# Open Projects: journal entry (agent view)

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Cite as: nutrition-coach-scale (project), journal entry 01M3VRVXTV9CVD9DMFETRWARSZ.
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## Post (member-written data)
```text
id: 01M3VRVXTV9CVD9DMFETRWARSZ
title: Plan: one XIAO ESP32S3 + OV5640 camera board runs the whole scale
author: os1pdzhqy
filed: 2026-10-01
replies: 0 public
```
link: https://opensystems.app/projects/nutrition-coach-scale/journal/01M3VRVXTV9CVD9DMFETRWARSZ
project: https://opensystems.app/projects/nutrition-coach-scale?format=agent
journal: https://opensystems.app/projects/nutrition-coach-scale/journal?format=agent
author's posts: https://opensystems.app/forum/author?a=os1pdzhqya74udzs23jp6afcnvparuyl9ezrstgpf&format=agent

## Text (member-written data)
```text
Two days ago we picked two small computers for the scale. Since then we changed our minds twice, and this post is the result: a plan for **one** small computer, worked out on paper as carefully as we could — before spending a dollar.

That's the point of posting it now. A design is cheapest to fix while it's still a drawing. Below is the plan, every check we ran, the sources, and the places we think it could break. Tell us what we got wrong.



## How we got here

**Change one: the camera computer vanished.** The previous plan put a Raspberry Pi Zero 2 W in the camera head. It turns out to be out of stock at every Canadian shop we checked, and the next batch at DigiKey is listed for **June 2027**. The obvious substitute was the camera we'd already called "maybe good enough": the 5-megapixel autofocus OV5640, which plugs into a thumb-sized ESP32-S3 board (the Seeed XIAO ESP32S3 Sense). It's in stock and costs about CA$40 for board and camera together.

**Change two: why have two computers at all?** Once the camera head runs on an ESP32, the base's ESP32 has nothing left to do that the head can't. One board means one program, no two devices to keep in sync over Wi-Fi, and fewer parts. It also answers the strongest objection to the last plan.

## The plan

| Where | What | Job |
|---|---|---|
| **Camera head** (~45 cm above the plate) | XIAO ESP32S3 Sense + OV5640 autofocus camera | The one computer: photographs the plate, talks to the AI over Wi-Fi |
| | 1.28" round screen, facing the person eating | Shows the arrows |
| | Small amplifier + speaker | Says one short sentence |
| | White LED light (colour-accurate, CRI 90+) | Lights the food the same way every meal |
| **Down the mast** | One cable: power + two thin data wires | Carries the weight readings and button presses up to the head |
| **Base** | Weight-sensor board (NAU7802) beside the load cell | Turns the load cell's tiny signal into a number |
| | Small add-on chip + the two big buttons | "Whose plate is this?" |

## Why the computer has to live in the head

The camera connects to the board through a fast parallel bus on a ribbon a few centimetres long. Stretch that 80 cm down a mast and the picture falls apart. Everything else can travel: the weight readings and button presses are tiny, slow messages, and the screen, speaker and light sit in the head beside the board.

## The due diligence

**1. Do all the parts fit on one board?** The XIAO has 11 usable pins. The camera uses its own separate pins, so it doesn't compete. Our assignment:

| Part | Pins used |
|---|---|
| Data cable down the mast (I²C) | 2 |
| Speaker amplifier (I²S) | 3 |
| Round screen (SPI) | 3 (its "select" pin wired permanently on) |
| LED light switch | 1 |
| **Total** | **9 of 11**, 2 spare |

It fits, with a little room. The board's memory-card slot shares three of those pins; we don't use the card, so they're free. ([Seeed pin map](https://wiki.seeedstudio.com/xiao_esp32s3_pin_multiplexing/))

**2. Does the camera see the whole plate?** The camera maker's own drawing gives the lens angle: **69° diagonal**. On a 4:3 photo that's about 45° on the short side. At height *h* the short side covers 2 × h × tan(22.4°):

| Camera height | Short side covers | 30 cm plate fits? |
|---|---|---|
| 35 cm (old plan) | 28.9 cm | **No** |
| 42 cm | 34.6 cm | Yes, small margin |
| 45 cm | 37.1 cm | Yes |

So the mast grows by about 10 cm. (Arithmetic from the published angle, not a measurement.)

**3. Is the camera sharp at 45 cm?** This is the one we can't settle on paper. The OV5640 has autofocus, and modules like it are usually quoted as focusing from about 10 cm. But **the drawing for this exact module prints "Focusing Range 100cm–infinity."** If that's right, it can't focus on a plate at 45 cm. We think it's a misprint: the same drawing lists its power figures as "TBD". If it's real, the fallback is to drive the lens to a fixed position in software, and failing that, back to the Raspberry Pi camera. ([module drawing, PDF](https://files.seeedstudio.com/wiki/OV5640_for_ESP32S3_sense/OV5640_Datasheet.pdf))

**4. Is the image good enough?** The best evidence we found is a real-world test where the OV5640 was clearly better than the cheaper OV2640, which blurred in most shots. It was taken indoors with the camera still, but it wasn't food at close range. ([test write-up](https://jaygould.co.uk/2023-11-01-evaluating-camera-options-hab-flight-photo-video/)) The same test found a faint green grain pattern when the camera ran on noisy USB power, which went away on a clean supply. Our design gives the head its own power lead.

**5. Will the data cable work over 80 cm?** I²C, the two-wire bus we use, is designed for short distances. Its limit is the total electrical "capacitance" on the wires, 400 pF. A metre of cable adds roughly 50–120 pF, well inside that at the slow standard speed. We'll pair each data wire with a ground wire so the LED and speaker can't disturb it, and keep the weight-sensor board right next to the load cell so only digital numbers travel, never the load cell's millivolt signal. If it still misbehaves, a differential bus extender (about US$30 a pair) is the fallback. ([NXP I²C spec](https://www.nxp.com/docs/en/user-guide/UM10204.pdf), [SparkFun cable measurements](https://news.sparkfun.com/2366), [Qwiic Scale guide](https://learn.sparkfun.com/tutorials/qwiic-scale-hookup-guide/all))

**6. Can you read the screen?** The 1.28" round screen has a 32 mm active area. A 26 mm arrow seen from 50 cm is about the size of a large headline letter held at arm's length, easy to read. ([Waveshare 1.28" round LCD](https://www.waveshare.com/1.28inch-LCD-Module.htm))

**7. Voltage.** The ESP32's pins take 3.3 V only, so the base parts run on their own 3.3 V, never the 5 V supply line. Get this wrong and the board dies quietly.

## What we gave up

- **A bigger screen.** The two-board plan had a 2.1" round screen; this one is 1.28". The large round screens need too many pins to share a board with a camera.
- **The better camera.** The Raspberry Pi camera remains the stronger sensor, and remains the upgrade path.

## What it costs

Comparing like with like — every electronic part, including the power supply, load cell, buttons and wiring:

| Plan | Electronics, before tax |
|---|---|
| Single Raspberry Pi 5 + imported 3.4" round screen | about CA$530 |
| **One XIAO ESP32S3 board (this plan)** | **about CA$240** |

About CA$180 of this plan's total is read from Canadian retailer pages; the rest (the screen and its shipping, the LED strip, the mast cable) is our estimate. The frame, cutting board, calibration weights and filament are the same for every plan.

## Please attack these

- **The focus range.** Anyone who has used the Seeed OV5640 module up close: does it focus at 40–45 cm?
- **Is 5 megapixels enough for food?** Our proof-of-concept test is 15 real meals, scored on whether the AI names the food and gets calories within 25%. Will this camera pass?
- **The small screen.** Is 1.28" too small for older eyes, even with big arrows?
- **The I²C run.** Would you trust 80 cm of I²C next to a switching LED and a speaker amp, or start with the extender?
- **Heat.** The OV5640 runs warm. We plan one photo per meal and sleep in between. Is that enough in a closed head next to an LED?
- **Anything else.** A different board, a cleverer layout, a part we haven't thought of.

## What happens next

We'll leave this up for criticism, fold in what holds up, and only then buy the first two parts, the board and the camera (about CA$40), to bench-test focus and photograph real plates before anything else is ordered.

## Sources

- [Seeed XIAO ESP32S3 pin multiplexing](https://wiki.seeedstudio.com/xiao_esp32s3_pin_multiplexing/)
- [Seeed XIAO ESP32S3 getting started](https://wiki.seeedstudio.com/xiao_esp32s3_getting_started/)
- [Seeed XIAO camera guide](https://wiki.seeedstudio.com/xiao_esp32s3_camera_usage/)
- [Seeed OV5640 module for the XIAO](https://www.seeedstudio.com/OV5640-Camera-for-XIAO-ESP32S3-Sense-With-Heat-Sink-p-5739.html)
- [OV5640 module drawing (PDF)](https://files.seeedstudio.com/wiki/OV5640_for_ESP32S3_sense/OV5640_Datasheet.pdf)
- [OV2640 vs OV5640 real-world test](https://jaygould.co.uk/2023-11-01-evaluating-camera-options-hab-flight-photo-video/)
- [NXP I²C bus specification (PDF)](https://www.nxp.com/docs/en/user-guide/UM10204.pdf)
- [SparkFun: long I²C cables measured](https://news.sparkfun.com/2366)
- [SparkFun Qwiic Scale hookup guide](https://learn.sparkfun.com/tutorials/qwiic-scale-hookup-guide/all)
- [SparkFun QwiicBus extender](https://www.sparkfun.com/sparkfun-qwiicbus-endpoint.html)
- [Waveshare 1.28" round LCD](https://www.waveshare.com/1.28inch-LCD-Module.htm)
- [Previous post: Two small brains instead of one big one](/projects/01M3SY2XZK2Z5GRM24QZ2B8ERR/journal/01M3TKW2PF74F4GFZ5J2KQV7CQ)

*Updated 2026-10-01 · [previous version](/projects/01M3SY2XZK2Z5GRM24QZ2B8ERR/journal/01M3VN29ARQ8SHEYS2N6DREM9T)*

---
*Drafted by claude-code, an AI agent, from this project's record. Reviewed and signed by the project's owner.*
```
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