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“Can a scale under my coffee jar tell me when to buy more beans?”
A small kitchen-scale sensor under the jar weighs the beans. When they drop below a couple of days' worth, your phone gets a shopping reminder.
What to buy
| Part | Why this one | Rough cost |
|---|---|---|
| ESP32 or ESP32-C3 board | WiFi for the reminder. | $4–7 |
| Bar load cell, 1–5 kg | A metal bar that bends a tiny bit under weight; the bend is what gets measured. | $3–6 |
| HX711 amplifier board | Turns the load cell's tiny signal into numbers the board can read. Often sold together with the load cell. | $1–3 |
| Two small plates or boards, screws and spacers | The load cell sits between them, fixed at opposite ends, so it can bend. | $2–5 |
Prices are rough ranges, not quotes. Check the current price and stock before you order.
What to install
ESPHome has an HX711 sensor: give it two calibration points and it reports grams. Or use Arduino with a maintained HX711 library. Send the reminder with ntfy or Home Assistant, once, when the weight drops past your line.
Build it in three steps
- Build the sandwich. Screw one end of the load cell to the bottom plate and the other end to the top plate, with small spacers so the middle is free to bend. Wire the load cell to the HX711, and the HX711 to the board.
- Calibrate. Read it empty (that's zero), then with something you've weighed on a kitchen scale. Now the numbers are grams.
- Set the reminder. Weigh the empty jar and subtract it. Alert once below, say, 150 g of beans, and reset when the weight jumps back up after a refill.
What catches people out
- The middle must be free. If the middle of the cell touches a plate, the readings barely change. The arrow on the cell shows which way the weight should push.
- Slow drift. Load cells creep a little with temperature and time. Re-zero with the empty jar now and then, and don't chase single grams.
- Ignore the moment you scoop. Lifting the jar reads as empty. Only act on a weight that has been steady for a minute.
- The load cell's wires are thin. Don't pull on them; hold them down with tape or a clip.
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