Powering distributed sensors from mains - Page 1
Forum section: electronics / projects

author: pjakobs
author_rank: Newbie
timestamp: February 27, 2016, 09:24:12 am
content: I'll soon be moving into a new house and that'll give me the opportunity to completely geek out on IoT stuff - heating, lights, sound, everything that can be controlled by electronics will be controlled by electronics.I guess at the heart of it will be a Raspberry Pi running FHEM, and there'll be a boatload of ESP8266s for sensor intelligence / communications. Now, that's all not too hard, and where there's something to switch (like LED strips), there's also 12V/5V/3.3V power available from a decent switching power supply. But what about those points where there is only mains? I don't want to rewire the house with 12V supply, the idea being to use WLAN so that I don't have to run wires. So let's think about a light switch: there is 230V right there, I want the actual switches connected to the 8266es inputs (and it's ground, of course) and have the whole thing run off a minimal power supply. I won't need more than 300-500mA on 3.3V.Any good ideas?I found the Supertex SR036/SR037 which provide a non isolated 3.3V/5V supply by only cutting through the very bottom of the mains voltage. The concept seems simple enough, but I don't feel too good about it, somehow.I know of course that there are 5V USB power supplies that can be mounted into the space for wall switches / sockets, but they tend to be consumer centered, so they'll have USB plug outputs and I'd have to open them up and rewire them to use them for internal stuff.I could take apart 5V wall warts, probably the cheapest way to go...Any suggestion is welcomeThankspj

author: Ian.M
author_rank: Super Contributor
timestamp: February 27, 2016, 10:00:55 am
content: Distributed sensors/controls would normally be designed to run on a much lower power budget. To comply with the EU ecodesign regulations, as of Dec. 2009, you have a 1W total consumption limit in standby mode if actively displaying information and only 0.5W in standby mode with no display or if off. So at 3.3V you'd have well under 150mA available for a compliant device, depending on the power supply efficiency.Also there is the issue that most European lighting circuits are run point to point through the ceiling with a two wire drop (Live & switched Live) to each switch, so any standby power used has to flow through the light itself, which tends to make LED or CF lamps strobe periodically due to the reservoir cap in their PSU or ballast charging up to the point where the control chip starts up and turns the light on till the charge in the reservoir cap is exhausted. When the light is on, there is no power available unless you add some sort of voltage drop in series with the switch you can tap power across.This leaves you with a couple of options - either a micropower RF mesh or powerline network of some sort (WiFi is just too power hungry) or to put your nodes in the ceiling where a normal mains supply + the switch drop and lighting load connections are all available. You could then run low voltage signalling over a single wire bus down each switch drop. However the switch wallplate modules would have to be designed to either tolerate 220V AC on their connections without hazard, or at least to fail safe, to remove the risk of fire due to any mistakes in tracing the light fitting to switch drop wiring.When you go into the details, you are probably better off pulling Cat6 everywhere and using POE, or non-Ethernet compatible DC power+ a proprietary bus as that also gets you out from under most of the electrical safety regulations you would otherwise have to comply with.

author: dwpatter53
author_rank: Contributor
timestamp: March 01, 2016, 10:35:27 pm
content: How about the home automation light switches that use z-wave or zigbee?There are z-wave shields for Rasperry PI;http://z-wave.us/razberry.html