Detecting a telephone RING

(Circuits -> Telephone)
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From: unknow

I took ideas from schematics posted here a few days ago and constructed a telephone "line in use" indicator. Here's the circuit...

                                               ----> (to +5)
                       1M           1k        /   E
   >-----        -----\/\/\----+---\/\/\-----|
                               |               \  C
                               \               |
                               / 220k          \
from                           \               / 1k
phone      bridge              |               \
line                           |               |
                               |              LED
                               |               |
    >------      --------------+---------------+
                                               |
                                              GND

The transistor is a PNP Motorola 3638 with hFE of around 100 (probably doesn't matter). Also, you could use this with different supply voltages if you change the 220k resistor.

Also, in case anybody's interested, I found the on-hook open-circuit voltage of my phone line to be 48.7V, and the short circuit current to be 72.8mA. This leads to the conclusion that the line has a resistance of about 670 ohms. There have been a few calls recently in sci.electronics for phone in use circuits (ie a circuit that lights a LED when an extension phone is off hook).

Following are two circuits I archived some time ago from sci.electronics. The first appears pretty complete and requires an external 5V power supply. The second seems to be a loop current trap that enables you to move from one extension phone to another without leaving the first phone off hook. I don't know how well either of these circuits work as I haven't actually built them.


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