Skip to content

IR-Gate

Logo

IR-Gate is a LoRa-enabled 38 kHz infrared timing gate for lap timing and speed trap applications. It uses a TSOP34838 IR receiver and a bank of BIR-BM5381G IR LEDs with a Semtech LR1121-based LoRa module for wireless synchronization, powered by an onboard Li-Ion cell with USB-C charging.

Features

  • 38 kHz IR receiver: TSOP34838 — demodulated digital output directly to STM32
  • 6× BIR-BM5381G IR LEDs — N-MOSFET (PMV20XNER) switched, driven via LED_GATE signal from MCU
  • LoRa module: E80-900M (Semtech LR1121) — dual-band Sub-GHz + 2.4 GHz
  • 3-way RF switch: BGSA14GN10E6327XTSA1 (100 kHz – 5 GHz) — routes 868 MHz or 2.4 GHz to shared SMA antenna
  • Matching network: TBD inductors/capacitors for RF_MATCH between switch and antenna
  • MCU: STM32F103C8T6
  • USB-C with ESD protection (TPD4E001) and Li-Ion battery charging
  • Battery: 1S Li-Ion cell via onboard BAT1 terminal, TP4054-42 charger (500 mA)
  • 3.3V LDO: onsemi NCP114ASN330T1G
  • SWD debug header (6-pin, 1.27mm)
  • SMA edge connector for external antenna

Hardware

Microcontroller: STM32F103C8T6

  • 32-bit Arm® Cortex®-M3, up to 72 MHz
  • 64 kB Flash, 20 kB SRAM
  • SPI → E80-900M (MISO, MOSI, SCK, NSS, BUSY, LR_RST, IRQ) with 22Ω series damping resistors
  • GPIO: IR_RX (from TSOP34838), LED_GATE (IR LED drive), CTRL1/CTRL2 (RF switch)
  • Debug: SWD, SWO
  • 8 MHz crystal
  • Package: LQFP-48

LoRa Module: E80-900M (LR1121-based)

  • Dual-band: Sub-GHz 868 MHz + 2.4 GHz
  • LoRa / (G)FSK modulation
  • Dedicated 868M ANT and 2.4G ANT pins routed to BGSA14GN10E6327XTSA1 RF switch
  • SPI interface with BUSY/IRQ handshake
  • LR_RST controlled by STM32
  • +3V3 powered

3-Way RF Switch: BGSA14GN10E6327XTSA1

  • Operating range: 100 kHz – 5 GHz
  • CTRL1 / CTRL2 logic from STM32 selects RF_868MHz, RF_2.4GHz, or RF_MATCH path
  • RFC → antenna matching network → SMA connector

IR Receiver: TSOP34838

  • 38 kHz carrier frequency demodulation
  • Active-low digital output → STM32 IR_RX pin
  • 1µF decoupling on VS pin
  • +3V3 powered

IR LED Bank

  • 6× BIR-BM5381G infrared LEDs (850 nm)
  • Each LED has a dedicated 62Ω series current-limiting resistor
  • All anodes tied to +VBATT (unregulated battery voltage for maximum drive current)
  • Common cathode switched via PMV20XNER N-MOSFET (Q2), gate driven by STM32 LED_GATE with 62Ω gate resistor (R17)

Power Architecture

  • Battery: 1S Li-Ion via BAT1 onboard terminal + 22µF bulk cap
  • Charger: TP4054-42-SOT25-R, 500 mA (set by 2.2kΩ PROG resistor), charged via USB-C VBUS
  • Power switch: DMG2305UX-7 P-MOSFET (Q1), gate driven by USB-C FET_Gate / FET_Drain logic
  • 3.3V rail: NCP114ASN330T1G LDO — feeds STM32, E80-900M, RF switch, and TSOP
  • USB-C: USB4105-GF-A, ESD: TPD4E001RDBVR, CC: 5.1kΩ pull-downs (UFP)
  • Charge indicator: red LED (D1) with 1kΩ

Headers & Connectors

Connector Type Signals
Antenna CON-SMA-EDGE-S RF_ANT (50Ω)
USB-C USB4105-GF-A VBUS, D+/D−, CC1/CC2
SWD 6-pin pad (1.27mm) VCC, NRST, GND, SWO, SWCLK, SWDIO
Battery BAT1 terminal +VBATT, GND

Schematics / PCB / 3D

Keybinds

  • Select LMB
  • Zoom In/Out Scroll
  • Pan RMB Drag
  • Rotate in 3D LMB Drag
  • Reset View R
  • Switch to 2D 2
  • Switch to 3D 3
  • Next / Previous Layer +/-
  • Flip Board Top / Bottom F

Quick start

  1. Charge the Li-Ion cell via USB-C before first use
  2. Connect a UHF/2.4 GHz antenna to the SMA connector
  3. Flash firmware via SWD (STM32CubeIDE / OpenOCD)
  4. Align the IR LED bank and TSOP34838 receiver across the timing gate gap
  5. The MCU modulates the IR LED bank at 38 kHz — the TSOP demodulates and outputs a clean digital break-beam signal
  6. Timing events are transmitted wirelessly via LoRa to the paired timing system

Safety

  • IR LEDs are driven directly from VBATT — do not stare directly into the LED array during operation
  • Ensure antenna is connected before enabling LoRa TX — transmitting without antenna load may damage the LR1121 PA
  • Li-Ion cell must be a protected cell or have external protection — the TP4054 provides charge protection but not cell-level overdischarge protection
  • 38 kHz IR output is eye-safe at normal operating distances but avoid close-range direct exposure

🔜 Firmware

Firmware for IR break-beam detection, timing event handling, and LoRa synchronization coming soon.

Photos

Downloads

IR-Gate.zip 

git clone https://github.com/sanko0112/IR-Gate.git
cd IR-Gate