For a few months I made an alarm for my house, it is very simple: it is based on magnetic sensors DC-1561 (for windows and exterior doors), a few PIR sensors for rooms, buttons, LEDs and (of course) an ArduinoNano v3. Something simple!

The question is that lately I do not spend much time in my house and I would like it when I'm away from home the alarm calls me (when is activated) to my cell phone using the public switched telephone network (PSTN) because I live in Cuba and it's the easiest way and cheaper (the GSM modems/modules are very expensive for me). I do not want to leave any message or sound record, just call me and receive the call on my cell phone, using the RJ-11 connector (in my country only 2 pins are used) to connect it with the Arduino/some_IC.

I have been investigating since yesterday and have found quite a bit of information. The most recommended/successful that I found (remember that I want to continue using the system that I already have, I just want to add to the Arduino the ability to use the mark through the PSTN) is to use an IC (HT9200B) that is a generator of DTMF codes (as see in the figure) with serial AND parallel connection, aparently, compatible with the electronic logic of the Arduino (5V).

HT9200B connection

But of all the information that I found is to connect to this IC (HT9200B) a speaker and reproduce the DTMF codes. This is not what I want, what I want is to connect directly to the phone line and be able to "call" from it.

Anyone have any ideas, links, projects in progress, or anything that can help me?

  • 1
    You need an FXO device. The simplest of these is a MODEM.
    – Majenko
    Commented Apr 26, 2018 at 17:37
  • 1
    They are hard to find new now. You want an external MODEM (RS232 one - find one on eBay) and a UART to RS232 (e.g., MAX232 based) adaptor.
    – Majenko
    Commented Apr 26, 2018 at 17:38
  • 1
    You connect the TX of the Arduino through the adaptor to the RX of the modem, and the other way around as well. Then send something like "ATDT18005553827\n" at the right baud rate. Wait 10 seconds or so, then send "\n" to cancel the call.
    – Majenko
    Commented Apr 26, 2018 at 17:44
  • 1
    It's the predecessor. It uses the "Hayes" AT command set that GSM modems later on inherited.
    – Majenko
    Commented Apr 26, 2018 at 17:48
  • 1
    Sure. Looks good to me. Any one will do. They all work the same.
    – Majenko
    Commented Apr 26, 2018 at 17:54

1 Answer 1


As already written in the comments, you may use any RS232 modem you can find. They use the so-called AT commands, which is a very common way to exchange commands with this kind of peripheral. You can check on wikipedia or on google to get more information on it.

The main problem is that this kind of modems are a bit hard to find (or at least, they are a bit harder to find and a bit too expensive for my liking). That's why, for a project of mine, I wanted to switch to a cheaper alternative. Looking on the internet, a lot of people stated that it was much better to have an already approved instrument rather than designing your own. For this reason, I bought a mini-phone like this one:

enter image description here

You just have to open it, wire the keys (or rather, the rows and columns of the keypad) to the arduino and let it do the magic.

This solution is cheaper (I spent 4€ for this, but if you have an old phone it becomes free) than getting a RS232 modem, but needs more work on your side.

  • I like your solution more. Could you give me the link where you bought the phone that you show me in the image? When I search (Amazon, eBay and Aliexpress) I get many cell phones and landlines, and I would like to buy yours, because I know it works. Thanks for the advice!
    – k.Cyborg
    Commented Apr 27, 2018 at 20:40
  • Is this one?
    – k.Cyborg
    Commented Apr 27, 2018 at 20:49
  • I bought it from ebay, but it is the same as yours. Just note that I did not complete the project, and with respect to the modem this requires you to tinker with it a bit more. On the other side you will technically be able to send also audio and receive DTMF codes
    – frarugi87
    Commented Apr 28, 2018 at 10:58

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