Old door alarm: curiosity teardown

I found this old "security" door alarm recently, and was curious about how it works. I remember similar units in the old Radio Shack catalogues, but never got to play with one back in the day. ----- My tip jar: https://www.patreon.com/pileofstuff My other tip jar (channel membership) : https://www.youtube.com/channel/UCRiFz3gNKE3VysarZ7nmqIA/join Buy me a coffee (or a beer): https://ko-fi.com/pileofstuff If you are OK with me getting a commission on your purchases, feel free to use my affiliate links: Ebay: https://ebay.to/33IF7PA Banggood: http://pileofstuff.ca/r/9ajsf Some other links to products may be affiliate links https://odysee.com/@pileofstuff #ElectronicsCreators ----- This isn't the only way to do it. It's probably not the best way to do it. But it's how I did it.