One PCB, Many Uses: The Power of Solder Bridges

By adding or removing a few key components and shorting specific solder jumpers, the same board can function in six different modes:

(💡 Short = connect with solder)

1️⃣ Standard Relay Mode (The Original Design)

 ✔  Install: J1 (screw terminal) and K1 (relay)
❌ Remove: J2 (fan header) and R3 (10K pull-up resistor)
 ⚙  Behavior: The board switches a relay via USB commands.

2️⃣ Solid-State Relay (SSR) Mode

 ✔  Short: B6
 ✔  Install: J1 (screw terminal, now outputting 5V for SSR)
❌ Remove: J2 and R3
 ⚙  Behavior: Similar to option 1, but the relay provides a switched 5V between Pin 2 and GND.

3️⃣ Simple Fan Control (2- or 3-Pin Fan)

 ✔  Install: J2 (fan header)
 ✔  Short: B4
❌ Remove: J1, K1, and R3
 ⚙  Behavior: The board powers a 2- or 3-pin fan with PWM speed control applied to the power pin.

4️⃣ PWM Fan Control (4-Pin Fan)

 ✔  Short: B5
 ✔  Install: J2 (fan header) and R3 (10K resistor in relay coil holes)
❌ Remove: J1 and K1
 ⚙  Behavior: Speed control by PWM on Pin 4 (PWM input).

5️⃣ Servo Motor Control

 ✔  Short: B5 & B1
 ✔  Install: J2 (wired as a servo header)
❌ Remove: J1, K1, and R3
 ⚙  Behavior: TIM2 generates servo control PWM (50Hz, 1-2ms pulse width).

6️⃣ UART Communication Interface

 ✔  Short: B2 & B3
 ✔  Install: J1 (now a UART terminal block)
❌ Remove: J2 & K1
 ⚙  Behavior: J1 now provides TX/RX pins for serial communication.

Ditching the Crystal: How USB Syncs Itself

Most USB-enabled microcontrollers require a crystal (typically 12MHz) for accurate timing.
But here’s the cool part: The STM32F042K6U6 doesn’t need one. It can self-synchronize using its internal RC oscillator.

How USB Self-Synchronization Works

🔹 USB Full-Speed mode (12Mbps): The host sends a Start Of Frame (SOF) pulse every 1ms.
🔹 The STM32 uses these pulses to fine-tune its internal oscillator.
🔹 This eliminates the need for an external crystal, reducing component count without losing accuracy.
That means fewer parts, lower cost, and easier assembly.

The Magic Button: One Switch, Two Tricks

One of my favorite design choices was a dual-purpose tactile switch connected to the BOOT0 pin.

Feature 1: Enter Bootloader Mode (Firmware Update)

Hold the button while plugging in USB → The STM32 enters bootloader mode.
Now you can flash firmware over USB, no programmer needed!

Feature 2: Manual Override (Toggle Outputs)

When the STM32 is already running firmware, pressing the button toggles the relay or fan manually—without USB commands.
💡 BOOT0 is repurposed as a general-purpose input once firmware is running.

USB Command Interface

The firmware uses USB CDC (Virtual COM Port) to communicate via serial commands; accepts structured ASCII commands (easy to use in Bash, Shell and Serial Terminal).

Example Commands (Send via Terminal/Script)

How to use on Linux Systems in Bash

// To send a command
echo '#S1!' > /dev/serial/by-id/usb-IT_Logic_USB_Relay-if00

// To read a result and store in a variable FAN_TACH:
echo '#T!' > /dev/serial/by-id/usb-IT_Logic_USB_Relay-if00
read -d'~' -t1 FAN_TACH < /dev/serial/by-id/usb-IT_Logic_USB_Relay-if00
echo $FAN_TACH

How to use on Mac Os in Bash

// To send a command
echo '#S1!' > /dev/cu.usbmodem8301
...