Telephone system question

[russlk]

I want to make a telemetering system using the telephone line, but know nothing about the system. Can someone tell me how the central office knows the phone is off hook and should stop ringing? How much current can I draw from the line?

[Chris Smith]

Part # 62-1388 Radio shack's "Understanding Telephone Electronics". <p>When the voltage drops from around 48 volts "ON HOOK" to around 5 Volts "OFF HOOK" that is how they know.

[cmcduff]

A simple way would be to buy one of the cheap "Telephone Recorder Controllers". They are just a little box that plugs into the line, has a jack to connect to the microphone and remote jack on a stardard tape recorder. Just use the remote jack, which acts as a switch to operate you timing device. This would be the quick and dirty way of doing this, and have an FCC approved way of connecting to the line.

[Externet]

Hi Russ.<p>.>>>>> Can someone tell me how the central office knows the phone is off hook and should stop ringing? How much current can I draw from the line? <<<<<<<<<p>
In the central office (CO), the 48 Volt battery is fed to your line pair trough sensitive relay coils in SERIES with it.<p>With your phone on hook, there is no current flowing. It's an open DC path.<p>When you seize the line (off hook) there is a DC path and current flows and the relays at the central office click in.<p>The contact closure at those relays stop any present ringing generation if you were being called; or connect the line to the network for dial tone if you want to place a call.<p>How much current can flow? Typically 37 mA.<p>Variations from 20mA to 60 mA depend on how far your telephone is fron the CO, the wire lenght and diameter (resistance) limits the current availability.
You may draw more current, but your voltage will drop. You may get more voltage, but at a smaller current.<p>A better expression would be ¿how much POWER can be drawn from the telco line? I would say around a quarter watt in DC ; about a watt in (AC) ringing.<p>The voltage in off-hook condition also reflects the amount of resistance due to the lenght of the loop from the central office to your telephone.
Tipically, is around 7 Volts and also depends on the brand of telephone you use (its internal resistance presented to the loop).<p>Hope it is of some help.<p>Miguel

[russlk]

Thanks, Miguel, that's the info I need.