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Simple question on touch sensitive switches.

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swedishlf

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Hey everyone,

I'm new here as you can probably see and my knowledge of electronics is pretty limited. Ok, extremely limited. I've been searching for a few hours trying to find the answer to this question and stumbled across this message board, so I figure I'll try to tap the vast intellect here.

The question I have involves using a human to complete a simple circuit. I am trying to make a push-to-make style switch in which the "push" part involves a person touching two seperate pads. I have no idea what the technical term for this is or how to go about it. I will never be working with anything beyond a few standard 1.5v batteries, and always DC current.

I'm having trouble describing the concept, but I've seen several novelty toys that work on this principle. (One that comes to mind is a white ping pong ball size sphere with two metal contacts on the surface. When you touch the contacts,both with the same hand or with opposite hands doesn't matter, the thing lights up and plays a tune.)

Sorry for the long winded post, I'd appreciate any help on this!
- Daniel
 
Hi Daniel, what you want is very quite simple. Its a great thing you stumbled across this forum, I wish I had when I first got into electronics.

You can use a 555 timer IC to do this, although I'm sure there's probably simpler ways, I'll let someone else tell you the name of that circuit cuz i forget.

One thing you might look into if you're interested is a switch that senses extra capacitance (your body). You have probably seen this in action on touch lamps. Just something else to consider :lol:
 
Thanks for your quick reply. I did a quick search on capacitance switches and came up with some stuff that looked pretty good. One even used the 555 timer IC you mentioned.

The one that looks most promising (as it's the simplest to build) is this:

https://circuitos.tripod.cl/schem/r109.gif

Any idea on how close you can get without throwing something like this?

Also, my searching succeeded and I found exactly the item I described in my first post:
https://scientificsonline.com/product.asp?cs=p&pn=3052930

I may have to order a couple and rip them apart to play with!

Thanks again, I'll let keep working on this.
 
This is aculy realy simple to do.

There are many types of these toch switches.There are resistive ones (that detect wen you toch bouth plates and you make an conecton since your body conducts),Capacitive ones (That detect a change of capacity wen you toch an plate,your body has a small capacitence)and radio noise ones (This is wen you toch a plate you make an antena that guidea radio signals in it,the davice detects this change)


The resistive one is realy simple. You just need one transistors (or two if you want to switch somting)This is simply by conecting an NPN transistors emiter to 0V and its colector to an LED wich is conected to an 9V batery.Wen you toch the positive pole of the batery and the trasnistors base the LED will dimly light up. (if you want to use an PNP ransistors just put the LED on the emiter and you have to toch the 0V and the base to light up the LED)


If you have an water clear superbright LED you can just conect one side to the - and you toch the + and the LED.If the LED is conected the right way round you shod see the LED is lighting up in a realy dark room.
 
Dis you guys notice that we got a lot of newbys this month.(dont wory we will be glad to help out)
 
Hey guys, I found a few loose transistors lying in my project box, so I thought I'd give that route a go. Using a 9 volt, an LED and two transistors in a darlington pair, I got a weak effect. Touching the base pin of the transistor with one hand and the positive pole of the 9 volt with the other, the LED did in fact light up. The effect with a single transistor was too weak to see.

Hopefully I can put this to use in my original idea (a stopwatch mechanism with touch pads to start/stop). Thanks again for the tips!
 
Update: I've run into a bit of a snag. Here is the diagram of the working test setup (please excuse the poorly drawn schematics):

**broken link removed**

The transistors in this case are ones I had lying around, the only markings on them are "FD043 37-12." I have several others but I don't know anything about the specs on any of them (nor what the specs mean) this is where I assume I'm getting my problem.

The above system works great, I can touch one pad while someone touches the other pad and when we touch hands the light (where "load" is) lights up.

The problem arrises when I try to use this in the actual system I want to build. Basically I am replacing a simple switch with this system as depicted here:

**broken link removed**

The main difference is that there is no actual load, I just want the circuit to be complete when the two pads are touched. Notice also that the voltage is only 1.5v in this case, as opposed to the 9v rig I have working.

So what am I doing wrong? How do I get from the working light system to a working switch system?

Again, I really appreciate the help guys!
 
Take a voltmeter and measure the voltage @ the base of the transistor when you have your hand there, relative to ground. You will find that it is probably a VERY small value, much less than the .6 or .7 volts that your normal transistor starts conducting at.

The very simple solution would be to find a transistor that has very high gain.

Or you could set up a couple of transistors to amplify the very weak signal.

google the number on your transistor and you will come up with a datasheet for it. I'd do it for you, but its quite late and i need to hit the sack.
 
Your problem is the very low collector voltage. You can fix it in two ways:
1) Use a voltage divider resister in series with the load and the original 9 volt battery.
2) Put a relay between the voltage source and collector. Use a voltage source rated the same as the relay and put a reverse diode across the coil to protect the transistor. Run your device of the relay contacts. If you use a relay with multiple sets of contacts, you can do interesting things like latching in the on position with a momentary touch.
 
Hi, I am trying to use a USB relay controller to control devices in my house. I want to be able to additionally turn them on/off with a momentary switch. So If turn off a device thru software I can toggle its state with the manual switch and vice versa. Has anyone come across a relay controller board either USB, RS232, ethernet that includes the capability to integrate manual switches? Any help is appreciated
 
i think he may refer to a touch "on" and "off" switch. a flip flop in between or a 4017 can serve for that.
there are lot of circuits available for this. i hope he can find it on internet.
 
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