OpenScale Applications and Hookup Guide

Contributors: Nate, LightningHawk
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Hardware Features

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Power Cycling

When the report rate is above 500ms OpenScale will automatically turn off the power to the load cell between readings. This saves power and limits the amount of localized heating of the strain gauges. The HX711 requires around 500ms after power-up to stabilize. If the report rate is set below 500ms OpenScale will keep the HX711 on all the time. This will not damage the load cell but it may affect the accuracy of the readings over multiple hours of consecutive readings.

Rate Jumper

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The Rate jumper selects between two rates: 10 samples per second (SPS) or 80 samples per second. By default there is a short between the two pads on the board connecting the HX711 RATE pin to ground and setting the rate at 10SPS. This decreases the sample input noise to 50nV (makes the readings less noisy) but increases the startup time (from power save mode) to 400ms.

Opening the jumper will set the sample rate to 80SPS, increasing the noise to 90nV, and decreasing the startup time to 100ms. Open the jumper by cutting this jumper with a hobby knife. The jumper can be closed again with solder if necessary.

Local Temperature Sensor

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OpenScale has a built-in TMP102 digital temperature sensor. When enabled via the configuration menu OpenScale will print the temperature in Celsius with the user selected number of decimals. The TMP102 sensor is very precise but calibration will be needed for the greatest accuracy.

External Temperature Sensor

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There is a 3-pin 0.1” spaced footprint to connect a DS18B20 compatible sensor. A 4.7k Ohm resistor is connected between VCC and the signal pin to allow one-wire communication. The Waterproof DS18B20 Sensor is an excellent way to detect temperatures of a remote load cell.

Please note that the scale readings do not take the local or remote temperature readings into account. It is up to the user to post process these temperature readings to get the maximum scale accuracy.

Fixed and Adjustable Gain

The HX711 features an active low noise programmable gain amplifier with gains of 32, 64 and 128. Use channel B inputs for A+ and A- and you will get an automatic gain of 32. Please refer to the data sheet for specifications and diagrams.

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