Telephone Ring Circuit

[Aan]

Hi, i want to understand how telephone ring circuit work and how it is connected to line (virtual telephone like PBX, not connected to PSTN).

[audioguru]

Hi Aan,
The PBX's I worked with in Canada used ordinary single line phones and used a 90VAC/20Hz ringing signal.
Old systems used relays to connect the ring generator to the line, but newer ones used triacs.

[Aan]

Hi audioguru,

How to make the telephone on the other side ringing ?  how to detect if the phone hook on the other side is picking-up and disconnect the ring circuit to not interfere with the line ?

Best Regards.

[audioguru]

How to make the telephone on the other side ringing ?

It has a capacitor-coupled mechanical ringer or a capacitive-coupled ringer IC that drives a speaker.
Here they are driven by a 90VAC/20Hz ringing signal.

how to detect if the phone hook on the other side is picking-up and disconnect the ring circuit to not interfere with the line ?

When a phone answers the ringing then the phone draws DC current from the phone line. When the PBX detects DC current then it stops the ringing and connects the audio.

[Aan]

Hi audioguru,

On line telephone with two wires,  TIP and RING.

I have found 2 type of circuits to make simple telephone network with two telephones and with no ringing circuit.

1. One type of circuit is to connect TIP line on one side to the other side of line directly, and cut the RING line and connected in series with 300 ohm and 9volt battery.

2. The second type of circuit is to connect both TIP and RING in paralel with two resistor 300 ohm and connected the end of both of resistor with + and - pole of 9volt battery.

My question is how to modify these two type of circuits so that one telephone side can ring when telephone on the other side made a phone call ?

[audioguru]

Hi Aan,
Are you making an intercom with phones?
I don't know if phones in your country are the same as mine.
My phones need a 90VAC/20Hz signal across tip and ring to activate their ringers.

[Aan]

Hi audioguru,

Well, i am trying to understand how general system of PBX working, but first i need to understand the basic circuit/connection between two telephones, i found two simple circuits on the internet of how to make an intercom (as you said) of two telephones or connect two telephones to communicate each other (like on the pictures below).

I have made an experiment, i connected TIP and RING line of telephone unit to 60VAC (50Hz), and the telephone was ringing.

My problem is i don't know how to connect or to couple ring circuit to one or both of these two circuits.

Can you guide me ?

Best Regards.

[audioguru]

Use a double-throw switch to disconnect your phone and connect the 60VAC to the line to make the other phone ring. But you won't have a circuit to stop the ringing when the other phone answers so the 60VAC might make a very loud noise in the other phone's handset then destroy the electronics in the phone.

[Aan]

Yes it is aloud noise produce by telephone set, i reduce the voltage level to 40 volt and it work.

How to place double-throw switch in the circuit ? and how to detect if the telephone's handset on the other side was picked-up ?

Regards.

[audioguru]

You connect a double-throw switch so that the telephone is connected to the other telephone with the switch thrown one way, or connected to the ring generator with it thrown the other way.

To detect that the ringing phone has picked-up you must detect its DC current in the phone line.

[Aan]

:-\ sorry don't know how... :(

[audioguru]

Use Ohm's Law to measure current.
A small value resistor in series with the phone won't affect the phone but it will have a DC voltage drop when the phone draws current when it is off-hook. An opamp can amplify the voltage across the resistor and the output of the opamp can drive a transistor that changes the position of a relay so the ringing stops and the phone is connected to the calling phone.
You will need to filter the ringing signal out of the voltage across the resistor.

[Aan]

I think i know how to figure it out, i'll try it.

Thanks.