Analog thermometers on logic circuits. Encyclopedia of radio electronics and electrical engineering

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

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The thermometers described in the article are built in an unusual way: in the first of them, the temperature-sensitive element (thermistor) is included in the integrating circuit, in the second - in the differentiating circuit. The change in the time constants of these circuits under the influence of the ambient temperature thermistor is converted into a change in the duty cycle of rectangular pulses, as a result of which the effective voltage at the output of the device changes, which is recorded by a microammeter. The devices are made on widely used digital microcircuits and are available for repetition even for novice radio amateurs.

The temperature-sensitive element in analog thermometers is most often included in the measuring bridge. Such a temperature sensor has a significant drawback associated with the need to limit the current through the bridge to values ​​that exclude self-heating of the resistors that form it. In addition, rather high requirements are often imposed on the stability of the voltage supplied to the measuring bridge. To amplify the signal taken from the bridge and stabilize the voltage applied to it, many analog thermometers use operational amplifiers. This complicates the design and adjustment of such devices.

The proposed pulse thermometer is free from these shortcomings. It contains a rectangular pulse generator, an integrating circuit with a temperature-sensitive element, a pulse shaper and a pointer indicator that registers an effective voltage proportional to the duty cycle of the pulses. CMOS digital microcircuits are most suitable for such a device: their low-level voltage practically does not differ from 0, and the high-level voltage from the supply voltage.

Schematic diagram of the thermometer is shown in fig. one.

On the elements DD1.1, DD1.2, a rectangular pulse generator with a repetition rate of about 60 kHz and a duty cycle of 2 is assembled. From the generator, the oscillations are fed to the integrating circuit RK1R2C2. Depending on the resistance of the thermistor (hereinafter referred to as the thermistor) RK1, the time constant of the integrating circuit changes and, accordingly, the duration of the pulses arriving at the input of the shaper, made on the elements DD1.3 and DD1.4. The duration of the pulses at the output of the DD1.4 element is proportional to the temperature and determines the effective voltage recorded by the RA1 device. The tuned resistor R1 serves to set "zero", R2 - to adjust the sensitivity (it is maximum at its minimum resistance). With a thermistor nominal value of not more than 5 kOhm, the dependence of resistance on temperature is close to linear in the range from -20 to +50 °C. The measurement error does not exceed ±1 °С.

The stability of the supply voltage (and, consequently, the amplitude of the pulses) is provided by a parametric stabilizer on the elements VD1 and R3. The current consumed by the thermometer does not exceed 7 mA.

All parts, except for the RK1 thermistor and PA1 microammeter, are placed on a printed circuit board made in accordance with fig. 2.

The board is designed to use fixed MLT resistors, SP5-3 wire trimming resistors, KM-6 capacitors (C1 and C2 - preferably M47 or M75 groups). Thermistor RK1 - KMT17 with negative TKS. Microammeter RA1 - M4387 or any other with a full deflection current of the needle up to 1 mA and an internal resistance of at least 500 ohms.

When establishing the thermistor, the thermistor is placed in a bath with melting ice and the trimmer resistor R1 sets the arrow of the RA1 device to the zero mark of the scale. Then the sensor is transferred to water heated to a temperature of +50 ° C, and the trimming resistor R2 is used to achieve the deviation of the arrow to the last mark.

To measure temperature in a wider range, for example, from -60 to +150 ° C, a resistor with a resistance of 3R or 1/3R, respectively, should be connected in parallel with the thermistor with resistance R or in series with it. The sensitivity of the device after such refinement, of course, will decrease, and the measurement error may increase up to ±3...5 °С. If higher accuracy is required, the indicated temperature range should be divided into two or three subranges and the thermistor linearized in each subrange. In this case, the measurement error can be reduced to ±1 ... 1,5 °C.

In TTL, TTLSh microcircuits, in comparison with microcircuits of the CMOS series, the logical levels differ significantly from ideal values. In addition, the basic elements of microcircuits of these series have very significant input currents. Therefore, a thermometer on such microcircuits should be assembled according to the scheme shown in Fig. 3.

Rectangular oscillations with a repetition rate of 60 kHz, generated by the generator on the elements DD1.1, DD1.2, are fed to the inputs of the buffer elements DD1.3 and DD1.4. They eliminate the mutual influence of the differentiating circuits C2R3RK1 and C3R4 and reduce the load on the generator, which favorably affects the stability of its frequency. Element DD1.6 generates a sequence in which the duration of the pulses is determined by the "exemplary" differentiating circuit R4C3, and DD1.5 is a sequence in which it depends on the resistance of the thermistor RK1 included in the measuring differentiating circuit RK1R3C2. As a result, a pulsating current flows through the device PA1, the effective value of which is proportional to the ambient temperature. With the values ​​​​of the elements of the differentiating circuits indicated in the diagram, the diodes VD1, VD2 can be excluded. However, if smaller resistors and larger capacitors C1 - C3 are used, these diodes are necessary to protect inverters DD1.5, DD1.6 from breakdown.

The thermometer uses parts of the same types as in the previous one. Instead of K555LN1, it is permissible to use K155LN1, K155LNZ, K155LN5, K1533LN6 microcircuits. The KD521A diode can be replaced with another diode of this series, as well as the KD522 series.

All parts, except for the RK1 thermistor and RA1 microammeter, are placed on the printed circuit board (Fig. 4). Setting the thermometer comes down to setting the maximum temperature with resistor R3, and zero with resistor R4. In the temperature range from -20 to +50 °С, the measurement error does not exceed ±1 °С.

This thermometer can measure body temperature. The device must first be calibrated in the range of +36. ..+40°С. To do this, the thermistor is placed in vaseline oil heated to +36 ° C and the microammeter needle is set to the zero mark of the scale with a trimmer resistor R4. Then, after raising the oil temperature to +40°C, the arrow is set to the last division of the scale with resistor R3. These operations must be repeated two or three times for better reproducibility of the measurement results. (When calibrating this instrument, it is vaseline oil that should be used, and not water, since the measurement results are significantly distorted due to the high electrical conductivity of aqueous solutions).

After calibration, the thermistor is placed in a glass tube, sealed on one side, and filled with epoxy. This design of the sensor eliminates the error in temperature measurement caused by the electrical contact of the thermistor with the patient's skin.

In the temperature range from +36 to +40 °C, the temperature dependence of the thermistor resistance is almost linear. When using thermostable capacitors (for example, mica or fluoroplastic) as C1-C3, the measurement error in this range will not exceed ±0,1°C.

Author: I. Tsaplin, Krasnodar

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