Home Security Hardware Makes You The Monitoring Service


[Nick] and [Simon] both have home security systems with a monitoring service who will call whenever an alarm is tripped. For [Simon] this ends up happening a lot and he wanted to change the circumstances that would trigger a call. Because of company policy the service is inflexible, so he and [Nick] went to work cutting them out of the loop. What they came up with is this custom electronics board which monitors the security system and calls or texts them accordingly.

They started with the self-monitoring alarm system design we looked at back in September. This led to the inclusion of the SIM900 GSM modem, which is a really cheap way to get your device connected to the cellular network. It also uses a DTMF touch tone decoder to emulate the phone line to keep the security system happy. [Simon] highlights several changes he made to the design, as well as the reasons for them. One idea he has for a possible revision is to do away with the MT8870 chip which handles the touch tones. He thinks it may be possible to use the SIM900’s DTMF features to do that work instead.

16 thoughts on “Home Security Hardware Makes You The Monitoring Service

    1. I recently had a meeting with a local electronics assembler. When he asked what package i was developing in and i replied it would probably be Eagle he said exactly that “oh God”. Apparantly (and ill admit this is according to one person) about 98% of the electronics design and manufacture work done in Australia that is not hobbyists or students is done in Altium because it is ultimately a better package at a corporate level. Easier to share, better tools and well embedded in the industry.

      The news did not really surprise me though, when you consider many of the people driving open source hardware are companies such as sparkfun / adafruit etc who market to hobbyists as well as the hobbyists themselves developing equipment. It is well known that Dave from the EEVBLOG uses altium for many of these reasons as well. So I am biting the bullet and moving up to an admittedly expensive but higher level package.

      1. Well, Dave used to work for Altium, so…

        I once tried to use Eagle and quickly removed it and went back to old DOS version of OrCAD and AutoTrax.

        And if you want Altium to… uhm… “test” Altium visit ru-board for easily downloadable version. Quickly updated, clean and doesn’t call home too much. Full set of libraries also avaliable

  1. You don’t really need GSM.

    Most phones can accept text via email. (ex. 2335553245@txt.att.com)
    The message will only contain the subject line, but it works.
    I use it for server notifications at home.

    With an SMTP server (or fancy scripts) you can even text the server back and have it act accordingly.

    GSM will just add costs. if you’re tied to a wall outlet for power, you can pretty much rely on the lan for internet.

    1. One advantage to GSM is that with a battery backup, you can monitor and also alert on events such as ‘power outage’, ‘phone outage’, and ‘internet outage’.

      Then not only do you need to cut all the hard lines, but also carry a GSM jammer! Both raising the barrier and also accruing the wrath of the FCC and local hams ;}

          1. Where did you hear this? Most burglars drive through a neighborhood during the day looking for houses with no cars in the driveway specifically to avoid having to deal with an occupant calling police.

          2. @booby The dialer that fartface is referring to is the alarm system’s triggered autodialer.
            If a burglar cuts the cable, telephone, and power to the house before they go in, most alarm systems can’t call out. (newer ones will use gsm modems though, which is nice)

          3. Which is exactly the reason GSM is needed.

            If I see power loss event, and 15 minutes later my server goes down, and an hour or two later my internet goes down… well that was just a power outage.

            If my power loss event and internet go down within seconds of each other, I now know someone cut my hard lines and is trying to break in (That or my UPS exploded and now my house is on fire…) either type of event I’d like a security message about right away!

      1. If you are the one doing the break and enter, don’t forget the old Russian radio tubes…
        Those will slow down the HAM operators long enough to confuse the police who was the actual perpetrator (s) if/when you can’t get away from the scene of the crime fast enough!

        take tongue, place in cheek, now talk like that ;)
        Seriously tho, nice concept, good implimentation if it comes with the MT8870 chip already, use it ( my opinion ). Ditching parts for the sake of removing parts is no good as it tends to mean more coding and more potential bugs ( FCC warnings on using the ‘ extra ‘ dialing codes as example )

  2. I’ve updated the firmware on several SIM900’s now, including the DTMF decoding part. All I can say is that it works great!! Simply pushes out +DTMF:x (x meaning the number or sign pressed). This is a neat feature if you’re going to have some sort of output turning on or off via the alarm.

  3. There are easier ways to do this. Most of the time you can find a hack or the backdoor password to your alarm system and simply reassign the phone numbers it dials. If the installer was not a dooshbag he did not set the installer password and it is still the CSU serial number.

    1. Did you just read the term CSU in a book somewhere? That is just gibberish. Yeah because everything hooked to a network has a CSU/DSU connected to a DTE connected to a DCE. Right. I’m sure that is how you think cellphones work too. Connect four boxes together every time you need to make a digital call.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.