Economical house-street thermometer with cell phone indicator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Economical house-street thermometer with cell phone indicator

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Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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The initial reason for this development was the desire to have a self-contained, small-sized device capable of long battery life. The first problem was the choice of an indicator to display the measured temperature values. The LED indicators did not fit due to the significant current consumption. A conventional multi-digit seven-element LCD consumes much less current, but it has to apply too many control signals, which requires the use of a multi-pin microcontroller. The LCD with an integrated controller, controlled by only a few signals, was the best. After a long search, I settled on the LCD from a Nokia 3310 cell phone. It has an LPH7779 or PCD8544 controller controlled via the SPI interface, the control protocol of which is known.

Indicator Specifications

Screen size, mm..........30x22
Resolution, px..........84x48
Supply voltage, V..........2,7...3,3
Operating temperature, °С.........-25...+70
Consumption current, uA .......... 300

I did not want to use a microcontroller of the PICmicro or AVR family in the device due to the relatively high current consumption. The choice fell on the family of MSP430 microcontrollers from Texas Instruments. It includes 16-bit microcontrollers with advanced peripherals. Most importantly, they are specially designed for battery operation and have ultra-low power consumption. The current consumed by the used MSP430F2011IPW microcontroller at a supply voltage of 3 V and a core clock frequency of 8 MHz does not exceed 2 mA. The thermometer circuit is shown in fig. 1.

Economical house-street thermometer with cell phone indicator
Fig. 1

To power the components of the device, the voltage of the galvanic cell G1 must be increased to 3,3 V. This is done by a boost converter on a specialized NCP1400ASN33T1 (DA1) chip. With its own current consumption of 60 µA, it maintains the output voltage unchanged when the cell voltage drops down to 0,8 V.

Sensors DS1821 (VK1, VK2) measure temperature in the range from -55 to + 125 °С with an error of no more than 1 °С. They can also operate in thermostat mode, giving signals to turn on and off the heater or cooler when the set temperature values are reached, but this mode is not used in the device under consideration.

The sensors are connected to the microcontroller via the well-known 1-Wire interface. But unlike other microcircuits with such an interface, these sensors do not have individual numbers, therefore, a separate line is required to communicate with each of them. Used lines P1.6 and P1.7 microcontroller DD1. The sensors are polled periodically, its results are displayed on the HG1 indicator.

Economical house-street thermometer with cell phone indicator
Fig. 2

The printed circuit board of the thermometer and the arrangement of parts on its two sides are shown in fig. 2. The device does not need to be adjusted and starts working immediately, but before installing the microcontroller on the board and connecting the indicator to it, it is advisable to check the operation of the voltage converter on the DA1 chip and make sure that the voltage at its output is 3,3 V in the entire range of possible voltage values galvanic cell G1.

The microcontroller program (termometr. txt file) and the thermometer printed circuit board file in Sprint Layout format can be downloaded hence.

Author: A. Nikolaev, Bogotol, Krasnoyarsk Territory; Publication: radioradar.net

See other articles Section Power regulators, thermometers, heat stabilizers.

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