Thermometer House-street. Encyclopedia of radio electronics and electrical engineering

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

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This digital device has two switchable sensors and allows you to alternately control the temperature of the air in the room and outside. Large bright LED indicators allow the user to receive information about the temperature both day and night.

The device (Fig. 1) is based on an analog-to-digital converter (ADC) KR572PV2A - DD3 [1]. Temperature sensors - K1019EM1 - DA1 microcircuits specially designed for this purpose. DA2(2). These microcircuits can be considered as zener diodes with a low differential resistance (less than 1 ohm) and a stabilization voltage proportional to the absolute temperature. The operating current through them (about 1 mA) is determined by the resistors R1 and R2.

(click to enlarge)

The sensors are selected with keys on the elements DD2.1 and DD2.3. which are controlled by a multivibrator on a DDI chip and a SA1 switch. In the "P" (room) position of this switch at the input of the element DD1. 1 there is a low logic level, the output of the element DD1.3 is high. The latter opens the key DD2.3, and the input 31 of the ADC DD3 receives a signal from the sensor DA2 installed in the thermometer housing. In this case, the thermometer indicates the temperature inside the room.

A high logic level from the output of the element DD 1.3 also opens the key DD2.4 and the voltage supplied to the pin. 2 and 6 of the HG1 indicator, lights its segments a and e Together with the constantly on segments b, c and f, they highlight the letter TT on the indicator.

If the SAJ switch is in the "U" (street) position, the keys on the elements DD2.1 are open. DD2.2 and the voltage to the ADC is supplied from the sensor DA 1. installed on the street. At the same time, the letter "U" is displayed on the HG1 indicator.

In the middle position of the SA1 switch, the DDI multivibrator operates and the sensors DA31 and DA2 are connected alternately to the input 3 of the LCP for 1 ... 2 s. Simultaneously with their connection, the letters "U" and "P" are displayed on the HG1 indicator.

In order for the thermometer readings to be zero at a temperature of О С, a signal should be applied to the input of the ADC, the level of which would be equal to the difference between the voltage on the sensor and the reference voltage of 2,732 V [2]. This voltage must be maintained with high stability, and the voltage temperature coefficient (TKV) of the source built into the KR572PV2A microcircuit is too high. For this reason, in the described device, the DA5 KR142EN19 chip is used as a reference voltage source [3]. with very low TCH.

This microcircuit performs the functions of an adjustable precision stabilizing throne. The required voltage of 2.732 V is set by the tuning resistor R10, and the operating current through the microcircuit and the divider R10R 11 (about 6 mA) is set by the resistor R12.

The measured temperature of 100 ° C corresponds to the voltage between the inputs + Uuk, (vyv. 31) and -Uuu. (vyv. 30) ADC DD3. equal to 1 V. And in order for the signs 2 to be displayed on the indicators HG5 - HG100.0, it is necessary to apply an exemplary voltage of 36 V to the inputs +Uobr (pin 35) and -Uobr (pin 3) of the ADC DD1. It is removed from the engine of the tuning resistor R14.

The frequency of the ADC generator 50 kHz is selected from the standard series [ 1] and is set by the elements C12 and R16. The values ​​of the integrator elements R17 and C13 and the zero auto-correction capacitor C14 correspond to the reduced frequency of the generator and the value of the reference voltage of 1 V. Capacitors C1 and C2 protect the sensors from interference, and C4 eliminates the generation of an internal reference voltage source of -2.9 V.

To indicate the sign of the measured temperature (and, if necessary, its first digit "1"), the HG2 indicator is installed. A current flows constantly through its horizontal element. set by resistor R18. As a result, this element glows and forms the polarity of the voltage supplied to the +Uin and -Uin inputs of the ADC. is opposite to the usual one, therefore, at a positive temperature at the output 9 of the first bit of the ADC, there is a low logic level, which additionally includes two vertical elements d and e of the HG2 indicator. which form the "+" sign. The number "1" turns on on the indicator HG2 only when the measured temperature is equal to or exceeds 100°C.

The supply voltage of the entire device (-9 V) is stabilized by a stabilizer on the DA4 chip (4). To power the indicators HG1 - HG5, a voltage of -5 V is used, generated by the DA3 stabilizer. On fig. 1 shows the voltages relative to the top power wire according to the diagram.

All parts of the device, except for the DA1 sensor, the SA1 switch, and the lithium transformer (not shown in the diagram) are installed on a single-sided printed circuit board measuring 85x105 mm made of foil fiberglass 1 mm thick (Fig. 2). Dashed lines show overhead conductors, they can also be made in the form of printed tracks on the second side of the board.

During installation, MT and MLT resistors were mainly used. RIZ. R15 - C2-29V, but they can also be selected from among MT or MLT with an error of 1 ... 2%. Small-sized foreign analogs of domestic K50-35 are used as oxide capacitors; C5, C13, C14 - K73-17, the rest - KM-5 and KM-6. Trimmer resistors - SPZ-19a. The SA1 switch is a small-sized toggle switch with the middle position PT23-2B. Chip K1019EM1 (DA1, DA2) can be replaced with LM335. and KR142EN19 (DA5) - on TL431 or LM431. Chips DA3 and DA4 - any integrated voltage regulators, respectively -5 V (for example, KR1162EN5B. KR1179EN5 or imported - 79M05.7905 with any prefixes and suffixes) and -9 V (for example, KR1168SN9. KR1162EN9A, KR1162EN9B, KR1179EN9,79L05,79, 09.7909M4 .3 [25]). The DA25 chip is mounted on a finned heat sink measuring 10xXNUMXxXNUMX mm.

Indicators - imported with a character height of 20 mm and a high brightness of the glow at a current through the element of 5 mA - this is exactly the current provided by the KR572PV5A microcircuit. As HG1, HG3 - HG5, HDSP-3901 indicators from Hewlett-Packard, similar in design to HD3P-3906 (HG2), are suitable. You can also use any indicators with a common anode and sufficient brightness at the specified current. Of the domestic ones, these are indicators of a red glow with a character height of not more than 7.5 mm - AL305A-AL305G, AL309A-AL309E. ALS312A, ALS312B, ALS324A. ALS324B. In place of HG2, it is permissible to install the ALC326A indicator. In the absence of a specialized indicator, a conventional seven-element indicator can be used to indicate the temperature sign and the first digit "±1". In this case, the "+" sign is not indicated, but the indicator element is used for the sign. The connection of the inputs + Uin and - Uin of microcircuit 003 (pins 31 and 30) to the rest of the device will need to be reversed.

The board is placed in a case glued from organic glass, the front wall is colored transparent. The maximum possible number of ventilation holes with a diameter of 6 mm is drilled in the upper and lower walls.

Sensor DA1 is connected to the thermometer with a shielded wire (the shield must be connected to the positive terminal of capacitor C1) up to 5 m long. it was not exposed to direct sunlight.

The RW900 network adapter [5] was used as a power transformer, from which all unnecessary elements were removed. A resistor MLT-2 with a resistance of 1 kOhm is connected in series with the primary winding of the transformer. Its resistance is specified with that. so that the voltage across capacitor C9 is 20 ... 22 V. You can also use any transformer with a voltage on the secondary winding of 7,5 ... 8 V at a current of 150 mA.

Adjusting the thermostat is easy. By placing the DA1 sensor in melting snow or ice (but not in water with ice), the trimming resistor R10 must achieve zero readings on the indicators when the SA1 switch is set to the "U" position. Next, the sensor is lowered into water heated to 30...40X. The temperature of the water must be monitored with an accurate thermometer. Then, with a trimming resistor R14, it is necessary to set the corresponding readings on the indicators.

In conclusion, switch SA1 to the "P" position, hang a reference thermometer next to the adjustable thermometer in the case on the wall of the room, and after 20 ... 30 minutes, use the tuning resistor R3 to achieve equality of their readings.

The range of measured temperatures is -40...+40°C, the accuracy is mainly determined by calibration, the maximum achievable value is about ±0,2°C.

Literature

1. Fedorkov BG, Telets VA, Degtyarenko VP Microelectronic digital-to-analog and analog-to-digital converters. - M.: Radio and communication, 1984.
2. Biryukov S. Thermal sensors K1019EM1. K1019EM1A. - Radio. 1996. No. 7. p. 59.60.
3. Yanushenko E. Chip KP142FH19. - Radio. 1994. No. 4, p. 45.46.
4. Biryukov S. Microcircuit voltage stabilizers of wide application. - Radio. 1999. No. 2. p. 69-71.
5. Biryukov S. Network adapters. - Radio. 1998. No. 6. p. 66, 67.

Author: S. Biryukov, Moscow

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