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Lighting a LED with ambient RF (was candle)

Discussion in 'Electronic Design' started by Billyb97113, Oct 2, 2012.

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  1. Billyb97113

    Billyb97113 Guest

    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
  2. Guest

    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.
  3. Guest

    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

    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.
  4. miso

    miso Guest

    I vaguely remember something about fluorescent tubes glowing near TV
    transmitters. Maybe Sutro tower.
  5. whit3rd

    whit3rd Guest

    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.
  6. Guest

    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.
  7. Martin Brown

    Martin Brown Guest

    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.

    A simple variant also serves as a thunderstorm alarm.
  8. Guest

    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.
  9. Guest

    apparently you can get ics for it,
    though power levels and distances a quite far from you usual AM

  10. amdx

    amdx Guest

  11. amdx

    amdx Guest

    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.
  12. Guest

    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 :).
  13. Martin Brown

    Martin Brown Guest

    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.
  14. Guest

    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.
  15. MrTallyman

    MrTallyman Guest

    AM radios receive femtowatts.

  16. We had that with a hydraulic servovalve!

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

    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.
  18. Guest

    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.
  19. WoolyBully

    WoolyBully Guest

    There is no "p", idiot.
  20. rickman

    rickman Guest

    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...


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