This series of monthly teardowns was started in early 2018 as an experiment, and since you fine folks keep reading them, I keep making them. But in truth, finding a new and interesting gadget every month can sometimes be a chore. Which is why I’m always so thankful when a reader actually sends something in that they’d like to see taken apart, as it absolves me from having to make the decision myself. Of course it also means I can’t be blamed if you don’t like it, so keep that in mind as well.
Coming our way from the tropical paradise of Eastern Pennsylvania, this month’s subject is an ADT branded Impassa SCW9057G-433 alarm system that was apparently pulled off the wall when our kind patron was moving house. As you might have guessed from the model number, this unit uses 433 MHz to communicate with various sensors and devices throughout the home, and also includes a 3G cellular connection that allows it to contact the alarm monitoring service even if the phone line has been cut.
From how many of these are on eBay, and the research I’ve done on some home alarm system forums, it appears that you can actually pick one of these up on the second-hand market and spin your own whole-house alarm system without going through a monitoring company like ADT. The extensive documentation from Impassa covers how to wire and configure the device, and as long as the system isn’t locked when you get it, it seems like wiping the configuration and starting from scratch isn’t a problem.
If it’s possible to put together your own homebrew alarm system with one of these units at the core, then it seems the least we can do is take it apart and see what kind of potentially modifiable goodies are waiting under that shiny plastic exterior.
Let’s rollback the hobby electronics calendar a few decades with [myvideoisonutube’s] alarm activation control circuit using a matrix style phone keypad. The circuit is quite old using CMOS 4081 with 4 ‘AND’ gates to hardwire the access code. [myvideoisonutube] references [Ron’s] “Enhanced 5-Digit Alarm Keypad” schematic for this build changing the recommend keypad with a more common matrix style keypad found in touch pad phones. These types of matrix keypads wouldn’t work outright for the input so he cut some traces and added hookup wires to transform it into a keypad with common terminals and separately connected keys. We love seeing such hacked donor hardware even when it requires extensive modifications. [Ron’s] source circuit included a simple enough to build tactical button keypad if you can’t find a suitable donor phone.
Learning how to use mostly discrete components instead of a microcontroller would be the core objective to build this circuit outside of needing a key-code access point or other secure 12 V relay activated device. Such a device would be quite secure requiring a 4 digit “on” code and 5 digits for “off”. You couldn’t just pull off the keypad and hotwire or short something to gain access either. The 4 digit on “feature” does knock the security down quite a lot. However, all keys not in the access code are connected to the same point so you could increase your security by using a pad with more keys.
On [Ron’s] site you will find a detailed construction guide including top and bottom view for placement of all the components on veroboard. Join us after the break to watch [myvideoisonutube] demo his version.