Flashing an LED with radio signals?

An ongoing project I’ve been trying to build is a simple circuit that will flash an LED under only radio frequency (ie, radio stations) power.

I realize that having it lit continuously is out of the question, but would it not be possible to have it flash occasionally? The idea being to charge a high capacitance capacitor. Ie, since the voltages available are so small (millivolt range), we can spend a lot of time charging a high capacitance capacitor to give the power necessary to flash and LED.

What do you guys think? Is such a circuit workable?

The most obvious problem is that no matter how long you charge a cap for, it’s going to be charged to the same voltage as the input, unless you employ some type of amplifier. You need a minimum of 2 V to drive a typical red LED.

Okay, so we have our LED requiring 2v and say 10mA. That’s 0.02 watts, or 0.02 joules / second.

So we need to store 0.02 j in our capacitor.

E = 1/2cv^2

So we have 0.02 = 1/2c(0.02)^2

We solve for c = 100 F

A huge amount, but this process seems valid. Sure, you can only ever get the same voltage across a cap, but the idea is we can store enough charge on it over time and then discharge it all at once giving more power.

Of course, if you can see any other way of doing, that’d be great too.

You’re missing the point, I think. Sure, you can store enough energy to light an LED over time, the problem is getting the voltage to the level the LED requires to turn on. You’ll either need a circuit to charge the cap to the required voltage, or one to take the charge ont he cap and step the voltage up enough to drive the LED. If you can do either of these things, it can be done. But it would make more sense to employ a broadband RF amp to drive the LED directly, and forget about fussing about with capacitors.

Well, we certainly don’t need to use capacitors. If a broadband RF amp can do the trick then that’s just fine.

Of course, keep in mind, I want no external source of power.

If by external, you mean a battery, then I think you’re hosed unless the RF signal is VERY strong. I just don’t see passive components doing the trick.

That is indeed what I meant. No battery.

Okay, so I guess my experiment it toast then. Dammit. Using a battery defeats the purpose as I may as well just hook the LED right up the battery in the first place :wink:

Cell phone blinky antennas do what you want them to, but that’s only because they are right at the RF source and the voltage is high enough to drive the LEDs. But if you want a weak RF sensor, you really need a battery in the circuit.

I know that Nikola Tesla was working on something similar (wireless electrical transmission?)

I seem to recall it involving massive high frequency voltages and how successful he was depends on who you talk to.

Ah, they’re…
oh, never mind!

A “crystal radio” which flashes an LED?

A couple of science museums have had these things as exhibits. Their LEDs flash about once per minute or less. I don’t recall what they used in their schematics, but their antenna probably powered a unijunction oscillator or one of those two-transistor NPN/PNP flasher oscillators popular with the solar-powered toy hobbyists. Obviously the oscillator itself needs to draw almost no current during the “dead time” between blinks. The less energy used to power the oscillator, the more ends up in capacitor, and the faster the LED can flash.

I received mail from one guy in Chicago who used a sharply-tuned resonant antenna to run tiny motors with the RF energy intercepted from a powerful downtown radio station. The secret is to add a tuned circuit to a longwire antenna (see Two Simple Diagrams: nearfield coupling and tuned circuits). It also pays to be a few miles away from a major AM transmitter.

An AM radio antenna connected to a tuned circuit acts much like the “director” elements of a Yagi television antenna: it distorts the surrounding RF field to direct energy towards itself. It intercepts far more RF energy than its small size would suggest. Even so, the larger the antenna, the greater the received power. A loop antenna with a tuning capacitor is good, but a long wire antenna connected to a tuned circuit should be even better.

I don’t think you’d have problems getting above two volts, but if your resonator didn’t put out enough voltage, you could always wind some turns around the inductor to form the secondary of a step-up transformer.

Now if you wanted to do some major work, you could experiment with arrays of VHF diploes and try to intercept energy from several FM radio and TV stations simultaneously. Or build a dipole array, a sort of “solar panel” tuned to a single powerful station. Take over a few acres of land, and you might be able to flash your LED every few seconds! :slight_smile:
“Energy Sucking” Radio Antennas

I received mail from one guy in Chicago who used a sharply-tuned resonant antenna to run tiny motors with the RF energy intercepted from a powerful downtown radio station. The secret is to add a tuned circuit to a longwire antenna (see http://www.amasci.com/tesla/nearfld1.html). It also pays to be a few miles away from a major AM transmitter.

Around 15 years ago, I put together a radio-powered radio that was able to pluck enough RF power from the air to drive an amplifier and speaker. Just a random wire antenna - recall around 10 feet - and ran it through a voltage multiplier made of germanium diodes. Created enough juice to power (IIRC) an LM386 amp driving a tiny speaker. Tagged onto the same antenna was a basic crystal set to supply the audio.

Similar to the link above, I was near Chicago, and a couple miles from a strong AM transmitter. My difference was it was all random with no tuning.

I know next to nothing about the electronics involved, but here’s another example that would suggest your project is feasible.

Sometime in the 80s or early 90s Monitoring Times ran a do-it-yourself article of a crystal radio that powered itself and its own high-impedance speaker. The only other detail I remember is that the design used one or two ganged tuning capacitors from old-style radios, and it was these that made the power available. Another effect of these was that the tuning was as sharp as any commercial radio. IIRC no amplifier of any kind was needed.

Earlier thread on this topic: Please help me find a circuit diagram for a radio-powered LED flasher

Doesn’t that pretty much say what I said?

Hmm, interesting. Too bad it deteriorated into a free energy joke thread; there were a few interesting ideas I’d like to have seen followed through (ie, the inductor idea).

You used an antenna AS THE BATTERY for an LM386?

This thread is about NO BATTERIES devices. If you use an amplifier, it must be powered by the antenna’s rectified output.

(People HAVE done such things; using one crystal set tuned to a powerful station to supply a couple volts to a one-transistor amplifier, which then amplifies the output of another crystal set tuned to a weak station.)

In theory this is not correct. There’s a workaround.

First, note that the “virtual area” of radio antennas is always different than the antennas’ physical cross-sectional area. An antenna made from thin wire works nearly as good as an antenna made from thick pipes. Antennas don’t act like solar cells. Making the antenna-wires thicker (wider) doesn’t let them intercept proportionally more RF energy. In other words, the radio waves always “think” that a thin wire dipole antenna looks like a big black disk which absorb the waves. The diameter of this “disk” is approximately the length of the antenna, and the width of the wire plays little part. Weird, huh? The shadow of a TV antenna is very different for radio waves than it is for light. The radio waves “think” that the wire is about 1/2 wavelength thick.

In antenna theory, the area of this “black disk” is called the EA, the “effective area” or “effective aperature” of the antenna.

Secondly, make note of three facts:

[li]The best simple antenna is 1/2 wavelength long (called a half-wave dipole.)[/li][li]Antennas for lower frequencies tend to be longer.[/li][li]Longer antennas have much larger Effective Areas. [/li][/list=1]
Suppose we’re trying to intercept power from a distant transmitter. A half-wave dipole for 1MHZ (AM band) will intercept 10,000 times more power than a 1/2-wave dipole for 100MHz (FM band). The AM antenna is 100x longer than the FM antenna, and hence has 100^2 times greater Effective Area (its absorbtion area, the “black disk,” grows as the square of antenna length, so if we double the antenna length, we get 4x larger EA and intercept 4X more incoming power.) If we use only a single antenna wire and not a more complicated antenna-array, then clearly the lower frequencies are much better as energy sources.

But isn’t this intuitively obvious? A 300-ft AM wire antenna should grab far more energy than a 3ft telescoping FM whip antenna. Yes, but the longer antenna doesn’t just grab 100 times more energy. Instead it starts sucking in waves over a larger area, so it pulls in ten thousand times more energy.

Lower the frequency, lengthen your antenna, and the received power increases much faster than you might expect.

Some years back, I knew some Citizen’s Band Radio geeks. It was cool for awhile, among them, to clamp a small fluorescent tube near the top of their long, whippy antennae. When the driver was transmitting, the bulb would light up, pulsing with the speaker’s voice. At a distance, it looked like a strangely throbbing entity following a pickup truck at a range of 2 feet. The geeks were severely amused. Soon, each one of these oddly pulsing lights died in a cloud of shattered glass, as the antenna voooped forward in a sudden stop. If the driver was talking at the time, it was a cloud of glowing glass.

Here we go: