Cheap Point Of Use Electric Power Monitor

The idea is to build a very low cost energy monitoring system. Price point for each sensor is less than $5, making it possible to economically deploy lots of them.


Sensors will be located at the point of use- typically either at an electrical outlet or light socket. Both types will be small and pass-through and easily user installable. Target price point for a sensor is single digit dollars.



Collector will collect data from all the sensors and relay to a PC or network connection. Only one controller per site it needed. Price point for collector is $10-$50 depending on functionality (cheapest being a simple USB gateway into an existing PC).


Communication between sensors and controller is over the existing power lines using a variant of the X-10 protocol.


Each sensor has a unique serial number and is transmit-only and is connected directly to the AC lines. It uses the same connection for internal power, for communications, and for monitoring.


Each sensor has three blocks- (1) power supply which provides operating power for the sensor itself, (2) power measurement circuit which measures the power being used by the connected load, and (3) communications circuit which lets the sensor send its measurements out over the connected power line.


These three blocks would be implemented using a single low-end PIC microcontroller and a handful of discrete components.


Notes on the implementation the power supply block here…


Notes on the implementation of the communications block here (only send stage needed for this application)…


Notes on the implementation of the AC power measurement block here (very simple and cheap, but relatively unknown technique)…


All of these technologies are old enough to be unencumbered by patents now.


There is some design work to integrate these blocks into this application, but it is straightforward. The sensors also need embedded software to make the measurements and send the results, but again this is mostly cut-and-paste work.


The collector would need only an X-10 communications receiver. The simplest one is just a gateway to an attached PC. A PC or a web application will present and publish the collected data. You can buy off-the-shelf X10 gateways for less than $20…


Higher-end collector models could have an embedded web server. Again, there are off-the-shelf products that would work here.


I think that’s it. No really hard parts. Very easy to put together working prototypes of everything.  Innovation comes mostly in putting the blocks together efficiently so you can hit the price point, and making the UI software compelling. Some challenges could also come in getting regulatory approval (UL) and scaling manufacturing, but they are well down the road.

 Let me know if you have any questions or if you want some samples of what the schematics might look like.

I’d imagined that the initial products would look something like these…



(pardon my horrible freehand mouse drawing).


First is a plug adapter. You’d plug this into the wall and then plug a load (desk lamp, computer, air conditioner) into it and it would watch the power flowing through it. Second is a screw in adapter for light fixtures. Again, you screw this into a light socket, then screw the light bulb into it and it watches the power flowing though. Both contain the PIC (a tiny one) and all the other necessary parts to measure the power and send the data over the power lines. They are molded plastic, hopefully much nicer looking than my horrible freehand mouse drawings.


11/2/2011: Published

11/08/2011: Added drawings