switching UK phone extension

Hi,

I am looking at switching a UK phone extension.

Before anyone goes mad at interfering with the PSTN, that's not what I'm doing. I'm merely turning on and off the bell wire of an extension AFTER the master socket.

Currently, I do this with a 12v DC RA4-12WM-K and a wall wart power supply on a timer, but it's ocurred to me that given I only need to switch one wire I could probably do this better with just the switching power of an Arduino pin.

Does anyone have any experiences to add?

Otherwise, any thoughts on how I can power this 12v DC relay from the Arduino?

THANKS

IMO a simple lamp timer and wall wart like you have now is the simplest way to do this. Otherwise you're looking at an arduino and a real time clock.

Sorry – should have been clearer. This is part of a wider Arduino system. Otherwise you’re quite right that there’s no advantage to using an Arduino. :wink:
I have posted about it in project guidance, but I was specifically wanting feedback about the relay.
Maybe if I use a 5v 4 pole relay instead of a 12v one I will encounter less difficulty?

Thanks

DANE:
Sorry – should have been clearer. This is part of a wider Arduino system. Otherwise you’re quite right that there’s no advantage to using an Arduino. :wink:
I have posted about it in project guidance, but I was specifically wanting feedback about the relay.
Maybe if I use a 5v 4 pole relay instead of a 12v one I will encounter less difficulty?

Thanks

yes use a relay with a 5 volt coil. Wouldn't hurt to put a resistor on it to limit current draw.

Bear in mind that the bell line from the master socket is hardly used these days. Most phones contain their own bell capacitor. If the phone line has ADSL broadband using microfilters, each microfilter contains a capacitor feeding the bell line to the phone(s) plugged in to it. In fact, disconnecting the bell line at the master socket is a standard technique for improving ADSL speeds.

If you establish that disconnecting the bell line does have the desired effect, then I suggest using a reed relay such as this one http://www.maplin.co.uk/dil-reed-relay-2613?c=froogle&u=2613&t=module - it has low power requirements, can be driven direct from an Arduino pin, and can comfortably switch the bell voltage and current. However, it's SPST, so the bell would be disconnected when the Arduino is not powered, which may not suit you. SPDT reed relays are also available.

Thanks. This is to switch the bell on an old fashioned GPO telephone so I hope it rings based on the ring signal on the bell wire.

Is the advantage to using a SPDT that the circuit would be wired NO and then when power was sent from an Arduino pin the circuit would break?

Thanks

DANE:
Is the advantage to using a SPDT that the circuit would be wired NO and then when power was sent from an Arduino pin the circuit would break?

Replace "NO" by "NC" in your quote, then the answer is yes.

Of course. Thank you.

I've been having a think. Can someone confirm that using a SPDT relay can be used like a push to break push button, only instead of pushing a button you apply current?

The circuit is closed and working. Current is applied and the relay breaks the circuit because there's nothing connected to the other pins?

Yes, if you use the common contact and the normally-closed contact, then it behaves like that.

DANE:
A SPDT relay can be used like a push to break push button, only instead of pushing a button you apply current?
The circuit is closed and working. Current is applied and the relay breaks the circuit because there's nothing connected to the other pins?

magnethead794:
Use a relay with a 5 volt coil. Wouldn't hurt to put a resistor on it to limit current draw.

I have acquired a 5v SPDT sub-miniature relay.

I would like to wire it up as advised above - with a resistor to limit current draw and so that it's a normally closed circuit but on application of 5v the circuit is broken. Can anyone give me some pointers? i.e. what size resistor, where to put it (gnd or 5v), and, crucially, which pins to solder which wires to? I could experiment with the pins but I don't want to damage my ethermega. Hope this is acceptable?

In that diagram, the bottom left and bottom right connections look like a normally-closed pair. The coil connections are the middle top and middle bottom ones.

if you are going to drive it from 5v then you should not use a series resistor. However, the coil resistance determines whether you can drive it direct from an Arduino pin. If it is 120 ohms or above, then you can drive it direct from an Arduino pin (but don't forget to use a diode in parallel with it to catch the back emf). Otherwise, you will need to drive it through a transistor.