SensingBench electronicsTest and identify
Load cell
Also called strain gauge, scale sensor, kitchen scale beam.
The aluminum bar in a digital kitchen scale is a real load cell, and new ones cost more than the scale did. Output is millivolts, not a 0–5 V signal. Bathroom scales are four separate half-bridges that have to be combined.
If you are new to this category
A sensor reports a condition such as weight, light, temperature, or position. Many sensors do not switch fully on or off. A kitchen scale bar, for example, changes by thousandths of a volt. Keep the small board that read the sensor if you still have it.
Keep the connector. Write down the wire colors before you cut them. If you do not know which wire is which, the sensor is hard to reuse.
Red and black feed the sensor a steady few volts. Green and white are the tiny signal, measured between them, not from either one to ground. Pressing the bar changes that signal by millivolts. A normal meter on DC volts can show that it responds. Turning it into a number on a screen wants a small amplifier board, often an HX711.
| What to look for | Usual figure |
|---|---|
| Kitchen scale | Usually one single-point beam, four wires, capacity equal to the scale's range (often 3–5 kg) |
| Colors | Often red/black excitation and green/white signal. Blue sometimes replaces green. Cheap cells improvise. |
| Output | On the order of 1 mV per volt of excitation at full scale |
| Excitation | 3–5 V is safe. 12 V is how you cook a small cell. |
Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.
What people actually use it for
- A scale
- A force limit on a small press
- A presence detector that is actually measuring weight
Leave this one when
The beam is bent or the gauges are torn. You cannot re-glue a strain gauge and get the old calibration back.
Buying a new one is the better idea when
A cell with a datasheet, once you need a certified accuracy rather than a calibration you did yourself.
How to tell if this one still works
Excite with 5 V, measure millivolts differentially, press with a known mass.
Step-by-step on the bench: Wire a salvaged load cellTools you will want: Meter with a mV range, A known weight, Amplifier such as an HX711 if you want a number in a microcontroller.
Where people actually find it
1 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.
| From | Call | How many | How often | Where it sits |
|---|---|---|---|---|
| Digital kitchen scale | Prize | 1 | always | The center. |
- In this particular machine
- Four-wire beam, capacity about the scale's range.
- Where to look
- The center.
- How to get it out
- Unbolt. Do not bend it further.
- The condition to expect
- Overload shows up as a zero you cannot calibrate out.