Author Options:

Simple Circuit Question LED light/ Metal contact Answered

I have two pieces of metal, pretty heavy stuff, probably stainless steel, and was toying of the idea of being able to wire them up so that if they were touched together it would light an LED.
See attached illustration for a general idea of what I am talking about.

I am not sure if this is even possible.
I think I may need to have some pad-style contacts wired to the very edges, so I'm limiting the surface area that is making contact... otherwise it seems like it may need an awful lot of voltage/current to get through all that metal, probably drain batteries fast too...?
Another thought is if the metal were left like it is in the diagram, separated, and if you could bridge the gap with a finger and have it light, that would also be very cool.. just don't know if it's possible.



For the metal, yes, the "type" of metal makes quite a difference as to just how much resistance is there. Normally, the thickness doesn't play a part in voltage however (thicker is for higher Amperages), as most voltage travels along the surface of any conductor.
As for "bridging the gap" with your finger, the circuit would only light "while" there is a connection. If you wanted it to "stay on", you'd need a latching relay or some other circuit to continue the contact. To turn such a think off, you'd need a manual break, or a flip flop, so that a second touch would deactivate the circuit.

I posted this below for "catilinsdad", but I did a test last night to test out how this would work. I was able to get the LED to light with 2 double A batteries and a 3v LED, by connecting the metal parts. I did notice that getting a larger surface area of my hand on each piece of metal DID light the LED to some degree, but may need a bit more current if i were going to use it that way (human bridging the circuit) check it out:

Thanks for your input!

Looking at your image, well it could work. It would depend on the voltage of the battery and the type of LED (low current or not)

You will NOT have to worry about the resistance of the metal parts. Comared to the resistance of your skin (even if wet), the resitance of any metall part, even if long and thin will be negligible.

So if the voltage of the battery divided by the resistance of the skin will yield a sufficient current (remember: I=U/R) the LED might light up. But take care not to electrocute yourself ;-)

But any way, the resistance of the skin will change wildly with the amount of pressure, area of contact and wetness of the skin - and so will the brightness of the LED. And - should the metal parts for any reason contact directly (R almost 0), well, goodbye LED!

If you really need to use the metal parts as a touch contact, use something like the darlington stage shown here:



Sorry, this is in German, but maybe the Google translator can help you, or search for 'darlington stage touch contact' or similar. I'm using a similar circuit for decades now with an 9V battery (changed it once or twice, though).

So, yes it is doable, just not s simple as in your sketch.

Look up capicitance switches or how some touchpads work. There are some lamps that you touch the metal surface and it turns on. That in turn activates a relay circuit to switch on something like a light.

I looked into the capicitance switches, and they do seem like they could be used for this application.

However, it seems like with the basics of electronics / circuitry, that the drawing (in theory ) is sound, without the need for an additional type of control or switch. Maybe a resistor, but i wouldn't think much more.

The metal pieces I'm working with would BE the switch, so to speak... no?
i.e., i want the light on, i push one piece of metal into the other... it goes on.
Separate the metal pieces... light off.

Mainly wondering if i have a 3v LED and a couple double AA batteries wired in there, if they would just die because the current would be sucked into the metal parts while NOT on... or if they have enough current to flow through the metal and light the LED when the two parts connect.

I guess there's the whole "try it and see what happens" .... ;)
I will play with this tonight and see what happens... (or doesn't)


You would need to take a voltmeter to see what the actual resistance is of your skin, wet skin makes a better conductor. Depending on how big the steel is, it will also contribute to the resistance and thus you might get a dim led to light up. See what happens. Look up LED calculator to see what kind of resistance is needed to keep your LED from burning out and go from there.

ok, i KNOW it isn't pretty... was just a quick test to see if the theory holds true.
Now that i know, i will clean up the oxidized areas of the metal and wire it up properly. get the proper battery enclosure.. maybe add a bit more power/light etc...

I also noticed (but was hard to capture without an extra hand) that if i put my hands on either metal piece, the LED would light up a small bit depending on how damp my hands were/how hard i pressed.. I imagine if I used an LED that could handle a bit more power, and used more batteries that would be even brighter...
It's kinda neat that a person completes the circuit and lights the lamp.

I posted video here: