Star Tracker H7 with STM32H723

Star Tracker H7 with STM32H723

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Capture the beauty of the night sky like never before with this innovative star tracker built around the STM32H723 microcontroller from STM.

The tracker device is designed specifically to allow for sharp, accurate photos of the stars using your telescope.

With advanced features and user-friendly controls, our star tracker is the ultimate tool for amateur and professional astronomers alike.

Read further to learn more and take your stargazing to the next level.

Star Tracker H7 is controlling 4x stepper motor drivers for Altitude (Alt) , Azimuth (Az) , Field Rotation and Focus control and is using several sensors as data input for calculations.

Star Tracker H7 PCB Top

Star Tracker H7 PCB Bottom

Star Tracker H7 PCB Top and Bottom view 

General presentation based on the below slides:

1. INA280 high-precision current sense amplifier - is used to monitor the power consumption. It is connected to the MCU using I2C1 pins PB8 and PB9 plus a interrupt connected to PE4 as EXTI4

2. Motors power supply 10V at 10A - made using 2 LD1084V connected in parallel with load balancing done by LT1678 Dual, low noise operation amplifier.

3. 5V power regulator

4. 3.3V power regulator

5. Fan speed control with NTC - one connected to the power regulation and one to stepper controller.

6. TMC5160 stepper controller - one for each motor. They are connected to MCU using SPI4 plus DIR x, Enable x, CS x and Step x.

The controllers mounted on Alt/Dec and Az/Ra have their Step x pin connected to 32Bit timers while the other 2 are connected to 16Bit timers.

TMC 5160 TopTMC 5160 Bottom

7. For accurate positioning the feedback loop will use 23bit encodes for Altitude (Alt) , Azimuth (Az) , Field Rotation

They are using RS485 as communication protocol and this is not available on STM32H723, so a conversion is needed.

To convert U(S)ART to RS485, the Star Tracker H7 is using ST485 Low power RS-485/RS-422 transceiver and TXS0104E 4-Bit Bidirectional Voltage-Level Translator

UARTs 2, 3 and 7 are used and they are set up with Hardware Flow Control enabled - see Star Tracker H7.ioc file and KiCAD Schematic & PCB.

14Bit magnetic encoder connected on SPI can be used to reduce the overall cost of the Star Tracker H7.

23Bit Encoder14Bit Encoder

8. Connection for ESP-01 or ESP8266 that provides WiFi for Android app connection on USART1 using pins PB6 and PB7 for communication and PB5 to reset the ESP-01

Star Tracker H7 Communication

9. Microcontroller (MCU) STM32H723VGTx in LQFP100 package

10. 5V GPIOs - they are PD0, PD1 and PE10  passed through TXS0104E 4-Bit Bidirectional Voltage-Level Translator

11. Buzzer connected to PA7 configured as Timer14 Channel1 PWM

12. Opto-insulated cameras or devices, they are controlled by PB12, PB13, PB14 and PB15 configured as GPIO output

13. USB C connector

14. External sensors connector


BNO080 - triple-axis accelerometer/gyro/magnetometer - connected on I2C1 and PE2 EXTI2 PE3 GPIO output.

This sensor is used to get a fast and rough estimation of the telescope angular position and North Pole.


MS5611 - Temperature and Barometric Pressure Sensor - connected on I2C1

This sensor provides the temperature and pressure at observer location and the data provide is used to compensate the atmospheric refraction.

GPS L80-M39

Quectel L80-M39 GPS Receiver - connected on UART8.

This sensor is providing the Latitude, Longitude, Altitude and UTC Time all used in astronomical coordinates calculation.

All sensors enumerated above are mounted in a separate enclosure and connected to Star Tracker H7 with an IDC cable.

The EagleCAD board and schematic are available for download here.

Star Tracker Sensors

The enclosure for Star Tracker H7 sensor assembly in stp format can be downloaded here.

15. I2C1 connector - used to connect SSD1306 OLED 128 x 64 Dot Matrix LCD


16. Limit switches - connected to pins PD8 to PD15 as EXTI8 to EXTI15

Limit Switch

Limit Switch Schematic

They are used as safety measure in order to avoid mechanical parts clashes.

17. Programming header is exposing the following pins: NRST, SWCLK, SWDIO, SWO

18. Power and Status LED - connected to PE15 configured as GPIO output

19. GPIO pins PA0, PC3, PE5 and PE6 - to be configured and used if needed

20. 3.5mm jack connectors and status LEDs - they are opto-insulated Focus and Shutter PB12 (Focus 1) , PB13 (Shutter 2), PB14 (Focus 2) and PB15 (Shutter 2) configured as GPIO output

21. Power connector 12-24V

22. Stepper motor connectors - I use 400steps/revolution stepper motors 42BYGHM810

23. Encoder connectors


Star Tracker H7 3D Top

Star Tracker H7 3D Bottom

Star Tracker H7 enclosure Top and Bottom view 

The 3D model for enclosure can be downloaded here.

Star Tracker Pinout

Pinout Star Tracker H7.ioc

With stepper motors 42BYGHM810 capable of 400steps/revolution we have:

- 3240 arcsec/step

- we use 1/8 micro stepping setting and we get 405 arcsec/uStep

- with a gear ratio of 1/405 we get 1 arcsec resolution/uStep at output.

Star Tracker Mechanical parts 1

Both gear stages have Precision Anti-Backlash Gears

Star Tracker Mechanical parts 2

View inside one gear box - the overall gear ratio is 1/405


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