One of the challenges in this year’s Hackaday Prize is Power Harvesting where we’re asking everybody to create something that harvests energy from something. It could be solar, it could be harvesting energy from a falling weight. If you’d like to give a TED talk, it could be harvesting energy from sound waves. It could be harvesting energy from ambient RF, and where’s the best place to harvest ambient RF? That’s right, next to a microwave.
[Jurist]’s entry for the Power Harvesting Challenge in this year’s Hackaday Prize is a simple device that mounts to the front door of a microwave. The design uses a simple PCB antenna to harvest energy, an LTC3108 DC/DC converter that was lying around in a junk drawer, and a bunch of passives to suck down some photons escaping from a microwave. The idea for this whole device is to use the harvested power to send off a message over Bluetooth (or whatever) when the microwave is done. Really, though, this falls right into the ‘because I can’ category of weird builds.
So, does this power harvesting PCB work? The initial tests were iffy because there was no trimming of the antenna and no tuning of the circuit. However, after [Jurist] connected the board to a voltmeter and cooked some beans, he was seeing an entire volt across the circuit. It’s a start, and the beginning of a truly ‘smart’ microwave add-on. Really, though, it’s just cool to see a circuit harvest power from a leaking Faraday cage.
That mesh on the window is there to minimize the amount of microwaves exciting the microwave so it might make this project a bit difficult.
Ok. Actually read the project page (I know I should have before I posted) and the author does know there is a protective device to prevent escape of microwaves. However, the author believes it to be a special glass coating, instead of the metal mesh that is used. Furthermore, according to the US FDA standards (can/will be different in other locales):
“A Federal standard (21 CFR 1030.10) limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface.”
(https://www.fda.gov/radiation-emittingproducts/resourcesforyouradiationemittingproducts/ucm252762.htm)
So, I would have to say that the 1 VDC measured is being generated from some other source or under a load the reading is much lower.
5 mW / cm^2 is no small amount of power (density)! Sunlight at high noon rates around 1000 W / m^2 (https://en.wikipedia.org/w/index.php?title=Solar_irradiance) or 100 mW / cm^2. That’s only a factor of 20.
I’d be surprised if microwaves are actually leaking this much, but who knows?
Anyway, cool / crazy idea. You could power some kind of microwave-oven notifier device or something…
The 5 mW leakage is over the lifetime of the device, not every use of the device.
It is not. “Watt is a unit of power” https://en.wikipedia.org/wiki/Watt and power is energy per time. What is meant here is that the power leakage must not exceed 5mW, e.g. due to wear.
[quote]A Federal standard (21 CFR 1030.10) limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface.[/quote]
You can also read that as it may never leek more then 5mW even if the the shielding degrades over it’s live time. Since 5mW is not an amount of energy but rather an energy level that is even more plausible.
5mW is 5mJ/sec. That is a rate of energy per unit time. So saying “5mW over the lifetime of the device” means that it can never leak more than 5mW/cm^2 @ 2 inches at any time, but it can be leaking 4.99mW/cm^2 @ 2in the entire time it is running and be within the regulations.
This is because the hinges wear with time, the door latch wears, so the door will begin to not fit as well. It is not a Faraday Cage. It is a box designed specifically to prevent too much RF at 2.4GHz from escaping. So you may notice the door and frame are painted metal. The length of overlap is made to be as exactly 1/4 wavelength at 2.4GHz as possible. But as the hinges and latches wear, the door slowly shifts downwards so the top and bottom are not at exactly 1/4 wavelength any more.
The significance of 1/4 wavelength is that a 1/4 wavelength transmission line inverts the impedance at the other end. So the open circuit at the outside edge of the closed door looks like a short circuit from the inside edge.
But only at 2.4GHz. Which is one of the reasons a cell phone will have a chance of working from a closed microwave oven.
Hi,
only now I noticed this hackaday article…. that’s awesome! =)
I have attached the device to my mW-oven, and I already noticed several times that leakage energy is high enough to light up the LED at some moments (even without energy harvesting IC!). I guess, that reflected power depends on what food you place on rotating plate, and at what angle/position.
However, that doesn’t contradict to “5mW leakage standart” – because onboard LED have 1kOhm current limiting resistor. even if there is 2V drop on LED+resistor, then it’s only 4mW – close to standarts ;)
Soon I’ll make some updates on project files.
I would have assumed that the EM field inside a microwave oven is perfectly confined as long as the holes of the metallic grid embedded in protection-glass are smaller than 1.22 centimeters (i.e. one order of magnitude smaller than the wavelength, Bethe approximation). I think I will have to read more recent literature, e.g.: https://arxiv.org/abs/0808.2873
A “perfect” shielding is quite difficult.
One order of magnitude smaller than the wavelength doesn’t mean perfect shielding, it just means you are reaching the point of vanishing returns. Sufficient shielding for the amount of power involved.
Faraday refuses to comment..
No spinning detected outside the confines of said grave.
how could you, it’s a Faraday “cage”
No shielding is perfect. There is a maximum amount of RF escaping every microwave oven, it is very small, but typically every microwave oven leaks a tiny bit.
Physics Girl actually tested it. What surprised me the most is that it didn’t fully block Wifi.
https://www.youtube.com/watch?v=ot4_jVFXxUU
Some RF devices are very sensitive too. For WiFi -74.5dBm doesn’t seem to be too out of the ordinary, and -92.5dBm is entirely possible. Those are pretty tiny amounts of power (-74.5dBm is 35.5pW, and -92.5dBm is 5.6pW), so perhaps it’s less surprising that it doesn’t take much leakage to get a usable signal.
I’ve always wanted to try and make a device that can be placed INSIDE the microwave and be powered by the microwave radiation. And survive of course. No good reason – just something I’d like to see if it is possible. But it’s just another thing on my list of projects I never get around to doing…
Try an eggMcMuffin.
Sulfur vapor lamp.
If you need mechanical actuators on that device, consider popcorn: https://hackaday.com/2018/07/11/hold-the-salt-and-butter-this-popcorn-is-for-a-robot/
I actually purchased a circuit board with a built in IC that was engineered to harvest microwave energy. Spoke with the developer about how cool his IC was and how it might be fun for kids if it lit up or spun and made noises while the microwave was going. I think he took my advice.