Dual Object Digital Thermometer

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

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To measure the air temperature in the room and outside it, as a rule, two different thermometers are used, although it would be quite possible to get by with one having two temperature sensors. Such a device will be discussed. The device is based on K1019EM1 integrated temperature sensors and KR572PV5 double-integration ADC.

The thermometer (Fig. 1) allows you to alternately measure the temperature of two different objects, and the signals from the temperature sensors can be switched both manually and automatically. One of the sensors is located directly on the device board, the other is remote.

Rice. 1 Schematic diagram of the device (click to enlarge)

The presence of a stabilizer implemented on the DA1 chip and the VT1 transistor makes it possible to use a wide voltage range from 7,5 to 20 V to power the device, and the temperature stability of its output voltage is several times better than that of the internal stabilizer KR572PV5 due to the use of an adjustable precision zener diode TL431C (domestic analogue of KR142EN19). All thermometer circuits are powered from this +5 V voltage regulator, including a pulse voltage inverter, made on the elements DD3.4-DD3.6, VD1, VD2, C11, C12. It produces a voltage of -4,5 V, which is connected to the ADC pin 26. The variable voltage (50 kHz) necessary for the operation of the inverter is taken from pin 38 of the same microcircuit.

For the most complete realization of the capabilities of temperature sensors DA2, DA3, a constant current of about 1 mA must flow through them. This mode is provided by current sources on transistors VT2, VT3 and VT4, VT5. The current value can be adjusted by resistors R15, R16. Switching of sensors is carried out by analog keys DD2.1 and DD2.2, which are part of the K561KTZ microcircuit. Its third element DD2.3, together with the resistor R12, performs the function of a logic signal inverter that controls the keys, and the fourth, DD2.4, together with the DD5.3 element and the DD4 counter, is involved in the thermometer operating mode selection circuit

Each press of the SB1 button leads to a change in the state of the outputs of the ring counter, which determines one of three options for connecting temperature sensors to the ADC input, namely:

• connected internal sensor (DA2);
• a remote sensor (DA3) is connected;
• the sensors are connected alternately with an interval of 3...4 s.

In the latter case, the keys are controlled by the output signal of the multivibrator (DD3.1-DD3.3, R17, R18, C3). In order to be able to determine which of the sensors is connected at the moment, simultaneously with the connection of the DA2 sensor, the sign ":" (colon) appears on the HG1 indicator, which indicates that the sensor located on the board and measuring the temperature in the room is on. The mode of alternate temperature display is accompanied by the appearance of the sign "~" (tilde) on the indicator. The display of these signs, as well as the symbol "°С" and the decimal point in the second digit of HG1, is carried out using the elements DD5.1, DD5.2, DD5.4.

The output voltage of the K1019EM1 microcircuit in millivolts is proportional to the absolute temperature and is equal to 10-Tk, where Tk is the temperature in degrees Kelvin. The offset of the output signal level of the sensors by -2731,5 mV, necessary for the transition from the Kelvin temperature scale to the Celsius scale, is carried out by the input differential stage of the ADC itself. The offset value is determined by the position of the trimmer resistor R8. Resistor R10 sets the level of reference voltage DD1.

In the thermometer, you can use fixed resistors of the type C2-23, C2-33 (5%), however, to achieve maximum accuracy with significant fluctuations in ambient temperature, it is better to stop at C2-29V with a deviation from the nominal value of 1 ... 2%. First of all, this applies to resistors R6, R7, R9, R11 and R13. The device also uses variable resistors SDR-19a. Capacitors C6, C7, and C12-electrolytic K50-35 or similar imported, C4 - K73-16, K73-17. The remaining capacitors are KM5, KM6, K10-17. SB1 is a miniature button with a pair of closing contacts. The power source of the device is any, including unstabilized. Current consumption - 7 ... 12 mA - depends on the source voltage.

Remote sensor DA3, located outdoors, is shunted by capacitor C6 to protect against possible interference and interference. For the same purpose, it is desirable to use a shielded wire to connect this sensor to a thermometer, and the design should provide for its isolation from atmospheric influences in the form of coating with electrical varnish or epoxy resin. You can also use heat shrink tubing of a suitable diameter.

Before setting up the thermometer, make sure that the voltages at pins 1 and 26 of DD1 relative to the common wire are +5 and -4,5 V, respectively. ) and by placing the DA3 sensor in a mushy mass of melting ice or snow, the trimming resistor R8 achieves zero readings of the indicator. Further, the DA3 sensor is transferred to hot water and, measuring its temperature with a standard thermometer, resistor R10 sets this value on the indicator. At the end of this procedure, the readings of the internal sensor are set with the SB1 button (the ":" sign appears on the indicator), the reference thermometer is placed in the immediate vicinity of the DA2 sensor and the resistor R20 achieves equality of the readings of the indicator of the device and the reference thermometer when measuring ambient temperature. The last operation can be performed without a reference thermometer. In this case, it is enough to bring the remote sensor as close as possible to the internal one and, in the mode of alternating temperature display, set the readings of the internal sensor equal to the readings of the external one with resistor R20.

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