LoRa-Latch

A remote keyless entry system for ancient apartment door latches using Arduino and LoRa

Context

Older apartment buildings have hard-wired intercom systems that allow visitors to activate a buzzer in an apartment. The resident has a button inside that allows the main building door to be unlocked. This project takes those features and creates a type of keyless-entry system for the resident to use when they get home.

How it works

1. The remote stays in the resident's vehicle and acts as a beacon, transmitting an unlock code periodically.
2. The latch module is integrated into the apartment building wiring and listens for the unlock code.
3. When the latch module receives the unlock code, it starts a five minute timer.
4. After the vehicle is parked, the remote stops transmitting.
5. If the apartment is buzzed before the timer runs out, the door latch will be triggered, allowing the resident into the building without using their key.

LoRa Modules

Check out the LoRa Boilerplate project for details on how these modules are built. https://github.com/jonmon6691/arduino_lora_boilerplate

Latch Control Wiring

Designing the half-bridge rectifier

• The latch module uses a relay and a half bridge rectifier in order to detect that the buzzer is active.
• Buzzer power is 13VRMS at 60Hz
• Relay coil is 720Ohms and requires between 9 and 15.6 volts

Using that information, a simulation can be built to evaluate readily-available capacitor values:

Latch Remote Wiring

The latch remote waits until USB power is removed before it starts sending the unlock code. In order to do this, it uses an Adafruit PowerBoost 500 Charger and a tiny lithium battery so this it can keep running after power has been removed. A simple RC circuit formed by R1 and C1 above keeps the boost/charger module enabled for about a minute.

Note: The 340k resistor on the charger (R13 in its schematic) has to be removed so that the enable-pin timer can work.

Next steps

• Improve security: The modules implement a rolling code security system much like modern car keyless entry systems do. This hinges on the quality of the built-in AES encryption of the modules... but it's a million times better than the previous two iterations!
• More efficient remote: Add a battery and charger circuit so that the unlock code is only sent when the car is turned off.
• Unlock via internet: Having a web API available to unlock would open up possibilities for allowing delivery people in while away, or setting to unlock via the resident's phone in anticipation of visitors, etc.
• Pairing: A run-time procedure can be done by the user to synchronize the security between the latch controller and the remote.