Design based on HX711 digital display weighing instrument

The digital display weighing instrument designed in this paper is based on the resistance strain sensor and the weighing control display system with the single chip as the control core. The measuring range is 0-10kg, the measuring accuracy is ±2g, the LCD screen displays the measurement data, and it can be multiple times. The measurement data is sent to the computer through the serial port. The system features high precision, stable performance and easy operation. The design diagram of the weighing instrument is shown in Figure 1:

Figure 1 Weighing device design block diagram

1, hardware circuit design 1.1, load cell

The resistance strain type load cell is composed of several main parts such as a strain gauge, an elastomer and a detection circuit. The elastic body is elastically deformed under the action of external force, so that the resistance strain gauge attached to the surface of the elastic body is also deformed along with it. After the resistance strain gauge is deformed, its resistance value will change (increase or decrease), and then corresponding measurement The circuit converts this resistance change into an electrical signal (voltage or current), thereby completing the process of converting an external force into an electrical signal.

The detection circuit is shown in Fig. 2, and the resistance change of the strain gauge is converted into a voltage output. Because the Wheatstone bridge has many advantages, such as suppressing the influence of temperature changes, lateral force interference can be suppressed, and the compensation problem of the load cell can be solved conveniently, so the Wheatstone bridge is obtained in the load cell. A wide range of applications.

Figure 2 Detection circuit composed of Wheatstone bridge

The load cell generally has four input and output lines. The output resistance is generally 350Ω, 480Ω, 700Ω, and 1000Ω. The input terminal generally performs some temperature and sensitivity compensation. The input terminal resistance is 20~100Ω higher than the output end. The multimeter measures the resistance value to determine the input and output terminals.

1.2, the amplifier circuit

The strain gauge load cell output signal amplitude is small (mV or even μV) and is often accompanied by large noise. For such signals, the first step in circuit processing is usually to first amplify the small signal with an instrumentation amplifier. The instrumentation amplifier circuit has better common mode rejection than a simple differential amplifier circuit. The main purpose of amplification is not the gain, but the signal-to-noise ratio of the circuit. In this design, the instrumentation amplifier adopts the structure of OP07 three op amps. As shown in Figure 3.

When R1=R2, R3=R4, Rf=R5, the gain of the circuit is: G=(1+2R1/RG1)(Rf/R3). It can be seen from the formula that the adjustment of the circuit gain can be achieved by changing the resistance of RG1.

Figure 3 meter amplifier circuit

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