How to easily make a pointer thermometer using 4 components

　　Temperature measurement is often required in daily life, such as adjusting the thickness of clothes according to temperature changes, determining whether to turn on the air conditioner for heating or cooling according to the temperature, etc. Based on the principle of temperature sensor, this pointer thermometer is simply made.

　　1. Select temperature sensor

　　There are many ways to measure the temperature of a device. With the development of integrated circuit technology, integrated circuits that can easily convert temperature into electrical signals have been developed, and they have good performance. The integrated circuit temperature sensor I chose is the LM35. It is a three-terminal element, and the output voltage is proportional to the temperature. For every 1 degree increase in temperature, the output voltage increases by 10mV. Its performance indicators are shown in the following table.

　　2. Choose a temperature indicator

　　With a temperature sensor, the temperature can be converted into voltage, which can be indicated by a multimeter. Based on this idea, the meter head was selected to indicate the temperature. I spent 9 yuan to buy an old meter head from an electronics store. After testing, the current when the needle is fully deflected is about 95μA, and the voltage across the meter head is about 0.09V at this time. Based on this, the internal resistance of the meter head is about 947Ω. With these test data, the necessary technical data is prepared for the following production.

　　3. Choose the right resistor

　　The selected meter head can only measure a maximum voltage of 0.09V, which means it can only indicate temperatures below 9°C. Therefore, a resistor must be added to increase its temperature measurement range. According to the voltage-current relationship in physics, a suitable resistor can be determined to increase the temperature measurement range. A 3.9KΩ resistor was selected for this production. Here is a point to note: there is no need to choose a precision resistor, just a general resistor will do.

　　4. Select the power supply

　　Since the power supply voltage requirement of the sensor is 4-20V, a single 9V square battery, model 6F229V, was selected. With the power supply, you can start making it.

　　5. Make a thermometer

　　Prepare four components, and then prepare tools and auxiliary materials such as soldering iron, solder wire, black tape, needle-nosed pliers, wires, etc. Use the soldering iron to solder the components according to the circuit (see the figure below). The battery, LM35 and the meter head have positive and negative polarities, so do not connect them in reverse. Note that it is best to connect the battery with a snap button so that the battery can be disconnected when not in use, saving electricity, extending battery life, and making it easier to replace the battery in the future. After the circuit is connected, the next key step is to change the meter head and draw the dial.

　　6. Draw the dial

　　The original dial of the meter head indicated the voltage value, which made it difficult to tell whether it was a voltmeter or a thermometer, and the indicated value needed to be converted, which was too troublesome. For this reason, the dial of the original meter head was removed (the meter head is a precision component. Be careful when disassembling and assembling), and then installed it in reverse. At this time, there was no scale indication on the dial. In order to accurately indicate the temperature. I recorded the scale position when there was no power supply, marked it with 0, and then used other thermometers to calibrate the second point. For example, when another thermometer (red water thermometer) indicated 27℃, I recorded the position indicated by the pointer and marked it with 27. After that, remove the dial, use a compass and a protractor on white paper, and carefully divide the dial between 0 and 27 into 27 equal parts, each grid is 1℃, and then draw the dial above 27 in the same proportion, install the dial, install the shell, and the whole production is completed. Note: If the measuring range includes 100℃, boiling water can be used as the second recording point of temperature calibration, and the second recording point is best close to the full scale value of the meter head, so as to reduce the error of the dial.

　　After comparing it with the red water thermometer at home, I found that the accuracy of this thermometer is really not bad, and the difference between the two thermometers is less than 1°C.

　　After using it, I found the disadvantages of the electronic thermometer. Because it is powered by batteries, the temperature indication is inaccurate after a few months of continuous use. Later, I found that the battery was dead. The red water thermometer does not have this problem. Of course, it also has many advantages, such as the pointer indication is clear and easy to identify, it can measure the local temperature of a certain point, and it reacts faster than the red water thermometer.