pi to Read 7 - 240Volt signals?

[Ashnook13]

Hi, I want to monitor the activity of seven room stats controlling my underfloor heating zones. The signals are 240v (on or off).
What is the best interface hardware to use to interface these 7 signals into Pi acceptable signals. I think I will be using Python software to log (and display?) the results.

Many Thanks, Brian

[davidcoton]

You need some form of opto-isolator rated for 240V input. If you are not familiar with mains wiring, theory, practice, regulations and safety, don't even try. Mains voltage can break your Pi, burn your house, or injure or kill people (possibly but not necessarily you). If you must try, get a qualified electrician to check your work before connecting up.

[Ashnook13]

Hi, thanks for your reply, I was a qualified electrician (5 year apprenticeship) before retirement and installed all the plumbing and wiring in the house my wife and I built, so no problem there.
What I am looking for is a recommendation for the 240v to ... (3/5V) interface to the Pi. I am just trying to short circuit the process to see if anyone has a recommendation for some kit to do this before I start the research process or even design something myself.
I guess I am being lazy

Cheers, Brian

[davidcoton]

I guess I am being lazy

Maybe so. The problem is that while you know what you are doing, you may get a reply from someone who doesn't. So check carefully.
I don't specifically know of a suitable device for checking mains status with a Pi -- maybe I will find out when I need one myself!
EDIT: Google "240V opto isolator".
EDIT": See this

[DougieLawson]

I'd have thought a non-invasive current sensor would be easier to use than any kind of opto-isolator. That's the type of kit used by the current cost metering folks.

[Graymalk]

I'd have thought a non-invasive current sensor would be easier to use than any kind of opto-isolator. That's the type of kit used by the current cost metering folks.

Yes. That is a good idea.

Some quick googling turned up what might be a useful article:
http://archives.sensorsmag.com/articles/0799/26/

There are likely lots more out there as this technique predates the internet. There are lots of different sensors (or bits of sensors) you can buy, too.

[Ashnook13]

The only simple way I know of is 7 small 240V relays mounted on a proto board. This is what I might have to do, I was just trawling the use base to see if something like this already exists.

As I said, just being lazy....

Brian

[davidcoton]

My post above edited to include another link

[broe23]

I'd have thought a non-invasive current sensor would be easier to use than any kind of opto-isolator. That's the type of kit used by the current cost metering folks.

CT's (Current Transformer) is what is used for monitoring current flow. A whole lot easier than using the Pi to burn down your house.

[Ashnook13]

Interesting, thanks.

[BMS Doug]

The current through the room stat may well be very low, you might find it hard to detect it.

[broe23]

The current through the room stat may well be very low, you might find it hard to detect it.

For a 24 vAC, there is no need to monitor voltage. 240vAC you have to use Current Transformers to monitor.

[Ashnook13]

Yes, the stat just makes a connection in the UF heating controller to activate that circuit's valve so possibly very low current in fact, which is why I am going to look at a relay system.

[johnb_summers]

viewtopic.php?f=37&t=54024
read this 3 year old post

[johnb_summers]

I did this many years ago, you can use capacitors instead of resistors so no heat is generated, I use capacitors and zeners to bring the voltage down to 4.7 volts with no heat, if you use resistors on the mains side you will generate heat.