PCB Blog - About PCB board design of digital display thermometer

1 Introduction
About PCB board design of digital display thermometer, with the development of science and technology and the rapid improvement of industrial technology, we often need to accurately measure and control the ambient temperature and equipment temperature in daily production. Therefore, it is very important to study the measurement and control of temperature. We want to sense the temperature from the outside, the key is the temperature sensor, which is done here with LM35. After obtaining the temperature value of the outside world, a certain display device is required to display it. The current popular method is to convert analog quantity into digital quantity through A/D converter, which is done here with ICL7107. Then display it through LED or LCD. The thermometer described below is displayed by dual common anode digital tube (LED).

2. Draw the digital thermometer circuit diagram and PCB board design method, the problems that need attention
If we want to successfully design the PCB board diagram of a thermometer, we generally have to go through the following steps: First, learn to draw the schematic diagram of the thermometer. When drawing a schematic diagram, you need to know which components are needed, the components that are not in the library or are difficult to find, small steps, we must build a component library (Sch.Lib) to meet the design needs, when making components, we should put more Each component is placed in a library for easy calling, and if necessary, the component should be described in the library file. You can select the component in the Components in Browse Schlib, and then describe it in the Description (Description, Footprint, default Designator, Sheet part Filename). We must pay attention to some small problems when making components, for example: when making TL431 component symbols, the pins are not placed on the grid, when transferring to the schematic diagram, the wires cannot be connected normally, and no nodes can exist on the pins. At this point, we can right-click in the production window to select Doument options, and set the Snap and Visibie in the Gird option in the pop-up Library Editor Workespace dialog box. When making components, you should also pay attention to skills. For example, with the help of similar components in the existing component library, copy them to your own production library and make slight modifications. When making integrated chips Icl7107, Icl7660, S-DSP, you must pay attention to the properties of each pin, and you must set them carefully one by one, so as to avoid the wrong connection in the schematic diagram, which will cause the chip to burn out during debugging. The second small step is to transfer the components to be used to the schematic diagram for connection. During the connection process, pay attention to the connection of the bus. The bus is only a schematic electrical connection, and the real connection is a network label. Therefore, network labels must be placed when using the bus. The third small step, after the circuit diagram is connected, we must perform ERC, Reports/Bill of material, and Create netlist operations. Electrical rule checking to correct circuit errors in circuit diagrams. Run the component report to view the component information of the circuit diagram. Create a netlist in preparation for PCB drawing. The designed schematic diagram is mainly composed of the following parts. The second step is to complete the PCB board production. The step of making a PCB board, determining the physical size of the board frame, the wiring size and the required various board layers, can be completed through the wizard. The second step, after defining the PCB board, call the menu bar command Design/Load Nets to transfer the components into the PCB board. When placing components, it should be noted that different components need to be placed on different layers, such as placing names ( Pinyin) in the BottomLayer layer, and the horizontal mirror image, can not use Chinese characters. The third step is to select the via form of the jumper. The fourth step, select the component form and the number of copper film lines passing between the pads, and call the menu bar command Design/Rules to set. The fourth step is to set the routing parameters. The fifth step, after connecting the PCB board, you need to check the design rules, and call the menu bar command Tools/Designs Rules Check. This can be done, which can avoid design errors and electrical errors.

3. Printing, transfer printing and corrosion processing and problems of copper-clad PCB board diagrams.
1) Print the PCB board diagram to the thermal transfer paper with a laser printer (use the thermal transfer paper to make the PCB board)
Add a printer in the start menu, and follow the steps of the add printer wizard (if there is no actual printer connected, you can choose a printer) PCB board map print settings: switch to the PCB board map to be printed in the window, under File, select Print/Preview, a document with a suffix of PPC format is displayed under Explorer, that is, the print document: Preview PCB PPC, switch the manager to Browse PCB Print, select the layer to be printed and set: the bottom layer of the printed graphics Set the MultiLayer to the upper layer in the Layers column, nCheck Show Holes in the Options column, nSelect Black & White (black and white graphics) in the Color Set column. When designing the PCB board diagram, place "String" on the Bottom Layer layer, and write your name (Chinese Pinyin) and design date in it. The name and date should be mirrored horizontally. Once set, print the underlying graphics onto thermal transfer paper.
2) Cutting material: cut the copper-clad plate according to the size, polish it with fine sandpaper, clean it
3) Transfer the bottom layer of the printed PCB board to the copper-clad board with a thermal transfer machine
4) Repair the board: repair a small amount of unprinted lines with quick-drying paint
5) Corrosion: Put the copper-clad board with the printed circuit board into the ferric chloride solution to corrode. If the solution concentration is high and the temperature is high, the corrosion rate is fast, but the concentration and temperature should be appropriate. Corrosion time to master.
6) Wash and dry
7) Drilling holes, select a drill with a suitable diameter: a drill with a diameter of 0.8mm for integrated circuit pins and general resistors and capacitors. If the component pins are thick, you can change the drill with a larger diameter according to the measured value. During the drilling process, the drill bit should be inserted into the fixture as deep as possible, and the fixture should be tightened.
8) Remove the ink on the copper film: Use organic solvent (alcohol, banana water, acetone, etc.) to wipe off the ink covering the copper film line. When the above solvent is not available, it can also be wiped off with sandpaper.
9) Sand and check the circuit board: After removing the ink, polish the copper film wire with finer sandpaper. The better the grinding, the better the welding. However, do not polish too much to avoid excessive loss of copper film thickness. After sanding, clean with a tissue. Check the circuit board: Use a multimeter to check whether there is a short circuit or open circuit between the lines on the PCB board. If there is a short circuit, it should be cut off, and if there is an open circuit, it should be soldered and connected.
10) Flux: Dissolve rosin in anhydrous alcohol to make flux for use. Note that the concentration of flux should be appropriate. Too thick, not bright after drying, too thin, poor soldering performance. Use a brush to apply the alcohol rosin flux evenly to the cleaned PCB board. When brushing, place the PCB board vertically to avoid the flux blocking the drilled holes. Just brush it once. Too many times of brushing is easy to wrinkle and not shine. Place the PCB board coated with alcohol rosin flux in the sun to dry or naturally dry. ICL7107 is a decimal double integral 3-bit analog-to-digital converter with an accuracy of ±2LSB (effective bit). When its typical reference voltages are 100.0 mV and 1000 mV respectively, and the corresponding peripheral circuit integrating resistor and integrating capacitor are selected, it can be Two types of circuits with a full scale of 200 mV and 2000 mV are constructed. The analog circuit is composed of a switch and an operational amplifier to realize signal sampling and integration. It adopts differential input, and the input impedance is 1O10Ω. The digital circuit consists of counters, latches, control logic and display decoders. The input analog signal voltage value is first converted into a time width signal proportional to it, and then the fixed frequency clock pulses are counted in this time width, the counting result is proportional to the number of the input analog signal, and the digital lock is performed. save and decode display. The conversion cycle is divided into 3 stages: automatic zero stabilization (AZ) stage, signal integration (INT) stage, and de-integration (DE) stage. The times are:
(1) Automatic zero stabilization stage (AZ), TAZ =1O00*TCL-(2000*TCL-1000*TCL*lN/VREF
(2) Signal integration stage (INT), the input analog signal is integrated, and the time is fixed: TINT= l000*TCL
(3) Inverse integration (DE) stage: This stage is to integrate the reference voltage VREF with the opposite polarity to the input signal. The time of the inversion integration is: T(DE)MAX =2000*TCL, from the inverse integration to the The time for the integrator output to return to the analog common terminal (C00M0N) voltage VCOM is proportional to the input analog voltage, and its digital reading is:

5. Assembly and debugging of components
5.1 The test method is adopted: the TEST terminal of pin 37 is short-circuited with pin 1, and the meter should display -188.8. At this time, you can check whether the display is missing strokes. If there is, most of the pins and connections are soldered. First, short-circuit IN+ and IN-, the meter should display "0000". If it is not zero, check the reference source capacitor C2 and the auto-zero capacitor C4 for leakage. Short the 3l pin with the 36 pin, the meter reading should be 1000. If there is a deviation, adjust the potentiometer RP. It still doesn't work, most of it is because RP and integrating capacitor C3 are bad. Short-circuit pin 32 and pin 26, the bit on the meter should display -l, and the other bits are off. Otherwise, check the power supply or change the chip.

5.2 Repair according to the fault phenomenon: This method is direct, and the fault is judged faster. For example: the input signal is zero, the meter display is not zero and keeps beating. Reference capacitor C2 and auto-zero capacitor C4 should be checked. If the reading deviation of the meter head is large, it may be the problem of the integrating capacitor C3, or the reference voltage has changed.

6. Experiment summary
a. Place the name (pinyin) on the BottomLayer layer, and mirror it horizontally, and cannot use Chinese characters
b. Check the netlist: According to the Nets in the schematic diagram, check them one by one under the manager Nets in the PCB board to see if there are any errors.
c. After the PCB board is made, DRC inspection must be carried out, no highlight (green) display
d. When making components, the position of the package diagram is not paid attention to. Generally, the No. 1 pad should be placed at the origin of the coordinates. If it is far away from the origin of the coordinates, it will be far away from the placement position when it is called into the PCB layout, and it will not even be visible.
e. Know how to test whether ICL7107, ICL7660, MC78058T, TL431, and S-DSP are normal.
f. ICL7660 has the function of inverting the voltage, ICL7107 has the function of D/A conversion and decoding, LM35 can convert the temperature signal into an electrical signal, and MC7805T has the function of providing +5V voltage on PCB board.