Some time ago I remember finding a web page with a simple circuit that would flash an LED powered solely by radio waves (like a crystal set, I suppose). I’d like to make one but I can’t find it now. Can anyone help?
Hmmm, it seems difficult to do but if you tell me it was done then I’ll take your word.
I guess you would just take an antenna, maybe increase the voltage with a coil, rectify, charge a capacitor and have a simple circuit to discharge the capacitor through the LED when voltage reaches a certain level.
Further searching seems to indicate that it either needs to be close to a fairly strong RF source (you can get little flashing LED stickers to go on mobile phones, powered entirely by the RF emissions) or it needs a whacking great antenna and a good earth.
Ho hum.
You can do it with two components, but it won’t flash.
Get a Schottky diode, and trim its leads so that it’s about 60 mm long and symmetrical (i.e., if the thing was 80 mm long to start with, trim 10 mm from each end).
Get an LED with a low forward voltage (i.e., not a high-intensity one) and trim its leads very short. Solder it across the Schottky diode, anode to cathode.
Glue or tape the two diodes to a plastic or wooden rod.
Hold the rod so that the two diodes are close to an operating microwave oven.
Have fun.
P.S. I initially thought as sailor did, but then thought what the heck I’ll try a few searches. You come up with some really strange stuff. Power your home with free energy from radio waves - send $15 for plans. Split the earth with Tesla’s secret technology - send $15 for plans. Far out.
Oh, you’re looking for unlimited, free energy? Why didn’t you say so? Here ya go: http://www.freelectricity.com/ 
Unless there has been some amazing new LED breakthrough that I haven’t heard of, I don’t see how an LED can be lit by RF energy simply because the power isn’t there to do it. Radio waves arrive at our homes in microwatts at best, while an LED requires at least 5 or 10 milliwatts to light up. No matter what kind of clever RF detector you come up with, where does the raw energy to light up the LED come from?
So you’d have to be standing in front of an unshielded magnatron as previously suggested.
A neon bulb will glow in the presence (within a few inches) of a high voltage field (5KV+) without any circuitry at all, just attach it to the end of a long stick and thrust it into the area you want to check for high voltage.
I believe that the circuit I saw didn’t try to light the LED constantly, but charged up a capacitor, then emitted a brief pulse every few seconds or so.
sailor: Nah, I’ve got something better than that. Just send me $15 and I’ll let you in on it.
Attrayant: All you need is what’s in sailor’s first post. Antenna, rectfier, charge pump, threshold discharger.
Weird. Is that a spoof site or do they actually expect to be taken seriously?
OK, if all you want is for it to blink every once in a while, then no problema.
In my opinion that free electricity site either wants to be taken seriously or is blatantly trying to defraud people.
Mangetout: I think the point is, they seriously expect you to send them $15. And some people do.
Attrayant: BTW “unshielded magnatron” <> “operating microwave oven”. Don’t be a weasel.
But…but…but they promise they will send the money back if you are not satisfied, so how…how…how could it be a scam?
I agree for the most part. It just doesn’t seem possible to power an LED from regular ‘ol RF energy. Unless, of course, you have an unusually strong signal.
Let’s say I use one of the new “ultra efficient” red LEDs (voltage drop = 2V, current = 2 mA). Let’s also say I want to power the LED for 50 milliseconds. This will require 0.0002 J of energy.
A 100 uF capacitor charged to 2 V would work, but you’d (obviously) need to convert the microvolt-level RF voltage to 2V. Assuming the small-signal resistance of the rectifier is around 1000 ohms, and taking into account all of the other inefficiencies (high source impedance on secondary of transformer, heat generated by boosting circuit, etc.), I’m not even sure if it would work. You might be able to get it to flash once an hour…
A better solution might be to store the energy in the magnetic field around an inductor. Once the energy exceeds 0.0002 J, the current through the inductor is abruptly interrupted, thus creating a large voltage. But… using a 100 uH inductor, you would need a whopping 2 amps through the inductor. Hmmm…
For only $20 I’ll tell you how you can get your $15 back.
Actually Dennis Lee has a long history behind him. Check out http://www.phact.org/e/dennis.html and Skeptic News
For more free electricity sites check out:
http://www.electricity4free.com/
http://www.teslaelectric.com/
http://www.ucsofa.com/Free%20Electricity.htm
This thread was going so well, and then you had to mention Dennis Lee…