[Guide] The principle of digital thermometer is very simple, and many people have DIY. But compared to other brands OMEGA's HH11B is slightly better in terms of product design, performance and cost. Is this the case? Here is a small series for everyone to disassemble the HH11B digital thermometer, to see where it is portable? How affordable? Compared with other brands of digital thermometer performance, how is the internal design?
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After reading the dismantling, I believe everyone knows about the HH11B digital thermometer. The price of OMEGA official website is 560RMB, which is similar to the price of similar products of other European and American brands. Of course, if compared with some domestic thermometer products, HH11B It is slightly upscale. Consumers who have high requirements for product details or who are interested in the OMEGA brand can buy one to see if it is worth the money.
At present, the commonly used contact thermometers can be roughly divided into thermal resistance and thermocouple. The thermal resistance is based on the characteristic that the resistance value of the conductor or the semiconductor changes with temperature, and the measurement accuracy is high, but the temperature measurement range is compared. Small, mostly used for medium and low temperature measurement; while thermocouple thermometers are different, the two ends of the conductor are joined into a loop, when the temperature of the two junctions is different, an electromotive force is generated in the loop. The electromotive force is used for temperature measurement, the thermocouple has a rapid temperature measurement, and the temperature measurement range is large, which is a product that is wider than the application range of the thermal resistance. OMEGA's HH11B is a digital thermometer based on the K-type thermocouple principle. Here is a portable economical digital thermometer HH11B from OMEGA, which is also one of the common contact thermometers.
Figure digital thermometer HH11B and type K thermocouple
HH11B specific parameters:
■Measurement range: -200 ~ 1372 °C (-328 ~ 1999 °F), -199.9 ~ 199.9 °C (-327.8 ~ 391.8 °F)
■Accuracy (K-type chrome aluminum): ±60 ° -1372 ° (0.1% +1 ° C reading)
■± at -60 ~ 200° (0.1% of reading + 2°C)
■-76 ~ 1999° ± (0.1% of reading + 2 °F)
■-76 ~ 328° ± (0.1% of reading + 4 °F)
■Temperature coefficient: 0.1 times 0 to 18 ° C and 28 ~ 50 ° C, the relevant accuracy specifications per ° C
â– Display: 3-digit LCD, maximum reading is 1999
â– Sampling rate: 2.5/s
â– Battery: 9V (included)
â– Battery life: usually 200 hours
From the parameters of the HH11B digital thermometer, we can see that through different K-type thermocouple selection, we can carry out temperature tests of different ranges, and the measured temperature values ​​may have different degrees of deviation due to the selection of different K-type thermocouples. However, the error range can be judged by the simple calculation formula above. Similarly, depending on the 3-digit LCD of HH11B, the highest pixel of the LCD can only display 0 or 1, so the maximum range we can measure is 1999, but this is for general occasions. This is more than enough. Dismantle HH11B
Like the simple design of the HH11B, the structure of the HH11B is also simple. Disassemble the HH11B without any effort. Unscrew the battery of the back battery compartment and you can see the 9V power supply battery. Continue to unscrew the remaining 4 screws to remove the back cover. I can see the inside of HH11B.
Figure disassemble the HH11B back shell
The circuit composition of the digital thermometer HH11B is not complicated, mainly based on the single-chip solution of the rich crystal electronic microcontroller FS98O22. Fujing Electronics is a well-known semiconductor company in Taiwan. Founded in 1995, it is dedicated to single-chip measurement technology solutions, the undisputed first in Asia (because no one is competing), although compared to European and American measurement technology. There is a certain gap, but the price factor makes him competitive. In the same kind of products, the price is lower than 2/3 of the European and American products.
Figure Microcontroller FS98O22 Segment LCD interesting "connector"
Flip the board, there are fewer circuits on the other side. Except for a segment LCD module and several function buttons (the black knob is a calibrated temperature varistor), there is nothing. Interesting is the segment LCD module and PCB. The way the board is connected.
Segment LCD and PCB connection
I am used to the usual LCD connector or FPC soldering connection. The HH11B's LCD interface makes people shine. Two rubbers? NO, the fool knows that rubber can't conduct electricity. How can it replace the function of the wire?
Do not sell off, the black and white thing on the back of the LCD screen is called "silicone zebra strip", that is, conductive silicone, it may be very novel at first glance, it is very interesting, but once you figure it out, you understand it, in fact, this is also A very common "connector" ''''''''''''''''''''''''''''''''' '''''''''''''''', such as the physical button of the mobile phone, the remote control of the home TV, the point where the hard key or soft key is in contact with the PCB board It is made of conductive silicone, for example, the black circle on the left function button of the above figure.
However, another problem that comes with it is how to ensure that the conductive silicone interface of the LCD and the metal contact of the PCB board realize the PIN-To-PIN. If it is misplaced, it will easily cause a short circuit and burn the board. The structural design of the HH11B, a plastic screen card cover and the structure of the front panel are fixed, so that you can completely avoid this problem during assembly, and there is no worries when unpacking and reinstalling. In some product designs that are simple and convenient, and require cost reduction, a "connector" of conductive silicone can be considered. K type thermocouple socket
Finally, we look at the K-type thermocouple socket, this thing is relatively simple, you can see a warning sign at the external K-type thermocouple interface of HH11B: maximum DC voltage 60V, maximum AC voltage 24V, internal pass mainly A 4pin pin header is connected to the motherboard (see figure below).
Figure K type thermocouple socket
The triode next to the K-type thermocouple interface on the small board above together with the previously mentioned knob variable resistor constitutes the hardware circuit for the HH11B digital thermometer temperature calibration function. In the whole product design, considering the plug-in method of the K-type thermocouple, the HH11B is designed as a control board and a small board structure. Otherwise, a main PCB board can get all the functions.
Finally, after disassembly analysis, we can basically understand the hardware block diagram (see the figure below)
Figure digital thermometer HH11B hardware block diagram
The principle of digital thermometer is very simple. I believe that users with this design basis can learn from and choose better solutions, such as Ti's ADS1118 chip mentioned above, DIY, a more cost-effective digital thermometer product. Of course, the author does not deny OMEGA's excellent product design on the HH11B digital thermometer. Although the hardware scheme is generally selected, the product details are all in place, there are no obvious defects, and the reliability is guaranteed. I believe it will still be accepted by some people. I like it.
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