Lighting a LED with ambient RF (was candle)

To quote John Larkins:
"More practical would be to get usable LED lighting from ambient RF"

I tried several different types of diodes and Ferrite rods and coils and
could never get a measurable voltage across a .01 uf cap. I used a 10X scope
probe as the only load. I would be happy if the LED occasionally blinked
using any of the inductive circuits found on the internet.

Any ideas?? -bill

 

Hello,
There are apparently a few working RF energy harvesters,
one by a Joe <Something> who in fact has a patent on it.
This basically uses a voltage doubler, and if I remember
correctly a 30 feet antenna. Another design was discussed
on this newsgroup, and this person was having some
grounding issues. Be warned though that the output is
very low, in the milliWatts.

 

Did you try to activate your cell phone close to your system ?

The capture area of a dipole is proportional to the square of the
wavelength. Thus, much more power is available at lower frequencies
with the same field strength (V/m).

Repeat your measurement close to a medium wave (AM) broadcast station
using a tall vertical antenna. Since your antenna most likely will be
much shorter than 1/4 wavelength, it will be very low impedance and
highly capacitively reactive, thus a series inductance is needed, in
order to get any power delivered to the load.

A LED at 2 V and 1 mA would be a 2 kohm load, while an electrically
short antenna would have a few ohms or even milliohms radiation
resistance, thus some impedance step up is needed.

A full size dipole would have 50-75 ohm impedance and might drive
nicely a back to back LEDs at 20-30 mA. At 100 MHz, this would require
about 2 V/m field strength, thus the system must be quite close to the
transmitter.

Any systems using ferrite rods on the MW band would be practically
useless, since the typical ferrite rod antenna gain is -30 to -50 dB
below the dipole.

 

I vaguely remember something about fluorescent tubes glowing near TV
transmitters. Maybe Sutro tower.

 

The key here, is that RF thermal noise isn't harvest-able (unless
your receiver is cryogenic). You need to identify a frequency at which
the RF is strong (depends on your area, of course) and then build the
equivalent of a crystal radio - a tuned antenna with a rectifier that
isn't too lossy. It should be possible to use a voltage-multiplier
and neon lamp in most urban environments, if you tune to a local AM
station. The neon lamp will blink after enough charge gets deposited.

If the neon lamp lights when you're holding it in your fingers, there's
a real transmitter nearby. If an incandescent lights when you put
a rabbit-ear TV antenna onto it, the transmitter is perhaps
right there in the room... I've seen that happen.

 

The effective area of a half wave dipole is about 0.12 square
wavelengths, thus at 1 MHz with 300 m wavelength, the capture area is
about 10000 m² or 1 ha.

The typical ferrite rod antenna has a gain of -30 to -60 dB below the
full size dipole, thus the effective area is somewhere between 10 m²
and 1 dm². Multiply this by the local power density [W/m²] and you get
a ballpark figure of the available power.

 

I don't see how you can make this work short of driving a micro Telsa
transformer and prayer. There just isn't enough total power except very
close to the transmitter or voltage available even in the tuned circuit
tank and as soon as you try to draw any current the Q falls rapidly.

You have to get to about 80v for a neon lamp to break down and glow. By
comparison getting 2-3v at 0.1uA or less for a white LED to just start
glowing should be a lot easier but still a bit of a challenge.

A high long wire antenna isolated from ground will quite often have
enough DC potential at very low current to light a neon screwdriver just
from the atmospheric potential gradient. Uselessly low current except
during thunderstorms when the LED would be at risk of damage.

My TV aerial is high enough up that on most winter days it will light a
neon screwdriver at ground level! A book value atmospheric gradient of
100V/m is typical but some days it is much more and at high enough
current to notice. If you try to measure it with an ordinary DVM rather
than an ultrahigh impedance electrometer you tend to get much less.

I would expect it to be able to provide enough current to light one of
the more sensitive LEDs provided it was protected from the overvoltage.
Never tried it though.

You can drive electrostatic toys from the potential difference across
two decent sized plates at different heights in the same way as with a
Zamboni pile like the perpetual ringing clock in Oxford.

http://en.wikipedia.org/wiki/Oxford_Electric_Bell

A simple variant also serves as a thunderstorm alarm.

 

The issue is power. ...and impedance matching.

An LED will also leak off charge without lighting (visibly).

Yet above you express a problem getting enough voltage to light a neon?

Power. Impedance matching.

An AM receiver works better. A weather radio, even better.

 

apparently you can get ics for it, http://www.powercastco.com/PDF/powercast-overview.pdf
though power levels and distances a quite far from you usual AM
station

-Lasse

 

That is not correct!
Here's a very active group.

http://theradioboard.com/rb/index.php


And here are some well researched papers, by Ben Tongue of
Blonder-Tongue fame.

http://www.bentongue.com/xtalset/xtalset.html

Here's Dave's site with about 80 crystal sets you can build.

http://makearadio.com/crystal/index.php


Mikek

 

The first LED flashlight I ever bought was like that. It had a faint
glow so you could find it in the dark.
It came in handy, I bought it at the Orlando Hamfest, on the way home
that night I had a flat tire, my kids had been playing with the light
and didn't know where it was. I turned off the lights in the van and
found it glowing under the rear seat.
The flashlight has slightly larger than the 9 volt battery that
powered it. It had a pretty developed surface mount circuit that
developed 3 different intensities for the LED.
Mikek

 

Sounds about right. That 100 µW/m² is in good agreement with old CCIR
(now ITU-R) field strength diagrams above average soil for a few
kilometers at 1 MHz.

With an outdoor antenna, just tune out the capacitively reactance with
some loading coil and use some 1:100 step up transformer and you might
get some usable LED currents.

I remember seeing some articles (long before the Internet) about a
transistorized tunable receiver powered by rectifying the signal from
a strong local broadcast station

For those not so familiar with the metric system 1 m² = 100 dm².

At the low end, the available power is only 1 µW.

Just googled around and found a measurements of a small (5 cm)
loopstick with -80 dBi gain, thus, the available power would be 100 nW
or 50 nA LED current. Perhaps an eye, well adapted to darkness for
half an hour, might be able to see something .

 

Your figures are over 3 decades out of date. The best modern white and
some green LEDs are just about visibly lit on the die in normal room
lighting at 1uA. When dark adapted you can drop that by a factor 100 or
even more. ISTR the voltage drop is nearer 2v though and only green or
white ones are worth trying since you need peak scotopic sensitivity.

Actual power requirement is about 2V * 0.01uA = 20nW in total darkness.

It is getting the 2v potential difference that is hard.

Selecting the brightest diode from a batch would be worthwhile...

Actually for a sensibly chosen modern high intensity LED suited to the
task it is more like an impedance match to 2V/0.01uA = 200M.

Except it that should be sqrt(2x10^8/75) = 1700 turns

With 100uV on the antenna this gives 0.17V still not enough on its own,
but a clever boost converter might be able to store enough energy on a
low leakage capacitor for the occasional flash.

If you take the average current drawn down to below 1nA then it looks to
me like you would be in the right ballpark for ordinary transistors.
The 5000t resonant coil will be hard to make though.

 

The resonant circuit impedance in a typical MW receiver is about 100
kOhms, while a LED circuit impedance is in the order of 100-1000 ohms.

 

AM radios receive femtowatts.

Sheesh!

 

We had that with a hydraulic servovalve!

"this is the BBC world service...."

 

To be precise 70 m tall (vertical polarization).

at what impedance level ?

The radiation resistance drops inversely proportionally to the square
of wavelength below 1/4 wavelengths, thus the matching network not
only needs to tune out the antenna capacitively reactance, but also
transform the very low (a few ohms or less) to the standard 50/75 ohm
impedance levels.

 

Definitively _NOT_

While the provincial US organization "IHF" tried to introduce the dBf
(femptowatt decibels above 1 W) in order to make some sense into
advertisement, those 10 dBf figures apply _only_ to receivers in the
100 MHz band with +/- 75 kHz FM deviation.

Due to the band noise around 1 MHz, those dBf figures are useless.

 

There is no "p", idiot.

 

In this whole conversation the only thing you chose to comment on is a
typo and you have to call the guy an idiot at that...

What??!!!

Rick