Battery thermometer. Encyclopedia of radio electronics and electrical engineering

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

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Air temperature is one of the important environmental parameters that significantly affects many processes of human life. Therefore, its constant control creates undoubted convenience. The proposed thermometer is designed for indoor use and has a number of advantages:

• firstly, it is powered by one AA battery, which does not require replacement within a year (the assembled thermometer has been successfully operating for 8 months);
• secondly, it uses a digital temperature sensor. In this case, an accuracy of measurements of the order of ±0,5°C is achieved. The digital sensor is supplied already calibrated, so the device does not require any additional configuration;
• thirdly, this is the connection of a liquid crystal indicator (LCD) directly to the microcontroller. In this mode, the current consumed by the indicator is about 10 μA, which favorably affects the time of continuous operation from the battery.

The thermometer (Fig. 1) consists of four functional blocks.

(click to enlarge)

The temperature is measured by a DD1 type MCP9803 microchip manufactured by Microchip. It contains a temperature sensor and a device that converts the temperature value into a digital code with a resolution of 0,0625°C. This code is stored in the registers of the microcircuit and can be requested by the microcontroller via the l2C interface. Resistors R1 and R2 serve to implement this interface. The operating range of the temperature sensor is from -40 to +125 C, which significantly overlaps the temperature range of the used LCD (from -10 to +50°C). Therefore, the measurement range of the thermometer is limited by the temperature range of the LCD. It should be noted that almost all LCDs do not work at low temperatures without special heating. However, negative temperatures are very rare for residential premises.

The temperature is measured approximately once every 8 s. The rest of the time, the sensor and the microcontroller are in sleep mode (readings continue to be displayed), which significantly reduces power consumption. The digital temperature code is transmitted to the PIC2F16 type DD916 microcontroller for further processing and display on the HG1 LCD (Varitronix model VI-201). The LCD has two digits for temperature indication and two decimal points. The left point is not used, and the right one serves to separate integers and tenths of a degree in the range from 0 to +9,9 ° C.

Temperatures between -9 to 0°C and +10 to +50°C are only shown as an integer. The left digit of the LCD is also used to indicate the minus sign for negative temperatures.

As practice shows, determining the air temperature with an accuracy of whole fractions of a degree is quite sufficient for most applications. This allows the use of widely used large-sized LCDs for easy reading.

The device is powered from a single AA battery by a voltage converter made on a DA1 chip of the MAX1724 type from Maxim. The MAX1724 microcircuit is available in several versions for fixed output voltages within 2 ... 5 V. We used its 5 V version. However, the battery life can be further increased by using the microcircuit version for an output voltage of 3 V. operate under such pressure.

.Filtering capacitors C2 and C3 are necessary for the normal operation of the converter, and capacitor C1 serves to suppress high-frequency ripples of the converter and prevent self-excitation of the microcontroller. A distinctive feature of the converter microcircuit is the absence of a rectifier diode, characteristic of such circuits. This achieves high efficiency of the converter. Inductor L1 with an inductance of 10 ... 22 μH can be of almost any design, since the current consumed from the converter is very small. We used a small-sized choke SRU5016-100Y from Bourns.

The printed circuit board of the thermometer was developed for the MAX1724 chip in the QFN package. All other elements on the board are also designed for SMD-MOH (surface). Resistors R1 and R2 can be in packages 0806 or 1206. Tantalum electrolytic capacitors C2 and C3 - for an operating voltage of 10 V, capacitor C1 - ceramic, in package 0806. Chips DA1 and DD1 - in SOT23-5 and S08 packages, respectively. As DD1, you can use the MCP9802 sensor. It comes in a slightly smaller 5-pin SOT23-5 package. This replacement will entail a slight correction of the PCB, however, no changes to the microcontroller code will be required. The microcontroller itself can be replaced with a PIC16F914.

The printed circuit board is designed for the PIC16F916 chip in the QFN package and is made of one-sided foil fiberglass with dimensions of 65x39 mm (Fig. 2). Two wire jumpers are installed on the reverse side of the board, shown in the layout diagram. The shape of the printed circuit board is designed for its installation in a standard "JAL-1" case with dimensions of 68x22x42 mm. There are two posts in the case for fixing the board with two screws. In the side wall of the case, opposite the temperature sensor, a 02 mm hole is drilled to improve heat transfer.

The front panel is made of 2 mm thick Plexiglas and fastened with four screws to the corresponding racks of the case. A holder for an AA battery is glued to the bottom of the case. The battery is connected on the board to the sockets marked "+" and "-". Due to the low current consumption, a power switch is not provided.

When assembling the thermometer, the microcontroller should be installed on the board first. It is assembled by hot air soldering. After installation, write the program shown in the table into it.

(click to enlarge)

For programming, you can use any programmer connected via a 5-wire ICSP cable. The cable is temporarily connected to the controller outputs through the LCD sockets on the board. The long printed track connected to pin 26 (MCLR) is for easy soldering of the corresponding wire of the ICSP cable. After programming, this track is no longer used and the wire connection socket remains covered by the LCD panel.

A properly assembled thermometer does not need to be adjusted.

Authors: S. Bezrukov, V. Artists, Superior, USA

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

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