Modem Used In An Alarm System

This alarm system senses motion and then alerts you by phone. [Oscar] had an old external modem sitting around and, with some wise hardware choices, he came up with a simple circuit to use it. First up is the PIC 16F628A chosen because it doesn’t require an external crystal. This connects with the modem via a DS275 RS232 transceiver because it requires no external parts for connection. The final portion of the puzzle is a PIR sensor that triggers a pin interrupt in the sleeping PIC, which then dials your number to alert you. It doesn’t look like anything happens other than your phone ringing, but that’s enough for a simple system. We’re just happy to see how easy it was to use that modem… time to go hunting for one in dreaded junk trunk. Don’t miss the clip after the break.


22 thoughts on “Modem Used In An Alarm System

  1. Maybe he should connect the incoming line to a loudspeaker. Then if the phone rings, he picks it up and shouts: “GET OUT OF MY HOUSE, I GOT A SHOTGUN AND WILL USE IT!”


  2. This could be setup to contact your cellphone provider’s TAP gateway and send you text messages. I used to do it with a Nagios setup where we didn’t have a backup WAN connection. Worked a charm.

  3. at work we needed to monitor humidity at different points in our fairly large warehouse

    I bought a box of old modems, send some at commands and they will connect point to point, over fairly long distances

  4. Nice, now we can use old modems for something useful :D
    Curiously, i did today the same experience on a room, arduino with an PIR sensor, and i set the policies on server side like sms, email, and others. Also in the kitchen i have a gas sensor for detecting gas leaks with SMS alert :).

  5. Not sure why he needed the PIC at all. Many of those old modems can be set up to auto-dial a stored number when DTR is raised.

    But having the motion sensor call you is kind of clever.

  6. @Paul Potter – Not down the drain in my case. I live so far out in the sticks that cell phone reception is pretty hit-or-miss at home. The old land line equipment isn’t fast, or high bandwidth, but usually dependable.

  7. @rallen71366 yep, same here. And forget about cable getting out here within the next decade.

    Neat hack, only having the discussion the other day about “Why do we have an old external modem in this closet?”

  8. Agree with DTR dialing comment. Just mod the motion sensor to feed the output voltage to a switching transistor to loop the DSR pin back to the DTR pin. I’ve used a similar setup, but in reverse, to monitor for power loss in the past, i.e. notify you when switching to UPS power.

  9. Without knowing about this work, I have constructed a similar system. Currently my home alarm works as follows:

    – PIR detectors (230V modules) connectected to light bulbs in a closet.
    – the light is picked up by a led as light sensor connected to a PIC that drives a sirene by means of a transistor.
    – the sirene sound is captured by a mike that triggers an arduino to send out AT commands to a modem.

    I know it is a little too …, but
    1) the mains are separated from low voltage circuits
    2) there is a sirene warning the neighbors
    3) other sources of sound will also trigger the modem to call me.

    If I only knew how to connect and transmit sound messages…

  10. Unless I’m mistaken, you won’t be able to use a modem to dial you and play a messaage. You could patch into a cheap phone though. Just control the off-hook switch, then to dial the number either tap into the dial pad or playback sampled DTMF tones into the mic line, . Then playback your message after a time delay.

    It’s nothing fancy like trying to detect an answering voice, but should work without too much effort.

  11. Years ago, I did something slightly similar, but I had the PIC programmed to use the modem to dial my pager service, and then wait a few seconds, and send a number via touch tones. It was all done with one long ATDT string with commas for pauses.

    I don’t have a pager anymore — It’s replaced by a cell phone. But some cell service providers let a caller leave a numeric page by dialing your number, waiting a bit, and pressing the right keys. You might be able to use this trick to get a simple PIC with a modem to send you a variety of numeric messages, more than just a simple “HEY!”

  12. Who needs a message? If you use a number that will be dialed only from your security system & you have caller ID on the phone you dial, the appearance of that number on the phone you answer should alert you that something’s wrong on the other end. Could you use a disposable phone for this?

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