send and receive voice signals to old telephone line

Thread Starter

mohsenpsr

Joined Oct 23, 2023
3
Hi
I'm trying to design a circuit to connect with POTS line. at first I designed ring detection circuit with notice that it works correctly , but now I don't know how to receive the voice signals from line and give it to my ESP32 for processing. in other side I have to send a voice signal from my ESP to the line to implement the call.
I read some notes but I couldn't get anything. I only understood I should use some Hybrid-two-wire circuits to send and receive with quality but I don't know how to get voice signals from the line and send another voice on it simultaneously!

I would really appreciate if give me any information about the interface circuit.
Here is my ring detection circuit.
ring detector.JPG
 

Externet

Joined Nov 29, 2005
2,182
Welcome.
You have to load the telco line with a near-to 200 Ohm resistor (\/\/\/\/\) to seize it as 'off-hook'. 0.5 Watt is enough.
And in series to the telco line ( R and T ) , insert any audio output transformer 8 Ohm winding (uuuuuu) to only one of the conductor pair (not across or parallel). Inject-extract audio from the higher-other transformer winding below :

T----------------------uuuuuu-------------------\/\/\/\/-----------./ .-------------R

1698102113872.jpeg

Do not ground anything. The transformer can be canibalized from a very old transistor radio or buy a similar like this :
----> https://www.amazon.com/RadioShack-RadioShack®-Audio-Output-Transformer/dp/B007Z7PDNU or
----> https://www.amazon.com/AEDIKO-Transformer-Efficiency-Transformers-1300/dp/B0CB67NQG1/ref=sr_1_5?keywords=audio+transformer&qid=1698100938&sr=8-5&th=1

Do not use a 600:600 ohm transformer. If you cannot get one, canibalize a small linear wall wart transformer and use the low voltage 6VAC; 12VAC... winding as the uuuuu above in series, and its 120VAC winding to extract / inject your audio. A hybrid circuit is when you want to send and receive audio at the same time. If you only send or receive audio at a given moment, you do not need it.
The off-hook line seize switch can be instead relay contacts managed by your microcontroller. Inject the audio into the transformer so the telco gets a -9dBm level (~0.27Vpp ~80mVRMS) across the resistor.

Your schematic shows the telco lines R and T shortcircuited left of C4 !

This is the book that taught me all I needed to know about POTS and also explains hybrid circuits made with operational amplifiers and with transformers:
1698105688180.png
 
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Thread Starter

mohsenpsr

Joined Oct 23, 2023
3
that
Welcome.
You have to load the telco line with a near-to 200 Ohm resistor (\/\/\/\/\) to seize it as 'off-hook'. 0.5 Watt is enough.
And in series to the telco line ( R and T ) , insert any audio output transformer 8 Ohm winding (uuuuuu) to only one of the conductor pair (not across or parallel). Inject-extract audio from the higher-other transformer winding below :

T----------------------uuuuuu-------------------\/\/\/\/-----------./ .-------------R

View attachment 305719

Do not ground anything. The transformer can be canibalized from a very old transistor radio or buy a similar like this :
----> https://www.amazon.com/RadioShack-RadioShack®-Audio-Output-Transformer/dp/B007Z7PDNU or
----> https://www.amazon.com/AEDIKO-Transformer-Efficiency-Transformers-1300/dp/B0CB67NQG1/ref=sr_1_5?keywords=audio+transformer&qid=1698100938&sr=8-5&th=1

Do not use a 600:600 ohm transformer. If you cannot get one, canibalize a small linear wall wart transformer and use the low voltage 6VAC; 12VAC... winding as the uuuuu above in series, and its 120VAC winding to extract / inject your audio. A hybrid circuit is when you want to send and receive audio at the same time. If you only send or receive audio at a given moment, you do not need it.
The off-hook line seize switch can be instead relay contacts managed by your microcontroller. Inject the audio into the transformer so the telco gets a -9dBm level (~0.27Vpp ~80mVRMS) across the resistor.

Your schematic shows the telco lines R and T shortcircuited left of C4 !

This is the book that taught me all I needed to know about POTS and also explains hybrid circuits made with operational amplifiers and with transformers:
View attachment 305722





thanks a lot
It was really helpful . after looking for TRANSFORMER that you said I found this, and I designed the circuit below. the transformer I used named blue-choke as you see in the picture.
now I can capture the audio signal, but just for assurance should I use anything else after transformer ? in transmitting or receiving path I mean, a circuit which is located between this circuit and the microcontroller ?

audio signal.png
 

Externet

Joined Nov 29, 2005
2,182
Hi. Your design is wrong.

- C24 and C25 should not be used. They are loading the audio to GND via pins 5, 4 of the transformer.
- The connection between transformer pins 1 and 2 to 5 nullify the purpose of a transformer, you are connecting primary to secondary. Absurd.
- You connected the transformer pins 1 and 2 in parallel to telco. Should be in series to only ONE of the telco lines.
- If your transformer is very low resistance between pins 1 and 2, it will short-out the telco line or load it way beyond obtaining signal from/to it. You do not tell what transformer is that 'blue chock' which can make it not work.
-The value of C1 is way too low. Should be at least 1μF/250V or 0Ω resistor if goes to detect 20 hertz ringing.
-The connection 'voice line phone' should have only analog audio in or out or both; or from ADC/DAC converter pins in a microcontroller. NOT a data signal from a microcontroller.
- At the point 'voice line phone', you have to connect a preamplifier for audio input to the rest of your circuit if a higher level is needed. Switching from any audio outputted by your circuit has to enter to the same point 'voice line phone' at about 0dBm to present about -9dBm at the Telco.
- There is no proper seizing of the line other than the wrongly connected transformer. Should be about 200Ω
 
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