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We've all heard of crystal radios that essentially draw their power from the radio waves which induces a tiny current in order to generate the sound we hear from them.

There are companies that are working on device chargers that magically pull power from the radio waves and promise to charge our devices wirelessly, 24 hours a day so we might never need to plug them in again... I find that unlikely, but I live in hope.

What I'm trying to figure out is this:

Is it possible to harness enough power from radio waves to drive a simple LCD such as to display the current time and date... and perhaps the weather? These are all signals broadcast over the air almost continuously. It could be an e-ink device for all I care to save more power.

Is there a device out there that already does this that I'm not aware of?

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"Energy harvesting" from RF and other very low power sources (like motion and light) is becoming a big field in electronics and there are lots of parts coming onto the market that make this easier.

Here is an article specifically on harvesting power from ambient radio waves...

http://www.mouser.com/rf_energy_harvesting/

The amount of harvestable radio energy is widely variable and depends on factors such as how strong the transmitter is, how far away the transmitter is, what the frequency of the waves are, and what kind of antennas you the receiver and transmitter have. When I was a kid, a ham radio operator showed me a trick where he lit up a florescent light bulb from across the room using the power from his transmitter, which means that several watts worth of power made it to the bulb. On the flip side, modern radio receivers can be sensitive down to nanawatts (1/1000000000 or a watt). A very wide power range.

LCDs would be a good choice for an application because it is possible to drive them with very little power. That is why they are used in watches which run for years on a tiny battery. It is not uncommon to find small LCDs that can maintain an image with less than 10 microwatts of power. A microwatt is 1/1000000 of a watt!

Considering the tiny amounts of power you are likely to have available, you would probably have to make trade offs between the functionality of the device and how much power it took to support that functionality. One common way to reduce power requirements is to reduce the time the device stays on, so something that it only on for 1 second per day has the other 86399 seconds each day to collect the power it needs for the brief moment when it is awake. Of course, a device that is only on for 1 second per day might not be very useful.

Long story short- it is theoretically possible to create device that is powered only by radio waves, but it would take a lot of careful design and engineering to make one that is useful.

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  • \$\begingroup\$ But you could for example harvest enough to power an LCD displaying the time and at a specific time flick a switching circuit that powered up an amp to play a radio station? An alarm clock that draws no power from the mains except when it's playing audio to wake them up. My ultimate goal I think isn't to make it entirely powered... but that on standby, it draws zero power from the mains. I think there's a lot of value to this. By this thought, devices that are on standby needn't draw "phantom" power. \$\endgroup\$ – BenAlabaster Mar 2 '15 at 20:34
  • \$\begingroup\$ Again, it depends on a lot of factors. If you live down the street from the WLW tower or under some high tension power lines, then you could likely power the whole radio from the ambient RF power available. In a normal suburban home you could probably harvest enough power to run a simple timer and then turn on an (electronic) switch at an appointed time, but probably much, much easier to use mains power that is already there! \$\endgroup\$ – bigjosh Mar 2 '15 at 20:39
  • \$\begingroup\$ The idea is to remove the power draw from the clock when it's not in use. If you live off grid, the power draw on your batteries is unnecessary. Turning it off entirely but having it still function is desirable. But you don't want to unplug it when you don't need it only to have to plug it in and reprogram it every night when you go to bed, does that make sense? A wind up clock requires maintenance... winding. It would be nice to have a modern alarm clock that was more efficient when "off" \$\endgroup\$ – BenAlabaster Mar 2 '15 at 20:44
  • \$\begingroup\$ It would be much, much easier to design a very, very low power timer that ran off the mains. It would not be hard to design one that used so little power (microamps) that you would not care. Such a timer could run for decades on the power in a single AAA battery (assuming the battery could physically last that long). \$\endgroup\$ – bigjosh Mar 2 '15 at 20:49
  • \$\begingroup\$ I think there could be a great market for a clock radio that functioned this way because the two we have draw almost very nearly as much power when they're not being used as when they are. Electronics that are designed to be "off" whilst in standby would be a big market for off-gridders. \$\endgroup\$ – BenAlabaster Mar 2 '15 at 20:54

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