Design of automatic circuit for judging pins of low-power transistors

　　In electronic technology, transistors are a very common component. The parameters of transistors are closely related to the measurement schemes and results of many electrical parameters. Therefore, in electronic design, the judgment and measurement of transistor pins and types are very important. There are many ways to measure transistor pins. Among them, the laboratory often uses a multimeter and the characteristics of each transistor pin for measurement. However, due to the complex relationship between the voltage and current between the pins of the transistor and the small size of the transistor itself, it brings great inconvenience to the measurement. At present, there is no device on the market that can automatically distinguish the pins and types of transistors. Therefore, it is particularly important to design a circuit that can automatically distinguish the pins and types of transistors.

　　1 Hardware circuit composition principle

　　According to the types and pin arrangement of currently commonly used transistors, the designed automatic discrimination circuit includes four parts: central control unit, conversion circuit, detection amplifier circuit and display circuit, as shown in Figure 1, in which AT89C2051 is used as the central control unit.

　　2 Hardware Circuit Design

　　Figure 2 shows the schematic diagram of the hardware circuit for automatic identification of transistor pin types. The hardware circuit mainly includes microcontroller AT89C2051, inverter CD4069, photocouplers 4N25, 74LS06, 74LS07, several resistors and capacitors and other components.

　　Figure 2 Schematic diagram of the discrimination circuit

　　First, the P3.0~P3.2 port of the single-chip microcomputer sends out a three-bit binary code (different high and low levels) and sends it to the 1st, 2nd and 3rd pins of the transistor respectively. For different transistors, when the single-chip microcomputer sends out different codes, the current direction on the 1st, 2nd and 3rd pins is different, with two situations of inflow and outflow. Two photocouplers are connected in reverse parallel to detect which direction the current is passing. At this time, the three-bit binary code becomes a six-bit binary code. The detected electrical signal from the photocoupler is amplified. Since the output signal at this time is not a standard high or low level, it cannot be directly recognized by the single-chip microcomputer, and the phase does not meet the requirements, a first-level inverter CD4069 is added for inversion, and then the standard six-bit binary code output by the inverter is sent to the P1.0~P1.5 port of the single-chip microcomputer. The single-chip microcomputer compares the data read from the P1 port with the data pre-written in the single-chip microcomputer. When the corresponding conditions are met, the detection result is output from the P3.3~P3.7 port, and finally the corresponding transistor type is displayed by the light-emitting diode.

　　3 Software Design

　　Since the pin arrangement order of NPN transistors in the commonly used small and medium power transistors is EBC, ECB, and BCE (there are very few exceptions that can be ignored), and PNP has only one arrangement order of EBC. Therefore, the software is written according to this rule. The overall programming idea is to add different voltages to the three pins of transistors with various pin arrangement orders, test their current conditions and convert them into binary codes. These binary codes are written into the microcontroller, and the external input data is compared with the binary code inside the microcontroller. If the read data is equal to a certain data written in advance, the transistor being tested is the tube type and pin of the transistor corresponding to this data, and then the corresponding light-emitting diode is lit to indicate the tube type and pin.

　　The main program flow chart of the software is shown in Figure 3.

　　The corresponding program is:

　　ORG 0000H

　　AJMP MAIN

　　ORG 0030H

　　MAIN: MOV A,#00H

　　EBC: MOV P3,#0F8H

　　MOV P3,#0F9H

　　ACALL DEL1

　　MOV P1,#0FFH

　　MOV A,P1

　　CJNE A,#0E9H,BEC

　　S1: MOV P3,#0F4H

　　AJMP S1

　　BEC: MOV A,#00H

　　MOV P3,#0F8H

　　MOV P3,#0F9H

　　ACALL DEL1

　　MOV P1,#0FFH

　　MOV A,P1

　　CJNE A,#0E1H,ECB

　　S2: MOV P3,#0ECH

　　AJMP S2

　　ECB: MOV A,#00H

　　MOV P3,#0F8H

　　MOV P3,#0FDH

　　ACALL DEL1

　　MOV P1,#0FFH

　　MOV A,P1

　　CJNE A,#0D9H,EBC1

　　S3: MOV P3,#0DDH

　　AJMP S3

　　EBC1: MOV A,#00H

　　MOV P3,#0F8H

　　MOV P3,#0FEH

　　ACALL DEL1

　　MOV P1,#0FFH

　　MOV A,P1

　　CHNE A,#0D6H,E

　　S4: MOV P3,#7BH

　　AJMP-S4

　　E: MOV P3,#00H

　　ACALL DEL1

　　MOV P3,#0F8H

　　ACALL DEL1

　　AJMP E

　　DEL1: MOV R5,#01H

　　D1: MOV R6,#0FFH

　　D2: MOV R7,#0FFH

　　D3: DJNZ R7,D3

　　DJNZ R6,D2

　　DJNZ R5,D1

　　RIGHT

　　END

　　Figure 4 shows the PCB board diagram. After the physical object is successfully manufactured, take a transistor and insert the pins into the test hole of the product in the order of 1, 2, and 3 to ensure good contact. Then press the power button, the system automatically resets and runs, and the LED indicates the tube type and pin corresponding to the tested transistor. The order of LED lights corresponds to the tube type of the pins. If the first LED light on the left is on, the tested transistor is NPN type, and the pin arrangement order is BEC; if the second LED light on the left is on, the tested transistor is PNP type, and the pin arrangement order is EBC; if the third LED light on the left is on, the tested transistor is NPN type, and the pin arrangement order is ECB; if the fourth LED light on the left is on, the tested transistor is NPN type, and the pin arrangement order is EBC; if the four LED lights flash at the same time, it may be that the tested transistor is broken or the pins have poor contact, and the program corresponding to the tube type is not written in the single-chip microcomputer.

　　In the process of making the actual object, the corresponding pins and types can be marked on one side of the LED lamp from left to right, or different colors of LED lamps can be used to display different pins and tube types.

　　4 Conclusion

　　The circuit is welded and debugged according to the hardware circuit and software design. The designed discriminator can quickly and accurately determine the pin and type of the low-power transistor, and the judgment result is displayed by the corresponding indicator circuit. It is much more convenient and faster than measuring with a multimeter.

　　Since this design uses a single-chip microcomputer as the central control unit, it has strong scalability. For example, a circuit for measuring the β value of a transistor can be added to the basis of this work, and the β value can be displayed by a digital tube. In addition, this design can only measure common small and medium-power transistors. If a drive circuit and a current limiting circuit are added, and part of the source program is modified, high-power transistors can also be measured.