On 5 February 2013 09:17, Bob Dunlop <[email protected]> wrote: > Hi, > > On Mon, Feb 04 at 11:14, Tim Brocklehurst wrote: >> On Monday 04 Feb 2013 21:49:18 James Courtier-Dutton wrote: >> > >> > I have found lots of web site detailing how to drive a relay from a >> > Raspberry PI, for example, turning 240V AC mains devices on and off. >> > What I cannot find is how to have the PI detect if 240V is on a wire >> > or not. I.e. If a 240 AC wire is powered or not? >> >> > Done anyone know of any sort of "detect 240V AC" adapter for the GPIO >> > of the Raspberry PI? >> >> I don't know of any pre-made boards/kits, so it's probably a DIY job. > > Don't think you'll find a pre-made board. The risk to the supplier of > DIYers plugging things together wrong outweighs the profit on something > with so few components costing pennies. > > Actually there are a few boards out there but the suppliers are very > careful to label them for low voltage operation only. Working out > which are genuine low voltage and which might be subverted for high > voltage use is not something I'd like to go into. > > >> There are (at least) two ways of doing this. You can either sense voltage, in >> which case you need some form of rectifier, potential divider and buffer >> circuit >> (possibly an optocoupler). There is some info here: > > Not "possibly an optocoupler", isolation is a must not an option. It might > take the form of optoisolation, capacitive coupling or a good old fashioned > isolating transformer. > > A current detector is kinda an extreme case of an isolating transformer, > with a caviat. It will tell you when the target device is drawing current > (and probably need a large current to activate), not when potential volts > are being provided. This might actually be what you want in the case of > monitoring a heating systems. > > Of the three voltage detectors an isolating transformer (not an "auto" > transformer) converting the mains potential to a low AC voltage is > conceptually the simplest, and probably the only one I'd suggest for > home experimentation. > > Capacitive isolation is specialist and tricky, not one to experiment with. > > The opto isolator like the transformer is conceptually simple. The > circuit at [1] is good and would be safe for 110V or 230V operation. > I'd drop the output transistor as not required to drive a RPi input. > However the size of components, normally surface mount these days, makes > if difficult to build a safe unit. Mains in and low volts out with only > a few mm clearance is not for the regular home constructor. > > [1] > http://electronics.stackexchange.com/questions/50782/ac-detection-for-microcontroller >
Fortunately I am not a beginner when it comes to electronics. So I am experienced mixing 240V and 5V on the same board. I have a component I need to detect reliably. It outputs about 100V AC when "off" and proper 230V AC when "on". It is a Honeywell V4073A, if you are interested. This 100V AC when "off" is causing problems with the heating going on at the wrong time. So, I was thinking of making a detector for the on/off, use a raspberry PI to detect the signal and then drive a relay to control the boiler itself. (Just sending the boilder the "call for heat" signal, not touching the boiler internal circuit board because that is way too risky) I then get my "logging" function at the same time as fixing the boiler problem. I can then write a small android app to remote control my boiler. -- Please post to: [email protected] Web Interface: https://mailman.lug.org.uk/mailman/listinfo/hampshire LUG URL: http://www.hantslug.org.uk --------------------------------------------------------------
