In Fig. 79 given schematic diagrams of the simplest semiconductor thermometers on diodes(Fig. 79, a) and a transistor (Fig. 79.6), published in one of the American radio magazines. In the thermometer, the diagram of which is shown in Fig. 79, a, the sensitive element (sensor) is four silicon diodes connected in series and powered by a direct current of 1 mA. In this case, a shift of the current-voltage characteristic of silicon diodes towards zero by 2.11±0.06 mVI°C is used. Thus, with an increase in temperature from -18 to +100 ° C, the voltage acting on each diode decreases by more than 400 mV (from 688 to 270 mV). Consequently, the voltage on all four diodes will decrease by 1.6 V, i.e. it will be 4 times greater.
To measure voltage fluctuations on the diodes, they are included in one of the arms of the bridge, which generally consists of a voltage divider across resistors R3-R5 and resistor R1 connected in series with diodes D1-D4. The thermometer indicator is a microammeter connected to the diagonal of the bridge through a variable resistor R2. The bridge is powered by a constant voltage of 6 V, stabilized by a silicon zener diode D5.
Setting up a diode thermometer comes down to calibrating its scale, which is done as follows. Diodes coated with waterproof varnish are placed in a vessel with water, the temperature of which is controlled with a mercury thermometer. The length of the conductors connecting diodes D1-D4 to the meter can be several meters. When cooling or heating water, you can go through the temperature range from zero to 100 ° C, while making the appropriate marks on the microammeter scale. “Zero” is shifted to the desired place on the instrument scale by adjusting variable resistor R4, and the temperature measurement range is selected using variable resistor R2. To power the diode thermometer, you can use any DC source with a voltage of 12-16 V.
A transistor thermometer, the circuit of which is shown in Fig. 1, is significantly more sensitive. 79, b.
This is explained by the fact that here a transistor is used as a sensitive element, operating in an amplifier stage assembled according to a circuit with separated loads. Thanks to the amplifying properties of the transistor, the sensitivity of the thermometer increases tens of times. The controls and settings here are the same as in the previously discussed design.
When making a thermometer according to the diagram in Fig. 79, or you can use diodes like D105 or D106 (D1-D4), KS156A (D5). In the thermometer according to the diagram in Fig. 79, b transistor T1 can be of type KT315 or KT312 with any letter index. A thermometer with a transistor of the KT312 type will have less thermal inertia, since this transistor has a metal body, while the KT315 has a plastic body.
All described thermometers can also measure negative temperatures down to -70 ° C. In this case, it is advisable to install a microammeter in the thermometer at 100 μA with zero in the middle of the scale.
Semiconductor thermometers are very convenient for remote temperature measurement. For example, by placing several groups of diodes at different points of the refrigerator, by switching them you can control the temperature of the corresponding area. Another example is measuring the temperature of the earth's surface and the near-earth layer of air. In rural areas, this is of great importance, as it can warn of the onset of spring and summer frosts on the soil. You can monitor the temperature of the soil or air in the garden or vegetable garden using the readings of a device installed directly in the room. There are other possible applications for semiconductor thermometers.
Vasiliev V. A. Foreign amateur radio designs. M., "energy", 1977.
On the eve of the onset of winter, the question arose of measuring the ambient temperature “overboard”, that is, on the street. Moreover, I wanted to do this without bothering myself with looking out for an outdoor alcohol thermometer through a frosty window, but simply observing the outside temperature remotely in the comfortable warm conditions of my home. An electronic thermometer is ideal for these purposes. This is what the article will discuss...
Actually, a digital electronic thermometer is sold already assembled and ready for use. 
This digital electronic thermometer is assembled on an ATtiny 2313 microcontroller. The temperature sensor is a DS18B20 product from Dallas Semiconductors. The characteristics of the thermometer are visible in the photo, so we will not repeat them.
To check the functionality of a digital thermometer, we connect it to a laboratory power supply and apply a voltage of, say, 12V (acceptable from 7 to 15V). I don’t have standard temperature meters (and I don’t need them), so we compare the readings of a digital thermometer with a regular household one. 
As you can see, the readings are very close - almost 19°C on an alcohol thermometer, and 18.8°C on a digital one.
This accuracy of a digital thermometer is more than enough for household needs.
I immediately wanted to test the operation of the digital thermometer at sub-zero temperatures, but since the temperature outside was still above zero degrees, I had to look for an alternative source of sub-zero temperatures. It turned out to be an ordinary freezer compartment of an ordinary refrigerator. Without hesitation, we place the temperature sensor in the freezer and wait a couple of minutes to ensure the stability of the readings. The thermometer showed minus 19 degrees Celsius. 
From here there are two important conclusions:
Since the test stage has been successfully completed, we will proceed to the final assembly of the thermometer.
For the body of the digital thermometer, we chose a plastic case that was lying around idle from the Soviet radio designer (set) Start-7176 “Electronic watch”. The watches I assembled from this set are also lying around somewhere.
The case has external dimensions WxHxD - 140mm x 90mm x 30mm. The internal dimensions are correspondingly slightly smaller.
The stumbling block was the choice of power source. There were three options:
I immediately refused to use a battery as a power source, given the fact that the digital thermometer consumes current up to 40 mA. The battery won't last long at this current.
A thin case with a depth of only 30 mm would not seem to allow placing a network power supply inside it. Therefore, option No. 3 looked most likely - an external power supply with a step-down transformer. I didn’t like this option; I wanted to get a candy bar, without any additional boxes or wires.
And a solution was found!
While going through my amateur radio junk, I noticed a charger from an old Samsung mobile phone. The nameplate on it informed that charging produces a voltage of 5V at a current of up to 1A. There was plenty of current in terms of current, but five volts of voltage was not enough. I had to open the charger case to see if it was possible to somehow increase the output voltage...
The body halves were glued together, so the body was simply broken. Inside there was a switching power supply board, and at first it seemed unclear what and how to do here. The dimensions of the scarf turned out to be suitable for placement in the selected case. 
View from the elements.
The marking of the microcircuit on which the charger is assembled is visible - SC1009PN. Please note that this chip does not have pin #6. This is done so that the high voltage on leg No. 5 does not transmit to the other legs of the microcircuit located nearby (Google said this).
On the reverse side of the scarf there are a couple of dozen elements in SMD design, among which the PC817 optocoupler and a six-legged microcircuit with two-letter markings stand out for their size. 
Searching for the datasheet for the SC1009PN did not yield anything. Knowledgeable people write that this is a specific custom-made microcircuit. There is an analogue - TNY264P.
We managed to find a circuit diagram for such a charger 
And here we see that the operation of the switching power supply through the PC817 optocoupler is controlled by a TSM1051 type microcircuit. This is that six-legged SMD chip with an incomprehensible designation.
But for the TSM1051 the datasheet is available online. You can see a typical connection diagram 
From the datasheet it follows that this chip is specially designed for use in such devices. But, most importantly, the output voltage of the power supply on this microcircuit can be changed within certain limits by changing the values of the divider resistors R1 and R2 (see typical connection diagram), or R10 and R11, R14 (see charging diagram above). This is exactly what we need.
A search for voltage divider resistors on a specific board showed that the desired resistor is marked R15 next to the TSM1051 chip and corresponds to resistor R1 on a typical connection circuit. 
The value of this resistor was 820 Ohms. By selecting the value of this resistor upward (it seems to be up to 1.8 kOhm), the output voltage was raised from 5 to 8.5 V.
Just what you need!! A test test of the digital thermometer power supply from the upgraded charger was successful. All that remains is to place it all in the case. Inside the case we fix the thermometer board, the power supply board, and on the back wall we place a connector for connecting the outside air temperature sensor.
Assembly is almost finished 
During the work, a desire arose to make it possible to measure air temperature not only outside, but also indoors.
For this, another DS18B20 sensor was used, which is installed directly on the rear wall of the case. To switch the sensors, a regular toggle switch is used, which is mounted on the front panel.
The switching diagram looks like this. 
To protect the outside temperature sensor from mechanical damage, we make a container like this from a piece of tube. A bracket is attached to the tube to secure the container on the wall (or anywhere convenient) in a place protected from direct sunlight and precipitation. 
We place the DS18B20 sensor inside the tube 
The power switch is mounted on the side wall 
All that remains is to check it in action...
Outdoor temperature 
This device was assembled at the beginning of October 2016 and at the time of writing (end of October) it had passed, so to speak, a full test cycle. Everything works flawlessly.
The only important point: there is no data on whether long-term, round-the-clock operation of mobile phone chargers is allowed. Therefore, in order to avoid overheating and fire, I do not recommend leaving the power source based on the mobile phone charger unattended. I turn off the device at night. For the sake of the experiment, I ran the thermometer without turning it off for more than a day - everything was absolutely normal, no heating of the elements was observed.
P.S. When frost sets in, I’ll add a photo of measuring the negative outside air temperature.
Updated November 30, 2016. Morning, frost...Here is how the thermometer displays negative temperature: 
How to Make a Simple Digital Temperature Meter (10+)
Simple DIY digital thermometer
I needed to quickly make a simple temperature meter from scrap parts. The circuit turned out to be simple, easy to replicate by people with basic electronics skills.
The design is a classic bridge. One arm of the bridge is made of a resistor and a temperature sensor with a linear dependence of voltage on temperature. The second arm is a voltage divider
The circuit is powered by a stabilized 9V voltage source. Attention! You cannot use sources that produce high-frequency pulsations at the output. The measuring instrument will not operate under such conditions. You can power it from a Krona battery, but then, as it discharges and the voltage on it decreases, adjustments will be required
Resistor R1- low-power 6.8 kOhm.
Resistor R2- low-power 30 kOhm.
Resistor R3- low-power trimmer 5 kOhm.
Meter A- a regular digital tester.
The voltage on the VD1 sensor is directly proportional to the temperature. Moreover, a change in temperature by 1 GHz leads to a change in voltage by 10 mV, which is very convenient, as it simplifies the conversion of instrument readings into temperature values.
We carry out the adjustment like this. We measure the ambient temperature with a regular thermometer. We turn on the tester and switch it to voltage measurement mode with a limit of 2000 mV. Using the tuning resistor R3, we achieve readings on the indicator equal to the current temperature multiplied by 10. That is, if it is 21 degrees in our room, then the indicator should show 210 mV.
That's it, now you can take measurements. The indicator readings must be divided by 10. If the indicator shows, for example, -120, then the temperature is -12 GHz.
Unfortunately, errors are periodically found in articles; they are corrected, articles are supplemented, developed, and new ones are prepared.
A thermometer is a necessary tool with which many measure the temperature of the air in the house, water, and also the body. There are various models of devices on sale, differing in appearance, measurement method (mercury, infrared, electronic), and also in cost.
But if you wish, you can make a thermometer from scrap materials with your own hands. The process will require patience and endurance, and you will also need ingenuity.
Liquid thermometer
A device made by yourself will last a longer period.
But before you start making it, it’s worth considering the types of thermometers:
Model of infrared non-contact thermometer
You can make different types of thermometers yourself - liquid, with a mechanical principle of operation, with metal spirals or tapes, electronic or digital.
The simplest option would be a cardboard product; it is quite simple to make.
Electronic and digital devices require experience and knowledge of electronics. For their manufacture, various circuits can be used that need to be connected correctly. Such devices are often used for freezers.
The device can be made from scrap materials that are available at home.
A homemade thermometer made from a plastic bottle is simple, the main thing is to prepare for the process. To get started you need materials:
Homemade thermometer
The manufacturing diagram for a homemade device looks like this:
Trial
After the thermometer is fully assembled, it must be checked. To do this, you need to alternately lower it into bowls of cold and hot water. When placed in cold water, the liquid level in the tube should decrease, and when placed in hot water, it should increase. If this happens, it means that the device is assembled correctly.
You can calibrate the product using a regular thermometer. To do this, you should bring it to the paper, lean it slightly and use a marker to make marks. Calibration will help you use a homemade device to measure the temperature of air or liquid.
Device diagram
Decoding the scheme indicators
If you are into technology, you can make an electronic thermometer. But you will need to purchase special parts for it. For self-production, a simple device with the following indicators is suitable:
Electronic thermometer board (connection diagram).
To make an electronic device for measuring temperature, you will need to purchase a special board. If you want the readings to be clear and visible from afar, then it is better to use large and bright LED indicators. The correct connection and connection of external elements to the board is shown in the figure.
Board with external elements
If the thermometer will be used to measure temperature outside, it must be mounted in a special box with a network adapter inside the apartment. The temperature sensor itself is connected using a flexible cable.
Board with flexible cable
The advantages of a self-made device include:
But there are several disadvantages:
Homemade thermometers are a great way to save money on a new device. A DIY device will last much longer than cheap measuring devices.
This simple device allows you to quickly (in a few seconds) measure the temperature of the human body, water, ambient air and any other objects in the range of 20 ... 45 ° C. Despite the simplicity of the scheme, the measurement accuracy is quite high – ± 0.1°С.
The heart of the device and, perhaps, the only relatively hard-to-reach part is an ST3-19 type thermistor with a nominal value of 10 kW. It is thanks to its small size that the temperature measurement time does not exceed several seconds. As can be seen from the diagram, the device is analog and is a measuring bridge that is powered by a stabilized voltage. Transistors VT1 and VT2 are used as a low-voltage zener diode.
When the temperature changes, the resistance of the thermistor changes, and the amount of imbalance of the bridge, consisting of elements R2, R5 and R8, is displayed on a dial indicator, the role of which is played by the PA1 microammeter. To calibrate the device, switch SA2 is used, which replaces elements R5 and R8 in one of the bridge arms with standard resistors R4, R6 and R7.
The thermometer is adjusted as follows. With the highest available accuracy, the resistance of resistor R8 is measured at a temperature of 20°C. The accuracy of the device will depend on the accuracy of this measurement. Next, resistors R6 and R7 are selected with values such that they add up to the measured resistance. They will be included in the calibration chain. Then we set the sliders of resistors R2 and R3 to the middle position and supply power to the circuit.
1.
We turn on SA2 in calibration mode. Using resistor R2 we bring the arrow of instrument PA1 to the zero mark.
2.
We place the temperature sensor on an object with a known temperature that lies in the measured range. This could be, for example, the human body's armpit. We move switch SA2 to the “Measurement” position and use resistor R3 to set the reading of device PA1 to a level that will correspond to this temperature.
We repeat operations 1, 2 again until (usually 3-4 times) until in the “Calibration” mode the device clearly shows 20 ° C, and in the “Measurement” mode - the pre-known temperature of the body being measured. At this point, setting up the device can be considered complete.
Thermometer type ST3-19
In the design, in place of VT1, VT2, in addition to those indicated in the diagram, you can use KT3102 with the letters A, B, C, D; any microammeter with a total deviation current of 50 μA will be suitable as PA1, and the larger the scale dimensions, the more accurately it will be possible to read its readings . Since the thermometer scale is almost linear, it can be calibrated in advance in the desired range, which can be slightly shifted and even expanded, although you should not get carried away with expanding the range - the calibration will be smaller and the visual error will be higher.
The device is powered by two galvanic cells with a voltage of 1.5 V or batteries of 1.25 V each, the current consumption in measurement mode is 3-5 mA. It is highly advisable to install multi-turn resistors R2 and R3 (for example, SP5-2), which allow for fairly smooth adjustment of the resistance. It is very convenient to place the thermistor in the body of the felt-tip pen, filling it with epoxy resin so that its measuring tip is in place of the tip of the “lead” of the new “felt-tip pen”. The measuring unit can be connected to the circuit with any stranded wire, twisting it in pairs. The length of the harness can reach 1 m.
