High Impedance dial tone monitor

[lm555]

Hello,
I need to build a phone line monitor based on LM386 with speaker (battery powered). Preferably high impedance so it doesn't interfere with data transmission. Since LM386 it self is not a problem
then interfacing it to telephone line is a challenge for me. This device would be used only for several second at the time to determine dial tone presence and static on the line - if any. I know that butt phones that would do that are readily available but this circuit eventually will be a part
of bigger device - DSL tester.
Can you guys help?
Thanks in advance!

 

[Externet]

Telco line A----------------------------Transformer 8Ω winding------------------------------------------------------ DSL line A-----> to your tester
Telco line B------------------------------------------------------------------------------------------------------------------------ DSL line B-----> to your tester

Use any audio output transformer, the high impedance winding to provide audio feed to the LM386 amplifier circuit.

In other words, the audio transformer to be in series with one of the telco incoming wires.
There is other details to take care of, as diodes in counterparallel at the high impedance winding of the transformer to clip any large signals.

 

[dr pepper]

I tried this with some succes, allthough a bit heath robbo.

Take a 12v relay a larger one preferably an 8 pin one, rip out the coil, take the lamination out of the coil so its hollow, feed either one of the phone line wires through the centre of it, then connect the coil to the input of the lm380 circuit, you might need a 741 op amp to increase the gain a little, theres no connection the phone line this way.

You might find a proper 600 ohm telephone line transformer in an old modem or fax machine, one of these would probably work in the same manner as externet mentioned.

 

[canadaelk]

This is what I would do. It has the advantage that you do not need to cut into the phone line. The 350/440 Hz decoders detect the North American dial tone. Many European contries use a single 425 Hz tone. E

 

[lm555]

Thank you Externet, dr pepper and canadaelk for interesting ideas. You guys are the best!. I will give them a tray and post the results. Regards!

 

[canadaelk]

Go to **broken link removed** for a lot of additional info. E

 

[audioguru]

Don't forget that the telco battery is 50V and ringing is 90VAC on top of the battery so the telco line can have peak voltages of 177V which will destroy the LM386 unless proper protection is provided.

A telco line that is not is use will pickup crosstalk from other phone lines, mains hum and other interference (including static especially when a lightning storm is coming).

 

[colin55]

Don't forget that the telco battery is 50V and ringing is 90VAC on top of the battery so the telco line can have peak voltages of 177V

This is incorrect. The AC is not on top of the 50v.

 

[colin55]

The AC is not 177v. It is about 120v.

 

[audioguru]

This is incorrect. The AC is not on top of the 50v.
The AC is not 177v. It is about 120v.

Read about the NORTH AMERICAN telephone system here: https://www.epanorama.net/links/telephone.html
It does not say a word about Australia.
It says the ringing voltage depends on how many ringers are on the line and ranges from 40V to 150V.
It also says that the telephone company may or may not remove the battery during ringing but they are added in Canada.

I said that the 90V RMS ringing is added to the 50V battery causing a peak voltage of 177V.

 

[davenn]

Read about the NORTH AMERICAN telephone system here: https://www.epanorama.net/links/telephone.html
It does not say a word about Australia.
It says the ringing voltage depends on how many ringers are on the line and ranges from 40V to 150V.
It also says that the telephone company may or may not remove the battery during ringing but they are added in Canada.

I said that the 90V RMS ringing is added to the 50V battery causing a peak voltage of 177V.

to quote from your link .....

To ring your telephone, the telephone company momentarily applies a 90 VRMS 20 Hz AC signal to the line. Even with a thousand ohms of line resistance, this is still a bit of a shock if you happen to touch the wires, so be careful when you are probing around trying to find a POTS line. Telephone ringivers have some differences between countries.The ring signal is much the same, worldwide. It is around 90V at a frequency between 16 2/3 Hz and 50Hz (20-25 Hz quite common). But its timings are wildly different, as are the return tones it generates.

it is pretty standard in many countries .... USA, Australia and New Zealand to name 3. Having worked in the telecom industry in both New Zealand and Australia I can confirm this

Dave

 

[WTP Pepper]

Take a look at this.
The op-amp isn't like shown it's just what was in the library. Try a CA3140 or any CMOS input op-amp (TL081).
The diodes are Zenners. The idea of the circuit is that here in the UK, the Telcos send DC micro amps down phone lines to detect any early failures of their switched exchanges. Hence the need for an isolated HiZ interface circuit. Depending where you are you could add extra lightning suppression across the inputs, but I never had need to.

I used this circuit for years on a home made alarm system for interfacing to a DTMF tone decoder. It worked fine without any problem

The input caps should be rated at 1000V
View attachment 65277

 

[dr pepper]

Thats a similar circuit to what you find in cheap fones, the bridge is a good idea, here in the uk the line voltage is often reversed, the meters in old exchanges used to work from line voltage reversals as well as other things.

 

[dougy83]

I tried this with some succes, allthough a bit heath robbo.

Take a 12v relay a larger one preferably an 8 pin one, rip out the coil, take the lamination out of the coil so its hollow, feed either one of the phone line wires through the centre of it, then connect the coil to the input of the lm380 circuit, you might need a 741 op amp to increase the gain a little, theres no connection the phone line this way.

You will have better coupling if you wrap one of the phone wires around the circumference (ie so it follows the direction of the wire on the coil) of the relay coil; sounds like you have the phone cable at 90 degree to what it should be.