack of understanding, but why does it say to keep the voltage above 10v? I thought you were not supposed to exceed the rated voltage of a capacitor. Shouldn't it be kept under 10v?

[Wesley666]

There, I fixed it now! Woooo! That wasn't very performance of me! Thanks again for catching that! :D

[Wesley666]

OMG! Sorry, that was a typo! It should say, BENEATH 10v or the caps rated voltage! Thank you for catching that! :D

[misteravocado5]

Lol just making sure. Don't want anybody blowing anything up.

[steel36]

I really want to build this thing, but I do not understand the schematic.  Simple things like your 120ohm resistor, I can't make out how it's connected.  Please make a bigger/better drawing w/ more detail.

[Wesley666]

The 120 ohm resistor is connected in series to the collector of the transistor.  It really isn't that hard to see.  If you want to see it bigger, you can click under the "View" option at the top left side of the internet bar the "Zoom" function and zoom in.  All the transistor are Darlington Configurations.  Everything else seems straight forward.

If your still having problems comment again and I will do what I can to fix it...

Thanks

[pyro-jim]

Is that a 9V LED then? I would like to run a 5V laser diode off this from 4.5V batteries. Is the voltage stepped down, do you have a resistor in front of the LED or is it a 9V LED?

[Wesley666]

For mine I wasn't using a resistor, but if you need more power you can increase the wall adapter from the 9v 300ma one I was using to a 12v 500ma.  You may want to check what would be applied to the load (Laser) before connecting it just to be safe.  You could also probably just remove the 120 ohm resistor I used in my schematic.  It would be easier to keep the same adapter and remove the resistor because if you increase the power you may need to increase part values so they don't blow.

Try removing the resistor first and test the output, if you really need more up the adapter power.  You should be safe with 12v 500ma, all the parts should handle that, but make sure the capacitor is above 12v, the one I used is only 10v and would explode.

If you have any other questions feel free to ask.

[pyro-jim]

(removed by author or community request)

[andronikus]

unfortunately this didn't work for me, wouldn't you please make a good schematic so it would  be easier for us to do it? , tnx anyway

[Wesley666]

I can't fix something that isn't wrong...make sure your transistors, diode, cap, load...Everything that has polarity is in the correct direction.

[arhodes18]

can you use something like an op amp or something rather than 3 transistors?

[Wesley666]

possibly, but I have no idea...I never thought about that...if you have nothing to lose go ahead and try or ask as a question, but that is something that sounds very clever.

[arhodes18]

ya, I will definitely try it...

[oakironworker]

Cap will explode if given under 10 volts. Help a nervous newbie out . I am just starting to build a few circuits but haven't heard about this.

[Wesley666]

No, if rated 10v at ****uF it will explode OVER 10v. If you want this to handle more power up the cap to a higher voltage, at least 5v over the power you want to use, that's what I go by. Just keep the ****uF rating the same. It will be on the cap before or after the **v rating and will say **** and then uF. Also check the other components to make sure they will handle more power otherwise you may wreck other components even if the cap is fine.

[icefire279]

im under the impression that this sensor works even if you are not in physical contact with the pad...if my assumption is correct is there a way to only activate the led by making physical contact with the pad?

[Wesley666]

A relay would work. Only when its touch would it have enough power to switch the relay.