# What are some alternative power sources for wireless sensors?

I'm looking to power several small, Wi-Fi enabled sensors, in a domestic home or office environment. As such I'm interested in keeping them powered as long as possible between charges.

Obviously an expensive Li-xx battery would be simple solution, but I've also been looking for more 'inspired' alternatives, such as Micropelt thermogenerators.

What other alternatives are there that would provide a decent amount of power, in a small size?

Alternative wireless networking ideas are welcome, but I'm keen to use WiFi - most homes have a WiFi network and enabled computer - I'd like to keep the extra equipment needed to an absolute minimum.

[There is a related question, about energy storage. My question is about generation and my needs are fairly specific (size, high power etc.]

• How much power are you looking for? That's really going to be a major factor in selecting a source. Consider that some systems, like the TI CC430 (note: wireless SOC, but not Wi-Fi), can operate on 1uA or less. With currents like that, a coin cell will power it for years. – Kevin Vermeer Sep 30 '10 at 22:06
• +1 for using WiFi. Most people try and build an exotic network without thinking about what the End User is actually wanting to use. – Gustavo Litovsky Dec 27 '12 at 18:16

You should specify what sort of environment you're talking about. Most of the methods I've heard of also use batteries to store the energy, but you could also use an array of supercapacitors. However, that's getting expensive as well.

Ideas I've seen used successfully to some degree:

Solar is the first one, but it does require light. Any light will do of course, even fluorescents. Will it keep your circuit powered for a significant amount of time? Depends on where it's put.

Vibration/Piezoelectric generation has been used successfully to power sensors placed under busy stairs. The key is busy stairs - imagine the New York subway or the main stairs at a university between classes.

EM capturing - If there's a lot of ambient EM then you can put out an antenna, rectify the results and regulate it - boom, power. However, it's usually illegal to do this with significant sources. For example, you can power a light bulb if you're with a quarter mile of a large radio antenna, but harvesting that power is illegal in most countries. It WILL be noticed too.

As Endolith said, you'll probably want a less power-hungry module. Consider Zigbee instead of WiFi - it was designed to be used in low-power devices that transmit for short bursts then stop.

[EDIT] Ok, office/home/domestic is the application. If you control it significantly, maybe you can alter it. Say by adding inductive chargers? I don't think there's that many source of power in a standard office other than solar.

• A 60 W bulb would really be noticed? What circuit do you use to harvest the energy from the radio tower? – endolith Sep 30 '10 at 15:13
• It's not the power YOU get from it, but the power you block going to everyone else that gets noticed. It's not the most efficient way of receiving power, so there's a noticeable loss in power in a very specific direction. The closer you are to the tower, the larger the area of lower power, the easier it is to find you. I found an article about it a while ago, I'll see if I can find it again. – AngryEE Sep 30 '10 at 16:16
• Added networking and environment clarification to my OP. – Jon Hadley Sep 30 '10 at 19:37
• electronics.stackexchange.com/questions/4664/… – endolith Sep 30 '10 at 20:56
• What about harvesting power from the 60Hz AC in the walls? I don't think you'd take enough to be noticed there... interactivearchitecture.org/richard-box.html – Kevin Vermeer Sep 30 '10 at 22:17

Rachel's Electronics has been posting about a solar-powered Arduino solution for remote sensors lately.

Not sure if that provides enough power for Wi-Fi, though. I'd look at less power-hungry wireless if I were you. Are you really sending 10 Mbit/s? Or do you just need a few kbit per hour?

• Added networking and environment clarification to my OP. – Jon Hadley Sep 30 '10 at 19:38
• Also, only being picky because this site is young, but is the picture really needed? It doesn't add anything useful... Useful link though :) – Jon Hadley Sep 30 '10 at 19:40
• @Jon: It does. I'm learning and seeing things is invaluable. – Stefano Borini Sep 30 '10 at 19:56
• It shows the size of the solar cell and PCB (which was part of the question), the number of components, etc. – endolith Sep 30 '10 at 20:17
• Is there a reasonably cheap USB dongle to hold an XBee? The XBee itself was like $20 if I remember correctly, and the PCB and USB microcontroller should be$5 ish? I recall having a hard time figuring out how to get wifi for under \$50 (other than used or refurbished parts). XBee might be cheaper, lower cost, and just as easy for the end-user (once the USB-xbee is included)? I have a project with wifi needs, but I have settled for using XBees to transmit to a master node with real internet (probably wired). I am a beginner hobbyist, so I might be missing something important. :) – Jack Schmidt Oct 1 '10 at 1:20

How often/how much data do you need to send? If it's fairly intermittent (on the order of a minute or so), some AA batteries could last you a very long time.

For sensor applications, the most important thing to optimize is the sleep-mode power consumption. You can wake up and send data using several hundred mA (seemingly typical for a 802.11* module) for a few seconds, but if you can then sleep for a few minutes using less than 1 mA, plain ol' alkalines will work just fine. NiMH might be an alternative, but in an optimized wireless sensor design their internal discharge rate will limit their life more than capacity.

If you use some complex energy-harvesting thing, you will need to spend a great deal on supercaps, as using a solar cell, peltier junction, or RF harvester will give you nowhere near enough power. Batteries can happily deliver a watt each with no added cost.

A much more tried-and-true method for RF sensors all about is using, as mentioned before, ZigBee (well, 802.15.4--very few people use true ZigBee). They will run for years with a very modest battery, and you can easily bridge them to an 802.11 network with some Digi ConnectPort X2's. 802.15.4 modules can wake up and send data in milliseconds using several times less power with a much greater range.

• Some useful stuff in there thanks, hadn't heard of bridging. – Jon Hadley Oct 2 '10 at 17:55

There is the WISP http://wisp.wikispaces.com/ They use a large dipole and a cascaded rectifier to generate voltage.

For completeness, I thought I would quote the answer given in a comment below:

That is a full wave voltage doubler. It won't work on the charges in the sky because there is no zero crossing voltage. However this circuit can pick up the radio station's energy. Of course the radio stations spends thousands of millions dollars to transmit signals at very high power, but the energy density at the far end will be very low. On the other hand, intercepting this kind of energy could be illegal because the circuit is making a boardcasting shadow behind it. Newton's law of physics-- energy is conserved. Sor far the only free energy is the primary solar energy.

Here's an example circuit I found on a site previously registered as teslasecret.com that appears to have lapsed some time ago but the main page is still available on the Internet Archive.

You need:

• 4 Germanium diodes (1N34)
• 2 100F 50v electrolytic capacitors
• 0.2F 50v ceramic capaciters

Here is the electrical diagram (hopefully it makes sense):

And they powered a cell phone with it. I am not sure what kind of antenna to use. I ordered it and found that it seems quite simple.

• That website simply screams scam. – tyblu Dec 23 '10 at 18:41
• Site registered Dec 7, 2010 by "Henry Reinders" of Williams Lake, BC. Phone number (+1.2503982280) is disconnected – Nick T Dec 23 '10 at 21:00
• I haven't had such a good laugh in a while. @tyblu: 100% Scam, 100% Spam. Also, I love how the sun is showering us with only 5 megawatt-hours of energy.... It's all a ploy to sell their book. I'll bet someone a dollar that Arlen is a member of said company or of their scamming referral network. – W5VO Dec 23 '10 at 21:11
• Site is a free-energy scam. – Connor Wolf Dec 24 '10 at 4:00
• They have spent a lot of time making an explanation that seems believable but is not actually valid if you have the experience with Electromagnetics to judge it thoroughly. – Connor Wolf Dec 28 '10 at 6:45