The chances are good that you’ve got an old cell phone sitting in your parts bin. You don’t want to throw it away — there are electronics in there after all. But, unless you get the sudden urge to play Snake, it will likely continue to collect dust. So, you may as well follow Techgenie’s tutorial and turn it into a DIY security alarm that calls you when a door or window is opened.
You’ll need an old cell phone that has an actual physical keypad, and ideally one that isn’t a clamshell design. The idea is to trigger a speed dial to the phone you actually use when a door is opened. Those old phones usually have a very long battery life, so you can leave it in place for extended periods of time. You will, however, need a working SIM card for this project. But, it won’t be making many calls, so you can set it up with a very cheap prepaid plan.
To start, you’ll program the phone number you want it to call to one of the speed dial buttons. Then, crack open the case and locate the PCB with the keypad buttons it. Solder two very thin enameled wires to the buttons pads, and put the phone back together with those wires exiting the case. Next, attach the wires to screws glued to a clothes pin.
When the clothes pin is fully closed, the screws will touch and make contact. That shorts the button pads and triggers the speed dial. So, you just need something like an old credit card to place inside the clothes pin to stop that contact under normal circumstances. Then, just attach the card to your door frame with a magnet and the phone and clothes pin to the door. Whenever the door is opened, the card is pulled out, the clothes pin closes, and the phone calls you!