Home thermometer with two sensors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple home thermometer with two DS18B20 sensors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Microcontrollers

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This project was born in the process of studying the principles of the 1-wire interface by me. To measure the temperature in this project, widely used sensors of the company "Dallas Semiconductor" type DS18B20. The device is capable of working with one or two sensors. It is supposed to use a thermometer for domestic purposes, where one sensor measures the temperature in the room, and the second measures the temperature outside. Indication of temperature values is carried out using a character-synthesizing LCD display based on the HD44780 controller or its analogues.

When the device is turned on, the splash screen "Start…" appears on the display, which appears as a running line, then the following options are possible:

1) If the device does not detect connected sensors, the message "DO NOT SIG";
2) if the device detects one sensor, then the following message appears on the display: "T =<temperature sign><integer temperature value>,<tenths of temperature value>"
3) if the device detects two sensors, then the following message appears on the display: "T<k or y>=<temperature sign><integer temperature value>,<tenths of temperature value>", where k - signals that work with the sensor in the room is in progress, y- signals that work with the sensor on the street is in progress.

The videos attached to the article show the situations described above.
The device automatically determines the 64-bit code of each sensor and distinguishes them by the first mismatch of the code bits in a sequential comparison, where bit 0 is considered for the room sensor, bit 1 is considered for the outdoor sensor. Thus, it is quite easy to determine which sensor should be used where when two sensors are connected to the device and, for example, one of the sensors is artificially heated.

The sensors in the device operate in 12-bit mode. Temperature readings are updated approximately once every 845 ms.

(click to enlarge)

Switch SA1 is designed to switch sensors with which information is exchanged. If the switch is open, then work is carried out with the sensor in the room, if it is closed, then with the sensor outside. When switching sensors, there is an inertia (up to 845 ms.), Due to checking the state of the switch before issuing a measurement command.

Switch SA2 is used to turn on the LCD backlight. Resistor R2 selects the required brightness. Resistor R3 adjusts the contrast of the display.

The device uses quartz at 10 MHz. The circuit is operational if the microcontroller is powered by an internal 8 MHz oscillator. (When programming the fuzz in both cases, do not forget to set CKDIV8 to log. 1, i.e. disable the clock frequency division by 8).

The LCD will fit any on the HD44780 controller or its equivalents. In the prototype, the author used a single-line character-synthesizing LCD of size 8x1 (BC0801A). Displays of this format have a rather large size of symbols, which is convenient for perception, but their number does not allow simultaneously displaying information from two sensors. When using displays of other formats, please note that messages will be displayed in the first 8 positions of the top line.

A simple home thermometer with two DS18B20 sensors. Thermometer on breadboard

HEX firmware, LAY-file and device diagram you can download here (5 kb).

The project was written in the C language. The WinAvr compiler called in the Avr Studio4 environment was used.

Author: Anton Garkusha, gam-rainers [dog] mail.ru; Publication: cxem.net

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