STM32F103C8 Digital Thermometer with 74HC595 and 7-Segment Display

Project Overview

This project implements a digital thermometer based on the STM32F103C8T6 (Blue Pill) microcontroller. Temperature data is acquired from an LM35 analog temperature sensor and displayed on a 4-digit 7-segment display.

The display is controlled using two daisy-chained 74HC595 shift registers over SPI, minimizing GPIO usage. The ADC operates in single-conversion, software-triggered mode, with conversion results handled via end-of-conversion interrupts. All peripherals are configured directly at the register level, with STM32CubeMX used only for initial clock and pin configuration.

This project focuses on low-level embedded firmware design, real-time data acquisition, and efficient hardware interfacing.

Project Variants

• HAL-Based Version: This repository - CubeMX for initial setup + register-level peripheral configuration
• Bare-Metal Version: STM32_Digital_Thermometer_BareMetal - Complete register-level implementation (no CubeMX/HAL)

For a more advanced version of this project with multiple temperature sensors, Direct Memory Access (DMA), and data logging capabilities check out: 🔗 STM32_Thermometer_DataLogger

Hardware Components

• MCU: STM32F103C8T6 (Blue Pill)
  • STM32 Blue Pill Essential Circuits
• Temperature Sensor: LM35 (10 mV/°C)
• Display: 4-digit common-cathode 7-segment display
• Display Driver: 2 × 74HC595 shift registers (daisy-chained)
• Interface: SPI1 (data) + GPIO (latch control)

System Configuration

The system is configured for deterministic timing and stable ADC sampling.

• System Clock: 64 MHz (HSI PLL)
  • STM32 Clock Configuration
• SPI1:
  • Data Size: 16-bit mode
  • Baud Rate: 16 MBits/s (derived from APB2 prescaler)
  • Mode: Master, Full-Duplex
  • Clock Polarity: Low, Phase: 1 Edge
  • First Bit: MSB
• ADC1:
  • ADC Clock: 10.667 MHz
  • Resolution: 12-bit
  • Sampling Time: 28.5 cycles
  • Channel: PA0 (ADC1_IN0)

Development & Testing

• Firmware developed in Keil MDK-ARM 5
• Circuit simulation and validation in Proteus 8

Video Demonstrations

Proteus Demonstration

Complete_Proteus_Demo.mp4

Demonstration of the digital thermometer:

• Starts at 1.0 °C, demonstrating basic temperature display functionality
• Both positive and negative temperature readings
• Voltage divider circuit for negative temperature measurement
• Measurement accuracy within ±0.1°C tolerance

Project Schematic Diagram

*Schematic designed using EasyEDA*

The schematic shows the complete STM32F103C8T6 implementation including:

• Core Blue Pill circuits: Power supply, 8MHz and RTC clock, reset circuit, decoupling and BOOT circuit
• Display driver: Two daisy-chained 74HC595 shift registers controlling the 4-digit 7-segment display
• Temperature sensing: LM35 sensor with voltage divider for negative temperature measurement

Pinout Documentation

STM32F103C8T6 Pin Configuration

STM32 Pin	Function	Connected To	Purpose
PA5	SPI1_SCK	Both 74HC595 SH_CP (Pin 11)	Shift Clock - data synchronization
PA7	SPI1_MOSI	First 74HC595 DS (Pin 14)	Serial Data - segment/digit data
PA4	GPIO Output	Both 74HC595 ST_CP (Pin 12)	Latch Control - update outputs
PA0	ADC1_IN0	LM35 Output	Temperature sensor reading

First 74HC595 (Segment Control)

74HC595 Pin	Connection	Purpose
Q0	Segment A	Display segment A
Q1	Segment B	Display segment B
Q2	Segment C	Display segment C
Q3	Segment D	Display segment D
Q4	Segment E	Display segment E
Q5	Segment F	Display segment F
Q6	Segment G	Display segment G
Q7	Decimal Point	Display decimal point
Q7'/QH'	Second 74HC595 DS (Pin 14)	Daisy-chain connection

Second 74HC595 (Digit Control)

74HC595 Pin	Connection	Purpose
Q0	Digit 1 Cathode	Activate Digit 1
Q1	Digit 2 Cathode	Activate Digit 2
Q2	Digit 3 Cathode	Activate Digit 3
Q3	Digit 4 Cathode	Activate Digit 4
Q4-Q7	Not Connected	Unused outputs

Voltage Divider Circuit (Negative Temperature Measurement)

Component	Connection Points	Purpose
R1 (10kΩ)	3.3V → R1 → R2	Creates voltage divider for 1.65V reference
R2 (10kΩ)	R1 → R2 → GND	Sets midpoint voltage at 1.65V
LM35 GND	R1-R2 junction → LM35 GND	Shifts LM35 ground to 1.65V for negative readings

STM32 Pin Configuration

Quick Start

Prerequisites

• STM32F103C8T6 (Blue Pill) Microcontroller
• STM32CubeF1 Firmware Package (includes HAL drivers)
• Keil MDK-ARM 5 with STM32 support
• STM32CubeMX for initial project setup
• Proteus 8 to run the simulation file
• ST-Link Programmer/Debugger

Setup

1. Clone this repository

git clone https://github.com/rubin-khadka/STM32_Digital_Thermometer.git

2. Open CubeMX file STM32_Digital_Thermometer.ioc in STM32CubeMX

3. Generate code in CubeMX - this will open the project in Keil 5

4. Build the project and test it with:

   • Actual hardware, OR
   • Proteus 8 simulation Proteus_Simulation.pdsprj

Alternative Setup (Without Keil)

If you don't have Keil MDK:

1. In CubeMX file STM32_Digital_Thermometer.ioc, configure the project for STM32Cube IDE
2. Copy files from Core/Src and Core/Inc folders
3. Copy code from main.c file
4. Build in STM32Cube IDE and test

The project should work with either development environment.

Resources

• STM32F103 Datasheet
• 74HC595 Shift Register Datasheet
• LM35 Temperature Sensor Datasheet
• EasyEDA Website

What This Project Demonstrates

• Register-level STM32 peripheral programming without reliance on HAL drivers
• Interrupt-driven ADC data acquisition for real-time temperature measurement
• Efficient GPIO utilization using SPI-based shift registers
• Hardware–software co-design, including analog signal conditioning for negative temperatures

Project Status

• Status: Complete
• Version: v1.0
• Last Updated: December 2025

Contact

Rubin Khadka Chhetri
📧 rubin.robotic@gmail.com
🐙 GitHub: https://github.com/rubin-khadka