Off hook detection.

[dramrattan]

Hi,

The attached ciruit is used to detect when a phone goes off hook by using two voltage dividers...The pin 6 voltage is obtained by dividing the 48V across the line by 20 t0 be compared with pin 7 that is fixed at approximately 1V.

Why is such a large resistor like 10M is required as the first resistor in the voltage divider to pin 6 of the LM339 comparator?.

 

[colin55]

The designer of the circuit wanted to put no-load on the phone line. So he chose 470k for one of the voltage divider resistors. The other resistor had to be 10 or 20 times this value to get the voltage division he wanted. I would choose 47k for the lower value and the high value would be 1M.

 

[chemelec]

Even with Very High Value Resistors, Your Phone Company can Detect that resistance.
Its Not a very good circuit.

 

[colin55]

Many handsets leach current to keep back-up memory capacitors (batteries) charged.
You can put 47k across the line without any problems. My Infinity Bug leaches 0.5mA without any detection.

 

[KMoffett]

This is part of a telephone monitoring circuit I've had hooked up to my home phones for 20 years. No problems.

ken

 

[Externet]

A correct way to do off hook detection is with a photocoupler in series to one of the telco wires.

Polarity steering rectifier bridge+--------|>|-----------./ .-------------+Tel

---|>|--- = led from optocoupler.
---./ .--- = hook switch

If the telephone does not use polarity steering rectifier bridge, dual counterparallel leds on optocoupler should be chosen.

By the way; the "48V across the line" comment may be wrong.
There is only about 7V when off hook if that status is what you want to detect.

Miguel

 

[colin55]

A LED in one line only works when the extension is "past" the detection point.

 

[chemelec]

Many handsets leach current to keep back-up memory capacitors (batteries) charged.
You can put 47k across the line without any problems. My Infinity Bug leaches 0.5mA without any detection.

Once I Had a Detector that was 44 Meg resistance across the line. (22meg on each wire)
It detected and automatically recorded all conversations.

Than one time, I had a problem with my phone and asked them to check it out, They Detected that resistance.
But even though it was NOT Causing the Actual Probem I was having, I had to Remove it Or I was subject to recieving a Fine for Illegal equipment on there system.

Your "Bug" is probably in "Series with the Phone", So it only uses that power when the phone is being used. Thats Not a Problem and Not Detectable as not all phones have the same impedance to the line.

 

[colin55]

The Infinity Bug cannot be in series.
Many of the old phones had a "pea lamp" (50v) that worked off the line voltage.
Many phones charge a battery via the phone line. They monitor the voltage and take a short pulse of energy every few seconds. The relays in the exchanged are "slugged" so they don't detect this pulse.

Here are two simple circuits to detect when the extension is in use:
**broken link removed**
The first circuit needs no battery but must be placed before the extension. The second circuit needs a 3v battery but can be placed anywhere.