Light Doorbell Hardware

This repository contains the KiCad design files (schematics and 4-layer circular PCB) for the Light Doorbell prototype: a battery-powered, LED-signaling doorbell based on a STM32L051K8T6 microcontroller and RFM69HCW 433 MHz radio module.

A light doorbell is useful in many situations: notifying a user wearing headphones, assisting people with hearing impairments, or simply providing a silent visual alert...

Overview

The prototype operates both as a transmitter and receiver.
When the central button is pressed, it sends a coded message to another board listening on the same frequency. Upon receiving the message, the board flashes its dual-color LEDs, green or red, depending on the battery level.

The device is rechargeable via a USB-C connector, and works with a range of several tens of meters in open space.

Features

• Ultra-low-power STM32L051K8T6 ARM Cortex-M0+, 64 KB Flash, 8 KB RAM
• RFM69HCW 433 MHz transceiver module
• Li-ion / Li-po battery charger and management IC
• 3.0V LDO voltage regulator to step down USB or battery voltage
• Load sharing circuit to seamlessly switch power source between USB and battery
• Battery voltage monitoring
• 433 MHz helical antenna
• Dual-color LEDs, green and red for visual feedback
• USB-C connector for charging
• Large mechanical push-button
• 10-pin ARM SWD debug / programming port
• UART port for optional serial communication
• Multiple test points and jumpers for current measurements, debug, and isolation
• Designed for hand soldering and easy prototyping
• Optimized for low power consumption

The current version focuses on development and testing. A future version could be further miniaturized by removing debug features and using smaller components...

Related projects and documentation

• Light Doorbell SW: Firmware of the project, based on the STM32 CubeHAL
• STM32L051K8T6 Datasheet
• RFM69HCW Datasheet
• Adafruit Feather M0 Radio with RFM69 Packet Radio
• Microchip (AN1149) - Designing A Li-Ion Battery Charger and Load Sharing System With Microchip’s Stand-Alone Li-Ion Battery Charge Management Controller
• Zak’s Electronics Blog - A Lithium Battery Charger with Load Sharing.
• Hackaday - Embed with Elliot: Debounce your Noisy Buttons, Part I.
• RefCircruit - Standard Simplest Battery Voltage Monitoring Circuit.