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TeamRadio

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FS-TeamRadio is a compact UHF handheld radio board built around the SA818S module, designed for Formula Student driver-to-pit communication. It integrates a 1W PA, TPA6139A2 headphone amplifier, internal/external microphone switching, attenuated speaker output, and a 16340 Li-Ion battery with onboard charging.

Features

  • Radio module: SA818S (UHF, 400–480 MHz, 1W / 0.5W selectable)
  • Headphone amplifier: TPA6139A2 with volume potentiometer
  • Internal / External MIC select via onboard SPDT switch
  • Attenuated audio output (voltage divider) for Bluetooth speaker input
  • Speaker jack (3.5mm) and headset jack (3.5mm, TRRS)
  • High-pass filter on microphone input (C24 + R20)
  • PTT — dedicated tactile button + external JST-XH 2-pin connector
  • 2× WS2812B ARGB LEDs — RSSI indicator + battery voltage indicator
  • MCU: STM32F103C8T6 (controls SA818S, audio mux, PTT, LEDs)
  • Power: 16340 Li-Ion (3.0–4.2V), onboard TP4054 charging via USB-C
  • 5V boost converter: TI TPS61023 (feeds SA818S and headphone PA)
  • 3.3V LDO: onsemi NCP114 (feeds MCU and logic)
  • USB-C with ESD protection (TPD4E001) and battery charging
  • TX power select: 1W / 0.5W solder bridge (SA818S H/L pin)
  • SWD debug header (6-pin, 1.27mm)
  • SMA edge connector (50Ω, UHF antenna)
  • M2 mounting holes (×4)

Hardware

Radio Module: SA818S

  • UHF band: 400–480 MHz
  • Output power: 1W (H/L = HI) or 0.5W (H/L = LO) — selectable via solder bridge
  • UART configuration interface (RX/TX to STM32 USART)
  • PTT pin: active-low push-to-talk
  • AF_OUT: audio output → attenuation network → volume pot → TPA6139A2
  • MIC_IN: microphone input from onboard MIC or external headset (SPDT switch)
  • AUDIO_ON / AF_EN: controlled by STM32 IO
  • ANT: single-ended 50Ω → SMA edge connector
  • SA818_EN / SA818_PWR: enable and power controlled by STM32

Microcontroller: STM32F103C8T6

  • 32-bit Arm® Cortex®-M3
  • Up to 72 MHz
  • 64 kB Flash, 20 kB SRAM
  • USART1 → SA818S (RX/TX)
  • GPIO: PTT, SA818_EN, SA818_PWR, H/L power select, AMP_EN, MIC switch
  • ADC1_IN1: battery voltage sense (VBATT divider, 10k/24.9k)
  • PWM: ARGB LED data (WS2812B chain)
  • Debug: SWD, SWO
  • 8 MHz crystal
  • Package: LQFP-48

Headphone Amplifier: TPA6139A2PWR

  • Stereo capless headphone amplifier
  • Input: SA818S AF_OUT → attenuation network (10k/2.2k) → volume pot (RK09K113, 10kΩ lin)
  • MUTE pin controlled by STM32 (AMP_EN)
  • GAIN pin configurable
  • Output: 3.5mm headset jack (J4, PJ-320D TRRS) and speaker jack (J5, PJ-320D)
  • Powered from +VDDA (3.3V)
  • High-pass filter on MIC input: 1µF series cap + 27kΩ

Microphone Path

  • Internal MIC: CMEJ-0627-42-P onboard electret microphone
  • External MIC: headset jack ring contact (J4)
  • Selection: SS-12D10G4 SPDT slide switch — routes either source into SA818S MIC_IN
  • 1µF DC-blocking cap on both MIC inputs

Power Architecture

  • Battery: 16340 Li-Ion (VAPCELL T8 or compatible), 3.0–4.2V
  • Charging IC: TP4054-42 (500mA), charged via USB-C VBUS
  • Charge LED: red LED (D1) with 10kΩ
  • Power switch: SS12D10G6 SPDT slide switch → DMG2305UX P-MOSFET for load switching
  • 5V boost: TI TPS61023DRLR — 1µH, 4.5×4.1mm inductor, 6A peak, 27mΩ — feeds SA818S (+VDD) and headphone PA
  • 3.3V LDO: onsemi NCP114ASN330T1G — feeds STM32 and logic (+VDDA)
  • USB-C: USB4105-GF-A, ESD: TPD4E001RDBVR, CC: 5.1kΩ pull-downs
  • Solder bridge SB1: selects USB power path (3-way)
  • VBATT sense: 10k / 24.9k divider → STM32 ADC1_IN1

Indicators

LED Type Function
RSSI WS2812B ARGB Signal strength indication (controlled by MCU)
VBATT WS2812B ARGB Battery voltage level indication (from ADC reading)

Audio Signal Path

SA818S AF_OUT → 0.1µF DC-block → 10k/2.2k attenuator → 10kΩ volume pot → TPA6139A2 IN → headphone jack / speaker jack

MIC Signal Path

Internal MIC ──┐
               SPDT switch → 1µF DC-block → high-pass filter → SA818S MIC_IN
External MIC ──┘

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. Insert a charged 16340 cell into the battery holder
  2. Slide the power switch to ON
  3. Connect a UHF antenna to the SMA connector
  4. Select internal or external MIC using the Mic Switch
  5. Connect headphones or a speaker to the appropriate 3.5mm jack
  6. Press PTT (tactile or external connector) to transmit
  7. Configure SA818S frequency, CTCSS, and squelch via UART using STM32 firmware

Safety

  • Always connect an antenna before transmitting — transmitting into an open SMA will damage the SA818S PA
  • Maximum TX power is 1W — keep the antenna away from the body during extended TX
  • USB-C charges the 16340 at 500mA — use a quality cell rated for continuous charge current
  • Do not short the battery terminals — no cell-level short-circuit protection on this board
  • VBATT LED turns red when battery is low — recharge before the cell drops below 3.0V

🔜 Firmware

Firmware for SA818S configuration, PTT handling, RSSI readout, and LED indication coming soon.

Photos

Downloads

TeamRadio.zip 

git clone https://github.com/sanko0112/TeamRadio.git
cd TeamRadio