PicoBETH (Raspberry Pico Badminton Electronic Tension Head) is an open-source project that allows hobbyist stringers who enjoy stringing but only have mechanical stringing machines (drop-weight, manual crank) to create their own electronic tensioning head. If you have basic programming skills, this project can be easily completed.

Design philosophy: Economical, Intuitive, Accurate

• LB/KG display and setting
• Pre-Stretch function
• Constant-Pull system
• Knot function
• Tension adjustment manually during tensioning
• Tension calibration
• Stringing timer
• Tension timer
• Tension counter and boot counter
• Detailed recording of tensioning logs
• Pull speed of the string(Switch on the TB6600 stepper motor driver)

• 0.05LB High Precision
  • Achieved with the Sparkfun HX711 at 94Hz and software version V2.2 and later.
• Low Power Consumption
  • Uses a DC12V3A power supply.
  • Power consumption is 7W in standby mode and 15W when tensioning at 35LB.
• Real-time UPS Redundancy (Uninterruptible Power Supply)
  • Uses 3 x 18650 batteries to ensure at least one complete racquet stringing.
• Compact and Space-Saving
  • Dimensions are approximately 38(L) x 15(W) x 9(H) CM.(excluding clamp head)

A year ago, due to company club activities, I started playing badminton. Although my badminton skills weren't great, I became fascinated with stringing. I purchased a drop-weight stringing machine and initially planned to buy an electronic tensioning head. However, I later thought about using my knowledge to create this project on the Raspberry Pico, incorporating a load sensor, several microswitches, and buttons.

Drop-weight stringing machine and modification parts

Modification completed (Under Development)

Final machine How to make step by step

Stringing demonstration video

If your badminton stringing machine structure is not robust, I strongly advise against undertaking this project. A weak fixing platform can deform when under tension, causing the racket frame to become rounded and the tension to decrease. As a result, the machine compensates by reinforcing the tension, leading to a cycle that ultimately results in the badminton racket breaking.

1. Use left and right keys to set pounds, kilograms, and the tens, units, and decimal places of pre-stretch.
2. Use up and down keys to adjust the selected settings.
3. Stringing timer function: Start timing by pressing the exit key, stop timing by pressing it again, and reset the timer on the third press.
4. The Pre-Stretch function (PS) and Knot function (KT) are switched using the up and down keys. After using the Knot function, it will automatically switch back to the Pre-Stretch function.

1. When the set tension is reached, automatically enter the constant-pull mode. Increase tension if it's insufficient, decrease tension if it's too high, until pressing the button on the clamp or exit button to end the tensioning mode.
2. Press the center button of the five-way key to enter manual fine-tuning mode. The constant-pull mode will be canceled at this time, and tension can be manually adjusted with the up and down keys. Press the center button of the five-way key again to re-enter constant-pull mode.
3. When the specified tension is reached, the countdown timer will appear.

1. UN: Select pounds or kilograms when setting.
2. AT: Default Constant-Pull Switch.
3. BZ: Active buzzer Switch.
4. FT: Amplitude of fine-tuning when reaching the specified tension.
5. HX: Calibration of the load sensor for HX711 (see the Final Settings chapter for details).
6. I: System information.
7. T: Tensioning Count/Log interface

On the settings screen, use the left and right keys to select the tensioning count, then press the center key of the five-way key to enter the tensioning log recording page. On the page, use the left and right keys to browse through the log records.

TIMER: If the timing function is enabled, display the time of tensioning.
LB: Set tension/stop tension.
PS: Set pre-stretch value.
FT: Increase tension fine-tuning count/decrease tension fine-tuning count/fine-tuning parameters.
C/H: CC parameter/HX parameter.

Main materials

1. Raspberry Pico H
2. CBX/SGX 1610 ballscrew 200MM sliding table
3. NEMA23 57x56 stepper motor (2-phase 4-wire 1.8° 1.2Nm)
4. TB6600 stepper motor driver
5. NJ5 20KG load sensor (YZC-133)
6. HX711 module (SparkFun)
7. 2004 i2c LCD
8. Wise 2086 bead clip easy head
9. Five-way key module
10. Button
11. Micro switch
12. Active buzzer (active high trigger)
13. Tri-color LEDs
14. 12V 18650 UPS Battery Box

Main BOM List Price Reference

There are many styles of sliding tables, and this project uses the CBX/SGX 1610 ballscrew 200MM sliding table. It is recommended to purchase the CBX version with a bearing fixing seat. Some sliding tables without bearing fixing seat may experience issues under high-speed and high-tension conditions.

Bearing Fixing Seat

TB6600 is a small, economical stepper motor driver used for 42 and 57 type stepper motors. It is very cheap on online stores. It is recommended to buy TB6600 labeled as "upgraded version" or "enhanced version". Some very cheap TB6600s may have noticeable electrical noise, and I am not aware of any unknown issues.

HX711 is a simple and easy-to-use amplifier for weight sensors, commonly used in high-precision electronic scales. In this project, it is used to measure the tension of the strings. I have tested many HX711 circuit boards produced by various manufacturers and found a serious issue: many HX711 circuit boards from different manufacturers tend to drift. Of course, this drifting issue can be fixed. I will produce a dedicated episode on my YouTube channel on how to fix this issue. It is recommended to directly purchase the HX711 Load Cell Amplifier produced by SparkFun, as it has better quality. Before fabricating the stringing machine head, use the drift test program taught in EP. 3 to test the stability of this board. If you encounter any issues, you can leave a comment on the video.

I tested the HX711 circuit board

Video Series of the Production Process (Continuously Updating)

Use Thonny to save the following code files to the Raspberry Pico. The src folder contains relevant libraries for hx711 and 2004 LCD.

1. main.py
2. src\hx711.py
3. src\lcd_api.py
4. src\pico_i2c_lcd.py

This project demands a higher standard for the HX711, and it is recommended to use the more stable quality provided by SparkFun.

The default setting for SparkFun's HX711 is 10Hz. To enable 80Hz, you'll need to cut the connection wire at the green arrow as indicated below.

The quality of each HX711 unit varies. Before installing the equipment, it's advisable to test the stability using a breadboard. A normally stable board should not drift by more than 1G over the course of a whole day.

Components can be freely arranged. The diagram below shows the positioning hole diagram for reference. Refer to the tutorial video for usage instructions.

Gerber PCB DOWNLOAD

Gerber PCB BTN DOWNLOAD

Upon completing assembly and powering on the machine for the first time, please conduct tests on all buttons, front and rear limits, and HX711 sensors as instructed on the screen.

FT Parameter: It determines the magnitude of adjustments after reaching the specified tension. A too large value can cause repeated tension adjustments, while a too small value increases the number of fine-tuning iterations required to reach the specified tension.

SW V2.2 and later have the recommended FT parameters:

SW prior to V2.12 have the recommended FT parameters:

HX711 tension sensor calibration coefficient. It is necessary to recalibrate it the first time you use it or when replacing the tension sensor or HX711 circuit board.

After version V2.2, due to the improved fine-tuning accuracy, enabling the auto-calibration feature will result in more precise corrections.

Calibration Steps:

1. Go to the settings page and ensure the Constant-Pull feature is enabled.
2. Set the HX parameter to 20.00 on the settings page.
3. Return to the main menu and set the tension to 20.0 lb with a Pre-Stretch of 10%.
4. Attach one end of the external tension gauge to the stringing machine and the other end to the badminton string.
5. Start tensioning, and once it stabilizes, record the lowest reading from the external tension gauge.
6. Enter the recorded tension gauge value on the settings page and press the S button to save.

Reference video

Calibration steps for versions prior to V2.12:

Calibration method:

1. Temporarily disable the Constant-Pull function on the settings page.
2. Set the HX parameter to 20.00 on the settings page.
3. Return to the main menu and set the tension to 20.3 lb with a Pre-Stretch of 10%.
4. Attach one end of the external tension gauge to the stringing machine and the other end to the badminton string.
5. Start tensioning, and when the LCD displays below 20.0 lb(19.9), note down the reading on the external tension gauge.
6. Enter the recorded tension gauge reading on the settings page and re-enable the constant-pull function.

Reference video

After completing the tension calibration, check the degree of drift in the tension. The normal drift should be within approximately ±0.05LB, as shown in the video below. After passing the test, use the old racquet stringer to test a few times. If there are no issues, you can officially start using it.

Reference video

The Reliability Testing Mode, introduced in version V2.4, can automatically simulate stringing tensioning to check for any abnormalities in newly assembled or hardware-modified machines.

When the version information appears at startup, press and hold the Up button until you hear a long beep, then release it. After securing the string, press the Set button to start the test.

The system will automatically cycle through tensioning from 20LB to 30LB, with a fixed 10% pre-stretch. After holding the tension for 3 seconds, the tension will release. If any abnormalities are detected, the process will stop automatically, and an error message will be displayed. The test will continue indefinitely unless the Set button is pressed when prompted.

It is recommended to test at least 1000 tensioning cycles (approximately 4 hours) without any abnormal interruptions. A properly assembled machine should not experience any interruptions during this period.

Reference video

Observe for any abnormal vibrations or noises during each use. Visually inspect the screw's condition every month. Check for dust or other foreign substances on the screw and ensure sufficient lubrication. If yellowish-brown lubricating oil appears around the screw and the walking surface appears smooth, it indicates good lubrication. The recommended viscosity for the lubricating oil used in ball screw maintenance is 30~40 cSt.

After replacing any components during maintenance, enter the hardware function testing mode at startup to retest all components for proper functionality.

Usage: When the version information appears at startup, press and hold the middle setting button until a long beep is heard, then release it.

You can restore the factory default settings without a computer by recreating the config.cfg file (the original configuration file will be renamed to config.cfg.bak). After restoring the default settings, be sure to set the FT parameters and recalibrate the HX tension parameters.

Usage: When the version information appears at startup, press and hold the exit button until a long beep is heard, then release it.

Reference video:

Please follow the steps below to update the firmware:

• Back up all files from the Raspberry Pi Pico to your computer.
• Upload the new version of main.py to the Raspberry Pi Pico.
• Perform hardware testing
• Conduct reliability testing
• Test stringing with an old racket at least once.

A: It is recommended to first watch EP.1 ~ EP.3 of the project compilation on my YouTube channel. You will need to purchase materials such as Raspberry Pico, HX711 load cell amplifier, NJ5 load sensor (YZC-133), TB6600 stepper motor controller, and 57x56 stepper motor. These materials are easy to prepare and not expensive. If the example program runs smoothly, you can prepare the remaining materials. The subsequent production process leans towards mechanical machining. Additionally, you will need tools such as a bench drill, angle grinder, soldering iron, and some basic mechanical machining skills. Follow the tutorial videos step by step to complete the project.

A: In theory, you can switch to DM542C, but the driving method may need to be modified. For example, parameters such as MOTO_FORW_W, MOTO_BACK_W for controlling forward and reverse in the code, and MOTO_SPEED_V1, MOTO_SPEED_V2 for controlling speed may need to be adjusted. It is recommended to first modify the example program in EP.2 to ensure that this driver can drive the motor normally and that there is no abnormal noise from the slide during movement before transplanting it into the main program. Although I haven't tried it myself, there have been successful ports by other branch developers, which you can refer to in the Pico-Badminton-Stringer project.

A: No, you cannot. Initially, this project was also made with a sampling frequency of 10Hz, which can be used normally. However, after testing, it was found that a sampling frequency of 80Hz provides more precise, delicate, and faster response control of tension (the time difference between detecting the specified tension and instructing the Raspberry Pico to stop the motor rotation at 10Hz is about 1.3 times that of 80Hz). Therefore, in version 1.96, I added a check for the 80Hz action.

A: You can refer to the test program in EP.3. After testing, the normal drift value of HX711 at 80Hz is about 0.5 ~ 1 gram. Therefore, in version 1.96, I added a drift value sampling during startup for 1 second. If it exceeds 1 gram, it cannot be used. If the check passes, generally speaking, the real-time tension displayed in the lower right corner of the LCD within -10 ~ 10 grams during standby is normal linear drift.

A: Of course, you can use HX711 load cell amplifiers from other brands, but the premise is that they can pass the test program in EP.3. In my experience, other brands' normal HX711s will be as stable as SparkFun's. Unfortunately, other brands may have many defective products that drift. If the drift exceeds 50 grams, it is an error value of 0.1 lb, which will cause repeated fine-tuning of the Constant-pull system.

A: Recently, some people reported passing the EP.3 HX711 test, but still encountering this error during startup. This check is performed on a value obtained by the Wheatstone bridge on the load sensor (YZC-133) after being amplified by the HX711, serving as the initial reference for zero-point calibration. In the EP3 test program, this output is displayed using V0. During development, I found that the value of the HX711 I purchased that tends to drift was negative, and after repairing these drifting boards, the value became positive again. Therefore, I added this check condition. Perhaps my test sample size was not large enough, and positive or negative values do not affect usage. If subsequent confirmation shows that negative values are not a problem, I will remove this check. If you encounter this issue now, please comment out this check (lines 447 and 448 in version v2.12), and then report back to me if it works properly.

A: This has also been something I've always wanted to do. The Wise 2086 bead clip easy head is the most expensive hardware in the entire project, but it's also because it can be easily installed on the NJ5 load sensor (YZC-133) and has excellent clamping functionality. So far, I haven't figured out how to replace it. If you have a good bead clip head design, you can install it yourself. Just make sure to pay attention to the precautions mentioned in EP.9.

A: If all high-quality components are used, the durability should theoretically be quite high. Even if repairs are needed in the future, all the parts are relatively inexpensive and easy to replace. The machine is currently undergoing reliability testing. As of today (2024/08/17), the tensioning count has exceeded 50,000 cycles, and no abnormal failures have occurred. I will update the test results here once the testing is complete.

A: In theory, it is possible, but some hardware needs to be upgraded, such as changing the load sensor (YZC-133) from 20KG to 50KG, larger stepper motors, larger power supplies, stronger platforms and slides, and modifying some code parameters. If interested, you can develop your own branch project.

A: I plan to add English subtitles in the future, but because my usual work is quite busy, I only have some time to work on projects each day. If possible, please give the video a thumbs up and subscribe, as this is a great encouragement for me.

A: Please turn to the back and try adjusting the blue variable resistor to change the LCD contrast. Also, check if the jumper for the LCD backlight is inserted. If everything is functioning normally, you can try connecting only the 5V power, GND, and the 4 pins of the LCD to the Raspberry Pi Pico, if there are no issues, the boot screen should appear when powered on. If it still doesn't work, please check if the four necessary files—main.py, src/hx711.py, src/lcd_api.py, and src/pico_i2c_lcd.py—are present on the Raspberry Pi Pico.

A: Please check if you mistakenly purchased the Active Low Trigger version. The correct version should be the Active High Trigger.

Below are some of the future development plans for this project:

• Reliability Testing: Currently has reached 50,000 cycles, ongoing. (2024/08/17)
• Tennis Compatibility: Modifying hardware and software to support tennis racket stringing.

If you have any questions about making, please leave a comment in the YouTube video.

I also have my own stringing pattern, temporarily named the Pico Stringing Pattern. I'm not sure if anyone else is using it, so if there is a similar stringing pattern, please let me know the stringing pattern name.

The stringing pattern demo.

1. The short side is approximately 5 racket lengths, and the long side is approximately 8 racket lengths.
2. Follow the Yonex stringing pattern for main strings, pulling the two outermost main strings together for tension.
3. The cross strings are first the bottom of the short side, tying knots at the bottom, and then strings on the long side, tying knots at the top.
4. Increasing the tension of the cross strings will help the racquet maintain its original shape after stringing. Due to variations in stringing machine and individual techniques, you can experiment to find the tension increment that minimizes deformation.

1. The measured values were obtained after stringing with a 25lb tension, following a 10% pre-stretch, and allowing it to sit for 48 hours.
2. The measurements represent relative tension reference values for each string, NOT the actual tension.
3. Calibration method for the tension tester: https://youtu.be/xYqu03XBzFU