Phone in use indicator 1

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Runs off 9V battery, Plugs into phone jack, Lights an LED when any phone on the line is off-hook.

Phone Information

Measuring the voltage across the telephone line shows (typical numbers):

  • On Hook: 40 to 50 VDC
  • Off Hook: 4 to 6 VDC
  • Ringing: 100 VAC

The “standard” impedence of a telephone, when off-hook, is 680 ohms. Hanging a 680 ohm resistor across the telephone line will drop the voltage from 48V to about 5V, causing the line to go “active”. This is how HOLD switches work. This probably means that it is bad to load down the phone line when the phone is off hook. I wouldn’t want to hang less than a 100Kohm load across it. Should probably measure this, and see how it affects the on-hook voltage.

I haven’t seen any official documentation on these numbers. They’re empirically derived.

The next question is: What are these voltages referenced to? If anything? It’s possible that the most positive phone wire is tied to the GND in your house, or else maybe the neutral wire in your 120VAC outlet. So measuring the phone line voltages with respect to your household GND should show 0V and -48V when the phone is on-hook. But I don’t know. It’s probably best to not rely on this behavior.

Circuit Design

Use a CMOS 4049 Hex Interter because (supposedly) has high drive output. And since it’s CMOS, it can operate with Vcc from +3 to +15V. And it has a high input impedence.

     Phone+   -----+                                              |+ 
                   |                                             LED 
                   R 2.7Mohm                                      |- 
                   |                                              | 
                   |                                              R 680 ohms 
                   |         5 |\ 4          3 |\ 2               | 
                   +-----------| >o------+-----| >o---------------+- 
                   |           |/ 4049   |     |/ 4049            | 
                   |                     |                        | 
                   |                     |                        | 
                   R 0.56Mohm            |  14 |\ 15              | 
                   |                     +-----| >o---------------+ 
     Phone-  ------+                           |/ 4049 
     R = resistor.  Those other things are inverters. 
     Connect 9V battery across +9V and GND, above. 
     Tie all unused inputs (pins 7,9,11) of the 4049 to GND!  Don't let 'em float. 
     Tie Vcc (Pin 1) of 4049 to +9V 
     Tie GND (Pin 8) of 4049 to GND 
     Voltage going into pin 4 of 4049 is: 
             Phone voltage   Voltage at pin 4 
             6V              1V 
             48V             8V 
             100V            16V 

Fancy Features

Not all phone jacks are wired the same way. Some have the two wires reversed. In the old days, before touch-tone, it didn’t matter. In the early days of touch-tone, some phones didn’t dial when the polarity was backwards. Now days, most phones don’t care any more.

But the circuit above does. It requires the phone wires to be connected as shown. If you connect them backwards, it won’t work. The light will just stay lit. And the 4049 may eventually be damaged. (4049’s seem pretty resilliant). So it would be nice to have an easy way to switch the phone wires

Design Analysis

The 4049 probably takes a lot of abuse in this design. When the phone rings, the 4049 probably sees bursts of 16V. When the battery goes low, the voltage on pin 5 of the 4049 may exceed Vcc on the 4049, which is probably bad. It shouldn’t be hard to improve on this circuit.


  • Roger Petersen



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