Thermometer on DS1821 and PIC controller

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Thermometer on DS1821 and PIC controller. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Microcontrollers

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In the device brought to the attention of readers, a DS1821 temperature sensor from Dallas Semiconductor is used. This is a microprocessor-based device that does not require calibration and outputs the temperature value as a byte in serial code over a 1-Wire interface (single-wire, bi-directional interface). The use of the PIC controller made it possible to create an extremely simple digital device for measuring temperature in the range from -1 to + 55°С with an error of ±125°С.

The schematic diagram of the device is shown in fig. 1. It allows you to measure the temperature of two sensors (DD1, DD2), one of which can be placed, for example, on the street, and the other in the living room. The microprocessor DD3 provides interaction with the sensors and transmits to the LCD module HG1 the temperature value received from them via a four-wire bus. LCD module AC161B is a matrix Russified single-line LCD indicator for 16 familiarity with a built-in controller and backlight (the letters A and K indicate the outputs of the anodes and cathodes of the LCD backlight LEDs). Resistor R4 limits the anode voltage to 4 V at 70 mA.

Thermometer on DS1821 and PIC controller

Chip DA1 stabilizes the supply voltage of the device. Resistor R1 protects it from a short circuit that may occur in the communication lines with the sensors.

The program that controls the operation of the microcontroller is written in MPASM assembler from Microchip. It uses standard software modules from Microchip (PIC), Dallas Semiconductor (DS1821), Ampire (AC161B indicator). Codes "firmware" microcontroller ROM in HEX-format are shown in the table.

(click to enlarge)

A properly assembled device does not need to be adjusted. The only thing that needs to be done is to set the desired contrast with the trimmer resistor R6.

The thermometer is mounted on a breadboard and placed in a plastic housing (Fig. 2).

Thermometer on DS1821 and PIC controller

Detachable connectors X1 (1) and X2 (2) are small-sized coaxial (they are equipped with small-sized stereo phones). The number 3 indicates the power wires. If the sensor intended for measuring the temperature in the living room is supposed to be placed in the same place as the main unit, it can be mounted directly on the pins of the plug. It is not recommended to place the sensor inside the device case, since even a slight heating by the heat generated by the resistor R4 will unacceptably distort the readings.

For power supply, you can use any AC adapter with an output voltage of 6 ... 12 V. If this voltage is less than 12 V, you must select resistor R4 according to the current recommended for backlighting the indicator (70 mA).

The external sensor is placed in a glass drug ampoule with the narrowed part removed. The exit point of the cable from the ampoule is sealed with any of the commercially available sealants.

Instead of the PIC16F84 microprocessor, you can use PIC16C84, instead of the AC161B indicator - AC082 (two-line with eight characters per line) or similar from other manufacturers. We can replace the 78L05 microcircuit voltage regulator with any other with a stabilization voltage of 5 V. In extreme cases, you can use the simplest parametric stabilizer on the KS156A zener diode by choosing a ballast resistor.

Author: V.Troshkov, Novosibirsk

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