Circuit design for automatically identifying transistor pins

Introduction

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 the pins and types of transistors are very important. There are many ways to measure the pins of transistors. Among them, the laboratory often uses a multimeter and the characteristics of each pin of the transistor 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 arrangements of 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 the single-chip microcomputer AT89C2051, inverter CD4069, photocouplers 4N25, 74LS06, 74LS07, and several resistors, capacitors and other components.

Figure 2 Schematic diagram of the identification circuit

First, the P3.0~P3.2 port of the single-chip microcomputer sends out three-bit binary codes (different high and low levels) and sends them 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 microcontroller, 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 microcontroller. The microcontroller compares the data read from the P1 port with the pre-written data inside the microcontroller. When the corresponding conditions are met, the detection results are output from the P3.3~P3.7 ports, and finally the corresponding transistor type is displayed using a light-emitting diode. [page]

3 Software Design

Since the transistor pin arrangement order of NPN 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. Write these binary codes into the microcontroller, compare the external input data 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
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
CJNE 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
RET
END[page]

Figure 4 shows the PCB board diagram. After the actual production is successful, take a transistor and press the pins 1, 2, and 3. Insert them into the test holes of the product in sequence to ensure good contact, then press the power button, the system will automatically reset and run, and the LED will indicate 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 has poor pin contact, and the program corresponding to the tube type is not written in the microcontroller.

Figure 4 Physical PCB board diagram

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

4 Conclusion

According to the hardware circuit and software design, the circuit is welded and debugged. The designed discriminator can quickly and accurately judge the pins and types of low-power triodes, and the corresponding indication circuit displays the judgment result, which 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.