How to build...

[airwave]

I'm sorry guys, but I recently had an idea stuck in my head but I don't really know how I would go about this. I was thinking of building a circuit that when you touch some metal piece, would turn on some led lights.
My first impression on how to do this was to have the person touching the metal act complete the circuit (since he is connected to ground (Earth)).

Then I thought a little more about it and I'm not sure how to get this working.
Any ideas/help would be greatly appreciated.

 

[zachtheterrible]

You could do a static sensor. or u could do a thing where wen u touch the plate, it will change the capacitance.

 

[williB]

simple ..connect the metal plate to the gate input on a N-channel FET..
on the drain put a current limiting resistor connected to a couple of 1.5V batteries and a couple of properly placed LEDs ( they should point in the direction of current flow Plus to minus) .. and transistor source to the negetive of the batt..
V=ir .. so 3V/.02 A =150 ohms .. so 100 ohms should do it...

 

[Oznog]

You can't use a floating gate like this. One, the person often doesn't short to ground at all but present a static positive or negative voltage. Two, that voltage will frequently be large enough to destroy the gate.

A touch sensor measures the change in capacitance, not current to ground. Simplest way to pull this off might be to make an oscillator with a 555 using the touch plate tied to the + of a low value timing capacitor. The additional capacitance of the touched plate should be significant enough in relation to the timing capacitance so as to produce a noticible change in the output freq. You will need a decent way to measure the freq, so I don't know a real simple way to do that. You might want to search the net for touch circuits.

 

[stevez]

I'd guess that commercial (some irresponsible too) touch lamp manufacturers must use something like the 555 timer idea because some of them generate incredible amounts of RF noise that is spread across much of the spectrum from 200 kHz up to 30 mHz (my observations).

I've never done it but you might use the 555 to generate a tone and one of the simple tone decoder like the LM567 (obsolete I think). The 555 and tone decoder would normally be tuned to same frequency - with a touch shifting the 555 so that the output state of the 567 changes to toggle the lamp.

 

[williB]

it is a simple dinky little project he wanted to do.. probably for halloween
nothing technical..

 

[DirtyLude]

The QPROX chips are made specificly for this. If you don't mind a packaged solution.

https://www.qprox.com/products/qtouch.php

They are available from Digikey, and they aren't expensive. The datasheets are pretty complete with example circuits. I have a few here, but I've never gotten around to actually putting one in a cercuit.

EDIT: Wow, they would make good halloween toys. Maybe I'll make something up for next halloween.

 

[williB]

The QPROX chips are made specificly for this. If you don't mind a packaged solution.

https://www.qprox.com/products/qtouch.php

They are available from Digikey, and they aren't expensive. The datasheets are pretty complete with example circuits. I have a few here, but I've never gotten around to actually putting one in a cercuit.

EDIT: Wow, they would make good halloween toys. Maybe I'll make something up for next halloween.

perfect .... nice !!..

 

[zachtheterrible]

hows this?