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

-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  
-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

-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.

Step 2: Circuit

The schematic is shown below.

The only reason the resistor is needed is because the last transistor always began to over heat after about five mins. but with the resistor there it does not overheat anymore. You could try a similar transistor to the 2N3906 but with better power ratings and the resistor would not be needed, but as I said earlier, probably, I live in Canada and the Source by Circuit City doesn't have a good selection so I have no choice. So if you live in the States (USA) or buy parts online (thats what I should probably do) then test different transistors.

I would also like to add in an indicator LED to the board that way it turns on whatever the load is as well as a visual que that it is working.

UPDATE! - The transistors are all 2N4403 except for the one connected to the resistor. It is a 2N3906. Also I think the arrows in schematic might be on the wrong side or pointing the wrong direction, but they are PNP transistors. I did label the Collector, Base and Emitter in the picture in case I did the symbol wrong.

Step 3: Solder

The title of this step speaks for itself.

Also you can do a way better solder job then I can, probably.

Step 4: Done

Now was that not easy? A simple single wire touch sensor without complicated ICs for under $5 (mind you that that is the Canadian dollar!) ;)

one project that would be cool is to remove an on/off switch and replace it with a locking relay. You can buy small relays that are like pens, when they are switched on they snap closed and they lock and when they are switched again they unlock, then add in the touch sensor to control the coil of the relay.

Have fun and mess around with this design all you want. Also if anything is wrong in my Instructable, message me to fix it.
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
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.
<p>yes, it would still work if you were grounded, and it has nothing to do with the AC wiring. it's simply a matter of capacitance. being near the &quot;touch plate&quot; of the sensor changes its capacitance a very small amount. if it were caused by static, it would not sense you even a millimeter away from it, after touching it, because you would have just neutralized any difference of potential.</p>
This circuit works because you're in a house that has electric wiring. If you ran an extension cord out to the yard, it would become far less sensitive. The AC wiring in the walls gives off a small amount of electro-magnetic force (EMF) and your body acts like an antenna to pass it to the touch plate. Imagine a coil of wire connected to an electricit source; it gives off magnetism (specifically, electromagnetism). Uncoil the wire and it still gives off a magnetic field, it's just not as concentrated. This effect is similar to the noise you hear on an AM radio when your car drives under a high-voltage power line.
ceramic capacitor of that value is?
Hi there<br>how can i wire this into the input of a PIC?<br>pic 12f629 to be exact<br><br>thanks
Connect the Output load to the PIC. Was that so hard?
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. :(
@SMOKEDASPHALT-in DELHI !..that's damn cheap for us... for a touch sensor...!!!
Here , in india,it can be build in under 2 $(canadian )
I agree too...:)
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!
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.
Would a 3v battery work on a smaller led?
It should still work. If it doesn't use a 9v battery.
hi,<br>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.
Step 1 is the parts list. All are available from Radioshack.
HiYa! <br>This maybe can be used in another project to make it even cooler : Table-for-Electronic-Dreams (https://www.instructables.com/id/Table-for-Electronic-Dreams/) <br>What do you think, would it work?
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.
what is the size of your ceramic cap?
Its 220 pf.
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?
OMG! Sorry, that was a typo! It should say, BENEATH 10v or the caps rated voltage! Thank you for catching that! :D
Lol just making sure. Don't want anybody blowing anything up.
There, I fixed it now! Woooo! That wasn't very performance of me! Thanks again for catching that! :D
I really want to build this thing, but I do not understand the schematic.&nbsp; Simple things like your 120ohm resistor, I can't make out how it's connected.&nbsp; Please make a bigger/better drawing w/ more detail.<br />
The 120 ohm resistor is connected in series to the collector of the transistor.&nbsp; It really isn't that hard to see.&nbsp; If you want to see it bigger, you can click under the &quot;View&quot; option at the top left side of the internet bar the &quot;Zoom&quot; function and zoom in.&nbsp; All the transistor are Darlington Configurations.&nbsp; Everything else seems straight forward.<br /> <br /> If your still having problems comment again and I will do what I can to fix it...<br /> <br /> Thanks<br />
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?<br />
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.&nbsp; You may want to check what would be applied to the load (Laser) before connecting it just to be safe.&nbsp; You could also probably just remove the 120 ohm resistor I used in my schematic.&nbsp; 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.<br /> <br /> Try removing the resistor first and test the output, if you really need more up the adapter power.&nbsp; 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.<br /> <br /> If you have any other questions feel free to ask.<br />
unfortunately this didn't work for me, wouldn't you please make a good schematic so it would &nbsp;be easier for us to do it? , tnx anyway
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.<br />
can you use something like an op amp or something rather than 3 transistors?
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.
ya, I will definitely try it...
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.
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.<br/>
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?
A relay would work. Only when its touch would it have enough power to switch the relay.
Just an FYI but the 1n914 are the same as 1n4148. One might be easier to find than the other depending on where you are located. Just for curiosity sake what was the part number on your transistors? Nice project by the way. Mike <>< Owner SurplusGizmos.com
I just looked and they are all: 2N 4403 -062 (that's the exact way its written on the transistor.) except the the last one connected to the resistor. It is actually a 2N3906.
What's the ceramic capacitor capacitance? Thanks a lot for this instructable :)
It says 221 so it should be 220pF.
You need the resistor to limit the current passing through the transistor. Any transistor you choose will require this, and the value will depend on the transistor itself and the power passing through it.
Have you tried with only a single base-linked cascade transistor instead of 3? At some point all you're doing is amplifying random noise. You can also do it with a single 2N7000 mosfet to replace all the transistors. Connect D to the LED/load and G to the metal plate. @ jeff-o It's a classic Darlington configuration - no resistors are required. Besides, if there's a short, the LED will act as a fuse!
Ok, I get that I could have used something else to replace the transistors, but in my instructable somewhere it does say I live in Canada. My parents don't like me ordering off the internet and my only other option is The Source by Circuit City and they have the worst selection ever, so I used what I had. But feel free to use the parts you have.
Oh, don't worry about that! Your choice of transistors is just fine. I have no idea where you live, but check www.sayal.com. Maybe you live close to one of their stores. The selection is a million times better than the Source!
Oh cool, I will have to check that out, plus the Source has terrible prices as well, it is like $5 for 2 5mm red LEDs!
Oh! I recognize the Darlington configuration, and I agree that no resistors are needed between the transistors. But the last transistor, that actually has a notable amount of current running through it, still needs a resistor.
cool i made something like<sup>i was trying to make a ic probe</sup><br/><br/>i used ic's though <br/>you say you got a ten foot range?<br/>i got like 6 inchs<br/>
I added more transistors and I noticed when I walked into the room with the project about ten feet away it went off. Scared me a little but used it for a practical use......Park the car in the garage every time perfectly! It doesn't detect the car from ten feet, more like three but it works and that guides you in until the car touches a pressure pad. Both the proximity and pressure sensor are connected to LEDs mounted on a shelf in front of the car, and they are different colors. As you drive in the yellow one slowly comes on, and turns off when the pressure pad activates and this activates a red LED. I am going to post an Instructable on it soon! = )<br/><br/>Also I didn't use shield wire I and I think that may have contributed to the range, but I honestly don't know, I didn't even mean to build a touch sensor, but hey when life gives you lemons make........touch sensors? = )<br/>

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