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For a educational purposes, I want design a security system which works with a PIR sensor. The application is after detection of movement the processor will call a telephone number that defined.

I know approximately everything except module for calling.

I don't know how the application of home phone networks (which work with RJ11)

What kind of micro and modules are suitable?

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    \$\begingroup\$ The old phone network is disappearing quickly for many of us, so you'd want to be specific. What do you want this module to do once it's dialled the number? Talk? Send DTMF? Connect to a modem? As you've found a common solution is to use a 3/4/5g mobile module - this is much easier as you don't need to concern yourself with connecting to a telephone line which might be illegal in some jurisdictions. The mobile module can connect to the internet and send an email etc. I'd suggest this is a preferable method and one that is well travelled. \$\endgroup\$
    – Kartman
    Dec 13, 2020 at 6:38
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    \$\begingroup\$ If this is a one-off, you could always take apart an old phone. \$\endgroup\$
    – Drew
    Dec 13, 2020 at 7:04

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There are no microcontrollers suitable for connecting directly to an analog phone home network.

  1. Analog phone networks have a (relatively) high DC voltage present on the wires. "High" in comparison to microcontroller voltages less than 100VDC, but it'll destroy any microcontroller that you connect directly to it.
  2. An analog phone network is controlled by either tones (sounds played into the wites) or pulses on the wires. No microcontroller has built in connections to do either.

This page describes building a microcontroller interface to a regular telephone line. (Link courtesy of The Wayback Machine.)

You'll need several ICs that may not be commonly available anymore.

Alternatively, you might dig up an old analog modem. Connect the serial port of a microcontroller to an RS232 converter, then connect that to the modem. Your microcontroller could then send commands to the modem for dialing or decoding signals.

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