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# Load Cell (Straight Bar)

A straight bar strain gauge load cell that outputs 1.0 mV per volt of excitation at its rated capacity, read through an HX711.

Source page: https://www.tinkered.ai/components/load-cell-tal220
Datasheet: https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/TAL220M4M5Update.pdf

## In short

The TAL220 is a parallel beam (straight bar) load cell whose strain gauge bridge outputs 1.0 ± 0.15 mV per volt of excitation at full rated load, with ±0.05 % FS combined error (HT Sensor TAL220 datasheet, p. 1). Its output is only millivolts, so wire it to an HX711 amplifier: red exc+ to E+, black exc- to E-, green sig+ to A+, white sig- to A- (p. 1). Keep loads under its 120 % safe overload (p. 1).

A load cell turns force into a tiny voltage. The TAL220 is a parallel beam type: a bar machined so it bends in a controlled S-shape under load, with strain gauges bonded where it flexes and wired as a full Wheatstone bridge (HT Sensor datasheet, p. 1). Weight on the free end unbalances the bridge in proportion to the load.

At rated capacity the bridge outputs 1.0 ± 0.15 mV for every volt of excitation, with 1000 Ω input and output resistance (p. 1). That is a few millivolts at most, far below what a microcontroller's ADC resolves, so the cell is always read through an amplifier ADC such as the HX711, whose gain-128 channel spans ±20 mV with a 5 V analog supply (Avia HX711 datasheet V2.0, p. 4).

HT Sensor rates the cell for ±0.05 % FS combined error, non-linearity, hysteresis and creep over 3 minutes, ±0.1 % FS zero unbalance, and 5 V to 10 V excitation (TAL220 datasheet, p. 1). It survives 120 % of capacity safely and 150 % before permanent damage. Every unit's exact output differs within its ±0.15 mV/V tolerance, which is why a scale must be tared and calibrated with a known weight.

## Agent rules

Follow these when you wire this part or write code for it.

- Do: Wire it to an HX711: red → E+, black → E−, green → A+, white → A−.
- Do: Pick the capacity for the job (the heaviest load under the rating): the counts per gram fall as the capacity rises.
- Do: Tare at start-up and calibrate with a known weight (see hx711-module).
- Never: No wiring its leads to the board's pins: no ADC sees its millivolts.
- Never: No loads past 120 % of its capacity: past 150 % it bends and never reads zero again.
- Never: No more than 10 V across E+ / E−.

## Pinout

| Pin | Function | Type |
| --- | --- | --- |
| E_PLUS | Red: E+ (excitation +) · 1–5 V | power in |
| E_MINUS | Black: E− (excitation −) · 1–5 V | power in |
| S_PLUS | ADC bridge + output, about 1 mV per V of excitation at capacity | output |
| S_MINUS | ADC bridge − output | output |

## Wiring

| Load Cell (Straight Bar) | Connect to | Note |
| --- | --- | --- |
| E_PLUS | HX711 Load Cell Amplifier E+ |  |
| E_MINUS | HX711 Load Cell Amplifier E- |  |
| S_PLUS | HX711 Load Cell Amplifier A+ |  |
| S_MINUS | HX711 Load Cell Amplifier A- |  |

## Specifications

| Spec | Value | Source |
| --- | --- | --- |
| Rated output | 1.0 ± 0.15 mV/V at capacity | HT Sensor TAL220 datasheet, p. 1 |
| Excitation voltage | 5 V to 10 V DC | HT Sensor TAL220 datasheet, p. 1 |
| Bridge resistance | 1000 ± 15 Ω input, 1000 ± 10 Ω output | HT Sensor TAL220 datasheet, p. 1 |
| Combined error | ±0.05 % FS | HT Sensor TAL220 datasheet, p. 1 |
| Zero unbalance | ±0.1 % FS | HT Sensor TAL220 datasheet, p. 1 |
| Creep | ±0.05 % FS over 3 minutes | HT Sensor TAL220 datasheet, p. 1 |
| Overload | 120 % FS safe, 150 % FS ultimate | HT Sensor TAL220 datasheet, p. 1 |
| Operating temperature | -10 °C to +55 °C (compensated -10 °C to +40 °C) | HT Sensor TAL220 datasheet, p. 1 |
| Wire colours | Red exc+, black exc-, green sig+, white sig- | HT Sensor TAL220 datasheet, p. 1 |
| Capacities listed | 3, 5, 10, 20, 25, 30, 50 kg (aluminium) | HT Sensor TAL220 datasheet, p. 1 |
| Interface | Analog | Tinkered component registry |
| Dimensions | 80 × 12.7 × 12.7 mm | Tinkered component registry |
| Weight | 28 g | Tinkered component registry |

## Circuit notes

- Wire it to an HX711: red to E+, black to E-, green to A+, white to A- (HT Sensor datasheet, p. 1).
- The HX711 module excites the bridge from its own regulator, about 4.3 V from a 5 V supply. The cell's output scales with excitation (it is rated in mV per volt, p. 1), so the HX711, whose ADC works from the same analog supply, reads it ratiometrically.
- At full capacity and 4.3 V the cell gives about 4.3 mV, well inside the HX711's ±20 mV range at gain 128 (Avia HX711 datasheet, p. 4). Choose a capacity close to your heaviest load for the most counts per gram.
- Mount it so it can bend: screw the cable end to a base on a spacer and the platform to the other end on a spacer, and load it in the direction of the arrow on its label.
- Keep the load under 120 % of capacity; past 150 % the bar is permanently bent (p. 1).

## Common mistakes

- **Wiring it straight to an Arduino analog pin.** The output is about 1 mV per volt of excitation at full load (HT Sensor datasheet, p. 1), around 5 mV on 5 V, which is one step of a 10-bit Arduino ADC. It needs an amplifier ADC such as the HX711.
- **Swapping the signal wires.** Green is sig+ and white sig- (p. 1). Crossed, the weight reads negative as you load it. Swap them, or use a negative calibration factor.
- **Bolting it flat with no room to flex.** A parallel beam cell measures by bending. Clamped flat along its length, or loaded at both ends of one face, it barely deflects and reads little or nothing. Mount each end on its own spacer.
- **Overloading it.** The safe overload is 120 % of capacity and the ultimate overload 150 % (p. 1). Past that the bar is bent for good and no longer returns to its zero.
- **Reading before the creep settles.** Under a steady load the output creeps by up to ±0.05 % FS over 3 minutes (p. 1). Let a calibration weight settle before you record the factor.

## Questions

### How do I wire a load cell to an HX711?

Red (exc+) to E+, black (exc-) to E-, green (sig+) to A+ and white (sig-) to A- (HT Sensor TAL220 datasheet, p. 1). Then connect the HX711's VCC, GND, DT and SCK to the microcontroller, tare at start-up and calibrate with a known weight.

### Why does my load cell read negative, drift or jump?

Negative as you load it: green and white are crossed or the load pushes against the label's arrow. Slow drift under a steady weight is creep, up to ±0.05 % FS over 3 minutes (HT Sensor datasheet, p. 1). Readings that jump by millions mean a loose excitation or signal wire at the HX711.

### What does 1.0 mV/V rated output mean?

At full rated load the bridge outputs 1.0 mV for each volt across E+ and E-, ±0.15 mV/V from unit to unit (HT Sensor datasheet, p. 1). With 5 V excitation, a full load gives 5 mV; half the capacity gives half that.

### Can I power a load cell from 3.3 V?

HT Sensor rates the excitation at 5 V to 10 V (datasheet, p. 1). A lower excitation still works with an HX711, since the output scales with it, but the signal shrinks with the voltage, giving fewer counts per gram. Recalibrate on the supply you use.

### Which capacity load cell should I buy?

The smallest one that covers your heaviest load with margin: the cell's full output, 1.0 mV/V (HT Sensor datasheet, p. 1), is spread over its whole capacity, so a lighter cell gives more counts per gram. Stay under its 120 % safe overload, including drops and shocks.

## Related parts

- [HX711 Load Cell Amplifier](https://www.tinkered.ai/components/hx711-module.md): reads it
- [Arduino Uno](https://www.tinkered.ai/components/arduino-uno.md): the board its circuit is wired to

## Try it

Tinkered simulates the Load Cell (Straight Bar) in the browser: wire it on a 3D breadboard, run your real code on the board, and measure it. Free to start at https://www.tinkered.ai.