Precise magnetic measurement
Two 1 kg beam load cells capture the force on the left and right and convert it into a stable holding force in grams.
DIY Measurement Project · ESP8266
Slotcar Magnet Scale Short & Precise
The scale for the magnetic holding force of slot cars. Measure how many grams the magnets pull on the rail – create equal conditions for everyone at your event and reliably sort out tuning magnets.
- 2× Load cells
- 2× HX711 ADCs
- Web + Serial Control: web + serial
Live force readout
Features
Fair downforce for everyone
Measure magnetic holding force precisely, reproducibly and right at the track – for equal starting conditions in every race.
Perfect for slot car setup
See at a glance how many grams the magnets pull – ideal for comparing and cleanly setting up cars for the event.
Web & serial control
Operate the scale conveniently via the web interface or through the serial console at 115200 baud.
Sort out tuning magnets
Reliably detect magnets that are too strong and create equal conditions at events for fair competition.
Mechanical Setup
Two load cells, one platform
The car rests on two polished steel rails. Both load cells are rigidly connected to the same platform – the magnets pull the car onto the rail, and the total holding force in grams is calculated from a weighted combination of both sensors.
- Measures the pure magnetic holding force in grams
- Joint multi-point calibration instead of per-cell
- Force = factorLeft · netLeft + factorRight · netRight
Hardware
Compact & affordable build
Proven standard components – quick to rebuild and easy to wire.
2×
1 kg beam load cells
Capture the force at both ends of the measuring platform.
2×
HX711 ADC modules
A 24-bit converter per load cell for precise readings.
1×
Wemos D1 mini
ESP8266 board with Wi-Fi – controls and evaluates everything.
Wiring
Pin assignment
Both HX711 modules connect directly to the Wemos D1 mini.
Left HX711
| Signal | Wemos D1 mini |
|---|---|
| DOUT / DT | D1 / GPIO5 |
| SCK / CLK | D2 / GPIO4 |
| VCC | 3V3 |
| GND | GND |
Right HX711
| Signal | Wemos D1 mini |
|---|---|
| DOUT / DT | D5 / GPIO14 |
| SCK / CLK | D6 / GPIO12 |
| VCC | 3V3 |
| GND | GND |
Build
From print to finished scale
A look at the mechanical build – from the empty, 3D-printed housing to the ready-to-use SMS.
Printed housing
The 3D-printed base housing with compartments for load cells and electronics.
Load cells mounted
The two 1 kg beam load cells sit centered on the base plate.
Measuring rails fitted
The rail inlay is placed on the load cells – this is where the car rests later.
Finished scale
The fully assembled SMS with measuring rails in the black housing.
Web Interface
Everything at a glance – in your browser
The firmware ships with a modern single-page web interface. Live weight, load distribution, calibration and Wi-Fi settings in one place.
- Live holding force with left/right share in percent
- Sections: Dashboard · Measurement · Calibration · Wi-Fi · System
- Wi-Fi scan, select SSID and save credentials
- OTA firmware update straight from GitHub Releases
Calibration
Multi-point calibration
Because both cells sit on a shared platform, calibration is not done per cell but jointly across multiple positions using the same reference weight. The firmware solves the factors using least squares.
Unload platform & set tare
Completely unload the platform, then set the tare.
Start calibration
Start with a known reference weight, e.g. 1000 g.
Record points
Record the weight at multiple positions (left, center, right).
Calculate factors
Calculate the factors – automatically saved to EEPROM.
Example · 1000 g
TARA 20
KAL_START 1000
KAL_PUNKT LINKS 20
KAL_PUNKT MITTE 20
KAL_PUNKT RECHTS 20
KAL_BERECHNEN
MESSEN 10
Serial Interface
Command reference
Control the scale entirely from the serial console (115200 baud). A selection of the most important commands.
| Command | Description |
|---|---|
HILFE |
Show help |
STATUS |
Show current status |
WLAN <ssid> <pw> |
Save Wi-Fi credentials |
TARA [n] |
Set tare (optional: n samples) |
KAL_START <g> |
Start calibration session |
KAL_PUNKT <name> |
Record calibration point |
KAL_BERECHNEN |
Calculate and save factors |
MESSEN [n] |
Single measurement |
START [ms] [n] |
Start streaming |
RESET |
Reboot device |
Firmware & 3D Printing
Firmware & print files
The complete source code and ready-made firmware releases are freely available on GitHub. The 3D print files are on MakerWorld. Flashed onto the Wemos D1 mini in no time with PlatformIO.
Go to GitHub repository
Firmware releases
3D print files on MakerWorld
An open-source project for the slot racing community.