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Is there any way to light a LED using only one wire? I mean, can you use a capacitor or something similar on the other pin of the led to "close" the circuit. Keep in mind that I need to light the LED for a short period (~100ms) and every 1-2 sec.

I found something similar for a fluorescent bulb here.

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You can light an LED using zero wires providing you generate a magnetic field that can be picked up by the LED's associated coil. This is basically how an electric toothbrush gets charged when put into its holder.

You can make it work at several cm too if you resonate the coils and use hundreds of kHz. Then use a bridge rectifier, smooting capacitor and maybe a low power 555 timer to make a low duty-cycle oscillator and bingo you have a solution.

To make this work with one wire takes no extra brain power.

I've used this technique to transfer many wats across a few inches to power a circuit that was rotating and therefore couldn't use wires directly.

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  • \$\begingroup\$ "...to power a circuit that was rotating..." very interesting! Did you measure/estimate the efficiency of the system? Care to provide some references or links on the subject? TIA! \$\endgroup\$ – Lorenzo Donati Aug 1 '15 at 21:07
  • \$\begingroup\$ @lorenzo send me an email - best done this way if u r interested. \$\endgroup\$ – Andy aka Aug 1 '15 at 22:12
  • \$\begingroup\$ Interesting approach. The problem is that I want to fit the led inside a small ball (4cm diam), and I think that the coil would not fit (or it will be to small for powering the led). \$\endgroup\$ – yiannisk Aug 2 '15 at 7:44
  • \$\begingroup\$ It's all about distance. \$\endgroup\$ – Andy aka Aug 2 '15 at 8:42
  • \$\begingroup\$ I agree and I can't be to close to the led-coil. \$\endgroup\$ – yiannisk Aug 2 '15 at 9:05
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Sending the power over a single wire requires you to transmit it via an alternating current and have a credible counterpoise at the other end. If you go to a high enough frequency you could use half of your ball covered in mesh capacitively coupled to the atmosphere and then use HF recitifiers to extract power from the RF signal you radiate down the wire. It may be quite possible but would also radiate a lot of RF into the vicinity that may interfere with other parts of your project.

Unless there was some other compelling reason I would use a co-ax cable thin enough to look like a single wire or perhaps a mixed core multistrand wire such as used in typical eadbud headphones. They have two, three or four isolated groups of wire strands (often coloured by group) to take the signals from a cell phone or MP3/4 player to the earphones and mic and possibly mic switch.

If you just need to do this once and for a single demo then the battery solution proposed could work, a IR receiver as used on cheap indoor RC helicopter may be flexible enough to work, radio is also simple if you select a small module.

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