From: grarpamp <grarpamp@gmail.com>

Free power of course... https://science.slashdot.org/story/16/07/14/2049246/cleanspace-co-sensor-run...

A few years ago, a Kickstarter was set up to develop a locator tag powered by free radio frequency (RF) energy harvested from the environment. This was called a scam here on Slashdot and was shut down before it was funded on Kickstarter. However, it now appears that the concept is not as far-fetched as some predicted. A UK company CleanSpace has developed a carbon monoxide (CO) sensor which is powered by free RF. A review of the product has been posted on YouTube. It uses Freevolt technology to keep a battery charged and the CO sensor running. Since they have several thousand of these devices collecting data, they do appear to work and it seems to be in the 'not a scam' department.

I don't think there's necessarily a contradiction here.  While there do exist CO sensors which needto keep the detector heated to 400 degrees C, and therefore benefit from being connected tothe AC supply, "electrochemical cells" which have extremely low power consumption are nowavailable. https://en.wikipedia.org/wiki/Carbon_monoxide_detector  The device needs merely to 'wake up' occasionally, maybe every minute or so, interrogate the voltage on the electrochemicalcell, and sound the alarm if the limit value is exceeded. The main concern is ensuring that the detector has enough power at the time it is necessaryto alarm to sound that alarm.  The ongoing (non-alarm) power consumption is probably much less than the self-discharge rate of a battery.   Modern alkaline-cells have a shelf-life of in excess of 5 years, andperhaps as much as 10 years, although Wikipedia doesn't state this specifically.https://en.wikipedia.org/wiki/Alkaline_battery   http://data.energizer.com/PDFs/non-rechargeable_FAQ.pdf In contrast, a tracking device must actively emit some sort of signal to alert some remote system toits current location, which takes power.  It must also somehow know where it is:  GPS receivers use much less power than they once did, but even that is far greater than that which a tiny battery couldbe expected to provide.   And the amount of power available from "free RF" is obviously quite variable:  If you're 0.5 kilometeraway from a 50Kw television transmitter, that's a lot of signal strength.  Compare that with out inthe boonies, 1 km away from a cell-phone tower is not a lot.  It would make more sense to designa CO detector with solar cells on the bottom, and have people mount it on an outside window, with a rechargeable battery to store enough to get through the night.                      Jim Bell