I made several of these for a client that wanted more alarms like they already had. I made 3 prototypes that were tough enough for them to use in production.
It's a box that makes noise if any of the connections are broken. They use cables with RCA connectors, and there is a key to enable or disable the alarm. it's battery powered.
I did the hardware and software for this project.
The hardware was a 2 gang metal electrical box, a DigiSpark brain, some RCA connectors, a piece of perfboard, 2 LEDS (red, green), a key switch, and a battery pack that used AA batteries.
In hindsight, I could have made much more money per unit if I had done the stuff in hardware instead of software, but I started with a DigiSpark because it was cheap ($8) and I didn't have much EE experience at the time.