Complete circuit diagram of digital display thermometer design (DS18B20/89S51 microcontroller/LCD)

When the AD7705 operating voltage is 3V and the on-chip programmable amplifier gain is set to 1, the A/D accuracy is 16 bits and the minimum resolution voltage is 37.69μV (1.235V×2/65536). The output thermoelectromotive force of the thermocouple changes by 38 μV to 81 μV/°C for every 1°C change (-150°C to 1000°C), which is greater than the minimum resolution voltage of the AD7705. Therefore, the system resolution can reach 1°C, which can meet most industrial measurement requirements. Since the AD7705 can directly perform analog-to-digital conversion on -0.6175V ~ 0.6175V voltage, when the thermocouple measures a temperature less than 0°C and the thermoelectromotive force is less than 0V, it can work normally without additional circuitry.

AD589 is the voltage reference source for the AD7705. The AD589 is an inexpensive, two-terminal device that provides a 1.235V bandgap reference voltage output with temperature compensation. Its on-chip component matching and thermal tracking characteristics make the AD589 highly stable. In addition, the AD589's output impedance is 10 times lower than common low temperature coefficient Zener diodes, so the circuit can maintain high accuracy without the need for external components, even if the load changes.

This system uses MSP430F413 and AD7705 as the core to implement the design of a low-power, high-precision portable thermometer. For portable instruments, this design achieves low power consumption and high accuracy requirements under low-cost and wide temperature measurement range conditions, and has certain practical value. At present, this circuit has been put into use. Practice has shown that the entire portable low-power high-precision digital thermometer is easy to use, works stably, has long standby time, and has broad application prospects.

Digital display thermometer design circuit diagram (6)

Circuit diagram of making a four-digit digital thermometer using 89S51 microcontroller

Hardware wiring on the system board

(1) Connect P1.0-P1.7 in the "SCM System" area to the ABCDEFGH port in the "Dynamic Digital Display" area with an 8-core cable.

(2) Connect P2.0-P2.7 in the "MCU System" area to the S1S2S3S4S5S6S7S8 port in the "Dynamic Digital Display" area with an 8-core cable.

(3) Connect P3.0 in the "MCU System" area to the ST terminal in the "Analog-to-Digital Conversion Module" area with wires.

(4) Connect P3.1 in the "SCM System" area to the OE terminal in the "Analog-to-Digital Conversion Module" area with wires.

(5) Connect P3.2 in the "SCM System" area to the EOC terminal in the "Analog-to-Digital Conversion Module" area with wires.

(6) Connect P3.3 in the "SCM System" area to the CLK terminal in the "Analog-to-Digital Conversion Module" area with wires.

(7) Connect the A2A1A0 terminal in the "analog-to-digital conversion module" area to the GND terminal in the "power module" area with a wire.

(8) Connect the IN0 terminal in the "Analog-to-digital conversion module" area to the homemade AD590 circuit with a wire.

(9) Connect P0.0-P0.7 in the "SCM System" area to the D0D1D2D3D4D5D6D7 terminal in the "Analog-to-Digital Conversion Module" area with an 8-core cable.