4G Alarm System Scares Thieves and Notifies You When It Detects a Person

Keeping possessions safe can be as simple as scaring away potential thieves. While plenty of commercially available alarm systems exist, you can also make one! GreatScott! recently built a 4G-enabled alarm system with off-the-shelf parts, including a Nordic Semiconductor development board.

Instead of a pyroelectric infrared sensor (PIR) motion sensor, the 4G alarm uses a "Human Presence Detector." These sensors are a millimeter-wave (mmWave) radar technology that operates at 24 GHz. A key advantage of mmWave sensors over PIRs is that not only do they detect motion, but they can also measure distance and detect whether the object is a person! In other words, they are far more effective at detecting an unwanted intruder than simple PIRs.

Scott used a Nordic nRF9160-DK development board. It features an nRF9160 system-in-package (SiP) containing an Arm Cortex-M33 microcontroller, LTE modem, RF front-end, and a power management controller. The LTE modem supports LTE-M and NB-IoT, which gives the modem longevity in the ever-evolving cellular ecosystem.

Some might wonder why an Arduino is in the box when the nRF9160-DK is a competent board. The reason is related to software.

Out of the box, Scott demonstrated how to use the nRF9160-DK to connect to the cellular network and send messages via MQTT. MQTT is a lightweight protocol for sending simple messages from IoT devices. In this case, the dev board sends a message to a mobile phone via an MQTT broker in the cloud.

The trigger for the message is an onboard push button. So, Scott wired the I/O pin with a transistor to activate the message. For the sake of time, an Arduino handles activating the siren and monitoring the "Arm" switch. Moving those functions to the nRF9160 is just a matter of embedded firmware programming.

Overall, this 4G alarm is a clever use of off-the-shelf parts. The project is a great starting point if you wish to build one. You can see the extended explanation in the video above. The project files are available via this GreatScott! Patreon post.