People living in small towns and rural areas may not realize it, but pickpocketing is a genuine issue in big cities. Real-life pickpockets aren’t magic like those in movies, but they can pull an item like a wallet from your pocket without you noticing — it is surprisingly easy to do. The only reliable way to prevent that is to make sure nothing valuable is easily accessible. But Engineering Dads didn’t want to protect their belongings, they wanted to catch pickpockets red-handed.

If a pickpocket does their job well, you won’t notice your missing wallet until later when you go to pay for something. So this device needed to trigger the moment it leaves a pocket and to let everyone in the vicinity know about it. To achieve that, it needed a sensor to detect the theft (but not normal movement) and it needed a way to alert the owner — and, ideally, everyone else in the area.

In this case, the sensor is a photoresistor. An Arduino Nano monitors that photoresistor and expects to see darkness most of the time, because the gadget should be in a pocket. If it registers significant light, that indicates that the unit is no longer in a pocket and has therefore been stolen. When that occurs, it starts blaring the famous “Attenzione Pickpocket!” viral video audio through a speaker module. Engineering Dads doesn’t say what kind of module that is, but there are a number of suitable options. The hardware fits inside a 3D-printed enclosure that is close enough to the size of a wallet to fool some thieves.