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Topic: Ambient radio wave energy (Read 8963 times)previous topic - next topic

MarkT

#15
Sep 30, 2013, 11:42 am

I am trying to do a demonstration in which energy from ambient radio waves (it doesn't matter which frequency) is used to charge up capacitors and blink a red 3 volt 1 mA led every 10 seconds or so. Is this feasible? If so how can I go about building it.

The problem is that if you do find a frequency with enough power, there's no easy way to harvest it without
retransmitting harmonics (which may be illegal, potentially interferes with emergency services, aircraft, etc)
The energy retransmitted might only be 0.01% of the incident energy, but if that was enough to harvest in
the first place the retransmission will have a good range.  Radio receivers only need something like
femtowatts to pick up a signal, to power an LED needs milliwatts (12 orders of magnitude more).

You could place a small SMT 50 ohm resistor at the output of a large dipole antenna and use a pair of
temperature sensors to detect any raise in temperature of the resistor compared to nearby - thus
demonstrating the arrival of RF energy (and a simple resistor doesn't generate harmonics).

Replacing the resistor with a full-wave rectifier of germanium or shottky diodes would allow a DC voltage
to be generated, but trying to extract much current would then inject odd-harmonics back in to the antenna.

With a 50ohm load 1mW of RF is only 0.6V peak-to-peak though...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

mauried

#16
Sep 30, 2013, 12:03 pm
This is difficult to do.
Whilst there is a lot of radio frequency energy being transmitted from multiple radio transmitters all around
its simply not possible to somehow add it all together and generate any power.
This is because its all AC and the phase relationships of all the transmissions are not related to each other in any way.
This simplest way and the most likley is the humble MF crystal set, consisting of a simple tuned circuit to isolate a single
transmission, a diode to rectify it , and a capacitor to charge.
But dont expect to get much energy, unless you live across the road from a 50+KW transmitter.
The longer the antenna the better, and long wire antennas work well with MF broadcasting stations.
The single largest source of radio noise is the Sun, but its totally useless as its non coherant so the energy is spread
across a wide bandwidth.

cjdelphi

#17
Sep 30, 2013, 06:09 pm

Do you know an energy harvesting ic that collects energy from radio waves ?
I can only find ics that use motion, or peltier, or solar, or nearby inductance, and so on.

An antenna and a diode, now depending on the size of the antenna you can absorb as much RF as possible, who's less likely to track you down for stealing microwatts of power to charge a cap?

AM probably a good source, a ferrite rod + copper coil ? you could even steal power from GSM charge it by placing it near a power supply or something...

But peltier, solar, inductance, they all create a potential voltage, so would an antenna and a radio diode, anything that produces a voltage would work.

http://www.ebay.com.au/itm/Energy-Harvester-Breakout-20-mV-Startup-/261273592646?pt=LH_DefaultDomain_0&hash=item3cd51ea346

They're getting more popular, i hope to see prices come down and then I could use a speaker and convert sound waves to energy, infact your body would generate enough heat to drive this device!

UserNotLoggedIn

#18
Oct 13, 2013, 10:01 pm
RFID works on this principle.
The "tag" is bombarded with a low freq / low level energy long enough to transmit its data a few inches to a nearby receiver.
Not something that can power any microcontroller.

polymorph

#19
Oct 14, 2013, 01:11 am
Thermoelectric work by heat difference, I think in Australia your body would be the heat sink for a large part of the year! Snork.

A big problem with diodes is the voltage drop. As pointed out, 1mW into 50 ohms is only 0.6V peak, enough to turn on a germanium or shottky, but thereby wasting half the power. And a bridge rectifier doubles that drop.

Resonance boosts voltage, but at the expense of bandwidth.

Actually, there are a few projects going on where researchers are trying to use ambient RF fields to power a microcontroller, but they are -very- low power and spend most of their time storing energy, only coming on for a short time a few times a day.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

polymorph

#20
Oct 14, 2013, 01:17 am
BTW, I love how that chip works, the one used in that eBay link. The LTC3108. Fascinating. Uses a chopper to turn the low voltage modest current output from a thermoelectric generator into AC and sends it through a transformer to boost the voltage. I've had my eye on it for a while for a project I'm doing. I'm wondering how it would handle low frequency AC.

http://www.linear.com/product/LTC3108

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

polymorph

#21
Oct 14, 2013, 01:22 am
Linear also has the LTC3109 which is auto polarity:

http://www.linear.com/product/LTC3109

Not meant to take AC, but I wonder how it'd react to it.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

MarkT

#22
Oct 14, 2013, 02:27 am

RFID works on this principle.
The "tag" is bombarded with a low freq / low level energy long enough to transmit its data a few inches to a nearby receiver.
Not something that can power any microcontroller.

I dispute that - RFID readers run quite high power typically, and tags are a small microcontroller in the general case.  I've played with one RFID reader chip and it handles
upto 200mA of coil drive current at upto 8V.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Grumpy_Mike

#23
Oct 14, 2013, 10:18 am
If you strip an RFID tag down and connect an LED across the coil it will light up when presented to the reader. We used to have an RFID tag in a transparent case that our CEO used to have to impress his clients, I have made several.

MarkT

#24
Oct 14, 2013, 01:23 pm

If you strip an RFID tag down and connect an LED across the coil it will light up when presented to the reader. We used to have an RFID tag in a transparent case that our CEO used to have to impress his clients, I have made several.

For pete's sake don't let the "(microwave oven owning) keep WiFi out of our schools" crowd
hear that!
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

polymorph

#25
Oct 14, 2013, 06:22 pm
I demonstrated some of the effects of resonance by having an untuned coil connected to a signal generator, an untuned coil connected to a couple of antiparallel LEDs, then I insert an LC tuned circuit.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8