On 5 February 2013 09:17, Bob Dunlop <bob.dunlop@???> 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.
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