Telephone Line Switch

[BillRobinson]

I want to be able to disconnect a telephone line with a digital output from a PLC. Any ideas on how i can do this?

 

[MarkNightingale]

Bill,

I'm not sure about Canada, but in the UK we can disable the 'ring' of the phone.

This will mean the phone will 'ring' at the callers end but there will be no 'ringing' on the phone itself. I don't know if that is what you want to do.

Mark

 

[Earthscape]

There are number of ways to do this, whats your application ?

 

[Dravik]

Couldn't you wire a contact closure inline w/ the phone wiring and hook that to the PLC?

 

[The Plc Kid]

If you are using it for a phone signal do it as Dravik said but use a relay that is made for low voltage signals and has gold contacts to maintain a high integrity signal.

 

[The Plc Kid]

If you are just trying to prevent unauthorized phone use then these off the shaelf products may be better. we used to have link #3 at my old job in the plant to keep production people off certain phones that had a outside line.

http://www.smithgear.com/tec-040.html

http://www.familysafemedia.com/phone_linelok_telephone_line_l.html

http://www.payphone.com/Programmable-Phone-Jack.html

 

[BillRobinson]

So here is what I'm trying to do. I have a PLC that regularly communicates to HMI system via an Ethernet radio. In case the radio fails, I've installed a dialup Ethernet switch/modem which allows a person to dial into the Ethernet network remotely over the telephone lines, for a backup.

The problem with the dial up is that it is not secure; that is, anybody at anytime could dial up the modem and get into the network. So what I want to do is use the PLC to "break" the telephone connection when the radio is good so that nobody can call in while the radios are working.

I was also thinking of using a relay to break one of the two telephone wires, but I'm not sure which one to break. Should I break the L1 or the R1 wire or both?

Also I'm have space solid state relays on site? Will the solid state give me headaches?

 

[BillRobinson]

Wow, I'm reading that telephone lines carry 48VDC. So much for low voltage.

 

[OkiePC]

Wow, I'm reading that telephone lines carry 48VDC. So much for low voltage.

OT: I learned that when I was 18, hooking up an extension for a buddy. In those days, my electrical experience was mostly automotive and I usually stripped small wires with my teeth.

Yes, I had the phone wire in my mouth when I heard (and felt) the phone ringing in the living room.

 

[control_logic]

I used to be a Phone Guy back in the day. I think if it was me, I would hook up a relay in parallel and use it to short the two wires when needed, then open to restore dial tone. This will cause the line to appear busy on the other end.

 

[Brijm]

OT: I learned that when I was 18, hooking up an extension for a buddy. In those days, my electrical experience was mostly automotive and I usually stripped small wires with my teeth.

Yes, I had the phone wire in my mouth when I heard (and felt) the phone ringing in the living room.

I had a guy I used to work with tell the exact same story. Was crying I was laughing so hard.

Also, it looks like in the US, the common ring voltage is a 90V 20Hz. I would recommend a good quality contact, especially for data.