Two-zone thermometer on a PIC controller. Scheme, description

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Two-zone thermometer on a PIC controller. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The author proposes the design of a thermometer with two remote sensors, which allows you to measure the temperature independently at two points. The information is displayed on the LCD.

Being engaged in the development of power electronics, in particular switching power supplies, one inevitably encounters the need to measure the temperature of cases of power transistors, microcircuits and other elements. For these purposes, a standard multimeter with a thermocouple is usually used, for example, Mastech MY64 and the like. But here you can run into a number of problems.

Firstly, as practice has shown, a conventional bimetallic thermocouple is short-lived, a break often occurs at the junction (soldering) of the sensor and wires, especially with frequent use. Second, the thermocouple sensor must be insulated when measuring the temperature of an energized heatsink or transistor. Thirdly, it often becomes necessary to measure the temperature on two elements of the device at once, for example, on both transistors of a half-bridge generator.

So the idea arose to assemble a device free from these shortcomings. The diagram of a two-zone thermometer is shown in fig. 1.

Rice. 1. Scheme of a two-zone thermometer (click to enlarge)

Main Specifications

Supply voltage, V......6...15
Interval of measured temperature, ^оFrom .......-50...+125
temperature display resolution, ^оC .......0,1

The thermometer is based on the PIC16F873A-I/SO microcontroller. Common digital sensors DS18B20 are used as thermal sensors. Sensors VK1 and VK2 are connected to different ports of the microcontroller, which made it possible to use a simplified procedure for polling them. The sensors are interrogated one by one with an interval of about 1 s. The measurement accuracy of the applied temperature values is programmatically rounded up to 0,1 ^оC. Information is displayed on a single-line character-synthesizing LCD. Trimmer R5 adjusts the contrast of the LCD. The XP1 connector is designed to connect the programmer. Please note that the location of its contacts does not match the standard pinout of the PicKit programmer.

The thermometer is assembled on a board made of fiberglass foiled on both sides with a thickness of 2 mm.

Dual zone thermometer on PIC controller
Rice. 2. Board drawing

Dual zone thermometer on PIC controller
Rice. 3. Location of parts

Its drawing is shown in Fig. 2, and the location of the parts - in Fig. 3. The thermometer board is designed for a standard plastic case for amateur radio constructions; a rectangular hole must be cut in the top cover of the case for the LCD screen, and in the side walls - for the switch and wires of temperature sensors. Photos of the assembled board are shown in fig. 4 and fig. 5. Place heat-shrinkable tubes on the sensors, preferably in different colors. The appearance of the finished device is shown in Fig. 6. A properly assembled device starts working immediately and does not need to be adjusted. As an LCD, any similar single-line sixteen-character LCD of suitable dimensions based on the HD44780 processor is suitable.

Dual zone thermometer on PIC controller
Rice. 4. Board assembly

Dual zone thermometer on PIC controller
Rice. 5. Board assembly

Dual zone thermometer on PIC controller
Rice. 6. Appearance of the finished device

The program and firmware of the microcontroller can be downloaded from ftp://ftp.radio.ru/pub/2015/06/doubl_term.zip.

Author: V. Lazarev

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