robf@mcs213f.cs.umr.edu (Rob Fugina) (02/23/91)
I took ideas from schematics posted here a few days ago and constructed
a telephone "line in use" indicator. Here's the circuit...
1M 1k / E (to +5)
>----- -----\/\/\----+---\/\/\-----|
| \ 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.
If anybody has any ideas of how to do this without an external source,
please get back to me...
Robf robf@cs.umr.edujon_sree@world.std.com (Jon Sreekanth) (02/23/91)
In article <2231@umriscc.isc.umr.edu> robf@mcs213f.cs.umr.edu (Rob Fugina) writes: a telephone "line in use" indicator. Here's the circuit... 1M 1k / E (to +5) >----- -----\/\/\----+---\/\/\-----| | \ C \ | / 220k \ from \ / 1k phone bridge | \ line | | | LED | | >------ --------------+---------------+ | GND To repeat myself just a little, the 1M resistor is too small; FCC requires a minimum of 5M to qualify as a device with Ringer Equivalence Number of 5.0. In practise, I'm told, a low resistance might be interpreted by your local office as line trouble, and result in a (unnecessary) service call. Since you're using a power supply anyway, you might consider a potential divider made up of 22M and 1M, with a cheap JFET opamp (like the quad opamp LF347) as a comparator. Regards, / Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com