Laboratory Power Supply Meter and Protection

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Project author: Aleksei SUBBOTIN

Copyright © 2026 Aleksei SUBBOTIN.

This project is a low-cost, high-accuracy voltage/current meter and active protection controller for a laboratory power supply. It uses an STM32F103C8T6 Blue Pill, INA226, 160x128 ST7735 TFT, rotary encoder, and passive buzzer.

On the hardware for which this project was developed, communication with the INA226 through the STM32 hardware I2C peripheral could hang. The firmware works around that issue with a complete software I2C implementation (bit-banging) on PB8 and PB9.

Only the I2C communication is implemented in software. Voltage/current conversion, shunt-threshold comparison, and the physical ALERT output are handled in hardware by the INA226.

Project video: YouTube

Features

• voltage, current, power, accumulated capacity, and current-limit display;
• rotary-encoder current-limit adjustment;
• INA226 hardware shunt over-limit detection (SOL);
• active-low, latched (LEN) physical ALERT output;
• ALERT! screen and intermittent audible warning;
• manual reset with the encoder push button;
• automatic re-trigger if the overload remains after reset;
• software I2C workaround for the hardware-I2C lockup;
• ready-to-flash HEX and BIN files.

The assembled unit has been tested to keep protection and the physical ALERT output active until the reset button is pressed. After reset, the INA226 checks the current again and immediately re-triggers if the overload is still present.

Firmware settings

Setting	Value
Initial current limit	0.15 A
Minimum / maximum limit	0.01 A / 2.35 A
Normal / fast encoder step	0.01 A / 0.1 A
Shunt resistance	0.033 ohm
Current resolution	0.001 A
INA226 7-bit address	0x40
Address format used by the code	0x80
INA226 Configuration register	0x484F
Voltage reading correction	+0.015 V
Encoder debounce	15 ms
Passive buzzer frequency	approximately 2.5 kHz

0x484F selects 128-sample averaging, 204 us bus conversion, 204 us shunt conversion, and continuous shunt-plus-bus measurement. A complete averaged cycle takes approximately 52.2 ms. Other values such as 0x4127, 0x4527, or 0x4927 are alternative conversion-time and averaging settings, not newer firmware versions.

Blue Pill pin map

Blue Pill	Function	Connected signal
PA0	EXTI0 input	encoder CLK
PA1	digital input	encoder DT
PA2	active-low reset input	encoder SW
PA3	TIM2_CH4, approximately 2.5 kHz PWM	passive buzzer or driver
PA5	SPI1_SCK	ST7735 SCL/SCK/CLK
PA7	SPI1_MOSI	ST7735 SDA/MOSI/DIN
PB0	digital output	ST7735 RES/RST
PB1	digital output	ST7735 DC/A0/RS
PB8	software I2C SCL	INA226 SCL
PB9	software I2C SDA	INA226 SDA
PB10	digital output	ST7735 CS/SS
3.3V	logic supply	display, INA226, encoder
GND	common ground	all modules and protection circuit

Although CubeMX assigns hardware I2C1 to PB6/PB7, the working INA226 code does not use those pins.

ST7735 wiring

ST7735 signal	Blue Pill	Purpose
VCC	3.3V	display supply
GND	GND	common ground
SCL, SCK, CLK	PA5	SPI1 clock
SDA, MOSI, DIN	PA7	SPI1 data from STM32
RES, RST, RESET	PB0	hardware reset
DC, A0, RS	PB1	command/data selection
CS, SS	PB10	chip select
LED, BL, BLK	3.3V	backlight
MISO, SDO	not connected	display data is not read

Use 3.3 V for the module supply and logic. Add a series resistor to the backlight if the display module does not already include one.

Encoder wiring

Encoder signal	Blue Pill	Purpose
+, VCC	3.3V	module supply
GND	GND	common ground
CLK	PA0	EXTI0 encoder pulses
DT	PA1	rotation direction
SW	PA2	protection reset button

The inputs use internal pull-ups. The push button must connect PA2 to GND when pressed.

INA226 wiring

INA226 VSSOP-10 pins

Pin	Name	Connection
1	A1	GND for address 0x40
2	A0	GND for address 0x40
3	ALERT	10 kohm pull-up to 3.3 V and external cutoff input
4	SDA	Blue Pill PB9
5	SCL	Blue Pill PB8
6	VS	Blue Pill 3.3V
7	GND	common GND
8	VBUS	measured bus, normally the load side of the shunt
9	IN-	load side of the shunt
10	IN+	source side of the shunt

Typical INA226 module

Module signal	Connection
VCC, VS	Blue Pill 3.3V
GND	common GND
SCL	Blue Pill PB8
SDA	Blue Pill PB9
A0, A1	GND for address 0x40
ALERT, AL	10 kohm pull-up to 3.3 V and external cutoff circuit
IN+, VIN+	source side of the shunt
IN-, VIN-	load side of the shunt
VBUS	load side (IN-) if not already connected on the module

The INA226 VS supply accepts only 2.7-5.5 V and must not be connected to the power supply's high-voltage rail. VBUS is the measurement input and is rated for bus voltages up to 36 V.

SCL, SDA, and ALERT are open-drain signals and require pull-ups to 3.3 V. Many INA226 modules already contain I2C pull-up resistors.

ALERT and reset behavior

1. The STM32 calculates threshold voltage = current limit * 0.033 ohm.
2. It programs the INA226 Alert Limit register and enables SOL + LEN.
3. The INA226 compares the shunt voltage against the threshold in hardware.
4. An overload sets AFF and drives the open-drain ALERT output low.
5. The STM32 detects AFF over software I2C, displays ALERT!, and starts the buzzer.
6. Pressing the encoder button on PA2 calls the reset routine and re-enables SOL + LEN.
7. Normal current resumes operation; a continuing overload re-asserts the alert on the next INA226 conversion.

The INA226 does not provide a separate "clear ALERT" I2C command. With LEN = 1, reading the Mask/Enable register (0x06) acknowledges the event and clears the latch according to the Texas Instruments documentation. This is what "software reset of ALERT" means in this firmware.

The physical ALERT pin is not connected to a separate Blue Pill GPIO in the current design. The MCU reads AFF through I2C, while ALERT feeds an external hardware cutoff circuit. Do not connect it directly to a power MOSFET gate or a high-voltage node; use a suitable driver, optocoupler, comparator, and/or hardware latch.

Buzzer

PA3 generates approximately 2.5 kHz PWM for a passive buzzer. A small piezo sounder may be connected between PA3 and GND. Use a transistor driver for higher-current or electromagnetic sounders.

Ready-to-flash firmware

The firmware directory contains:

• LBP_Indicator_v1.1.hex for ST-LINK or STM32CubeProgrammer;
• LBP_Indicator_v1.1.bin, flash address 0x08000000;
• a HEX-only archive;
• a complete forum package with source, PDF, and firmware;
• SHA-256 checksums.

The complete Russian wiring and operation manual is available as LBP_Indicator_RU.pdf.

Main source files

• Core/Src/main.c: main loop, UI, and software alert state;
• Core/Src/INA226.c: measurements and INA226 alert setup;
• Core/Src/soft_i2c.c: software I2C on PB8/PB9;
• Core/Src/encoder.c: encoder, push button, and buzzer;
• Core/Src/st7735.c: display driver;
• Core/Src/spi.c: display SPI1 configuration;
• LBP_Indicator.ioc: STM32CubeMX configuration.

Links

• Project video on YouTube
• GitHub repository
• Texas Instruments INA226 documentation