M5 Grader: Low-Cost Open-Source Fruit Sorting Scale

1. Problem

Fruit farmers cannot avoid the task of grading harvests, but existing grading machines cost at least US $750, making them too expensive for small-scale farms. In addition, a machine breakdown during the busy season can cause serious risks. Affordable and expandable grading machines that can be used by small farms and individual growers are in great demand.

2. Solution

We developed a low-cost, open-source grading machine using M5Stack and a load cell. The total cost of parts is under US $60. Multiple units can run in parallel, so even if one fails, work can continue. With a 3D-printed case, assembly and replacement are simple.

3. Features

• Weight measurement: 1g units, up to 1kg
• Feedback: LCD color display + voice guidance
• Data logging: automatic saving to Google Sheets
• Offline mode: can disable logging in unstable network conditions
• Auto calibration: drift correction every 5 minutes for accuracy

4. How it Works

The M5Stack reads weight from the load cell (HX711). When thresholds are exceeded, it classifies by color and provides voice guidance. When connected to Wi-Fi, logs are sent to Google Sheets via Google Apps Script. Buttons allow calibration and online/offline switching.

5. Impact

Running four units in parallel achieves a speed comparable to commercial graders. This project demonstrates how digital transformation can be advanced in farming without expensive machinery. It can also be adapted to other fruits (apples, peaches, pears). As an open-source project, it enables farmers around the world to customize and expand it.

6.Movie and Images

By using four of these graders, we achieved processing speed comparable to an existing commercial grader.

7. Resources

3D print files (STL) are available on Printables: M5 Grader on Printables

https://www.printables.com/model/1190976-m5stack-grader-as-open-source-agri-tech-project

For more details and the full background story (in Japanese), please check the following blog posts. You can use an online translator (e.g., Google Translate) to read them in English:

M5 Grader Blog Post 1

M5 Grader Blog Post 2

8. Legal Compliance – Japan’s Measurement Law

An acquaintance pointed out possible legal implications under Japan’s Measurement Law (Keiryōhō). To confirm, I consulted the Fukushima Prefecture Weights and Measures Inspection Office. They explained that the law mainly applies to explicit weight-based sales (e.g., “200 g for 300 yen”) or official reporting. Since M5 Grader sorts persimmons by size, it does not fall under regulation, and no certification is required. If used for weight-based selling or official records, consultation with local authorities is recommended.