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Regarding Microcontroller Design for a PBX Switch

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Full Member level 3
Jul 6, 2008
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Hello there.

I need a device which will use with Landline phone to provide IVR password security. For example:

Telephone line --- ( RJ11 Cable) ---> DEVICE --- ( RJ11 Cable) ---> Landphone

When a user calls then the DVEICE will auto Answer and play an IVR "Hey, thanks for calling, please enter password". If caller enter passwords correctly then Landphone will start ring. IVR can be stored into MicroSD Card as a media file.

Can anyone advise some outline about the schematic?

Thanks in advance.

This isn't as simple as it sounds. Before looking for a 'device', provide a breakdown of exactly what it has to do.
For example:

1. monitor the incoming line for a ringing voltage
2. seize the line (go off hook)
3. connect the incoming/outgoing audio paths. Note they have to be combined to a two-wire connection.
4. play the outgoing recorded message.
5. listen for the incoming password, maybe by voice or keypad tones
6a. if rejected, send a warning to the line and disconnect.
6b. if accepted, connect the phone and generate the ringing voltage to it
7. monitor for the phone going off hook, if it doesn't within a reasonable time, turn the ringing voltage off and disconnect the incoming line.
8. if is answered, turn the ringing voltage off and either directly (relays) or indirectly (through your circuit) connect the phone to the incoming line.
9. detect when the call is finished to reset the system.

You also need a method to program the password and answering message into the system.

There are commercial devices that do this but they are expensive. The electrical needs depend on the phone line requirements of your country and it varies from place to place. Typical phone lines are balanced around ground potential and constant current when off hook. You probably have to generate around 50V DC for the line and maybe 75 to 100V AC for the ringing voltage.


Hi @betwixt. Many thanks for your detail advise.

I made a basic Block diagram for the project but need some more suggestion on the Schematic.


-1. How the connection will be between RJ11 Port 1 and RJ11 Port 2 with MCU?
-2. For RJ11 port 1, how to seize the line (go off hook)?
-3. For RJ11 port 2, how make the Ring Volt?
-4. If the Password match (detected by DTMF) then how can I replay the Incoming Line to Outgoing?

Can you please tell or share any resource for the RJ11 Telephone Pinout and its operation?

You probably have to generate around 50V DC for the line and maybe 75 to 100V AC for the ringing voltage.

Would you mind to explant this more?

One more ques: Is it possible to detect the Caller Number?


Your RJ11 ports are not standard wiring so the signals they carry may not be normal telephone levels. What type of telephone do you have? It appears not to be a standard analog line.


Hi @betwixt, The phone will not be 1970's Dialing based Phone. My project will be mainly for USA so general Number Pad based Landphone which are commonly used in home residence. We can consider like below. All this types of Phone set will have standard RJ11 communication ... right?


Can you help to share more detail how I can get the RJ11's Pin and Signal information?


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