Microcontroller based Diode and Bipolar Junction Transistor (BJT) tester

Introduction

Most of the digital multimeters these days have built-in features for testing diodes and sometimes transistors. The purpose of this project is to demonstrate a simple way to construct a testing device for diodes and bipolar junction transitors (BJTs) using a microcontroller. The testing algorithm is based on a simple fact that a working PN junction conducts current in only one direction. A PIC16F688 microcontroller is used in this project that switches the bias voltage across the PN junctions of diode and transistors, and determines if a particular junction is normal, open or short.

Theory

The logic behind testing a diode is straightforward. A diode is a PN junction that allows the conduction of current only in one direction. Therefore, a good diode will conduct current in only one direction. If it does in both the directions, it means the diode is short, and if it does in neither direction, it is open. The circuit implementation of this logic is shown below.

This concept can be easily extended to test a transistor by realizing that a transistor consists of two PN junctions: one betwen the base and the emitter (BE junction), and the another between the base and the collector (BC junction). If both the junctions conduct in only one direction, the transistor is normal, otherwise it is faulty. We can also identify the type (PNP or NPN) of the transistor by considering the direction of the current conduction. Three I/O pins of a microcontroller are required to implement the testing algorithm for a transistor.

The test sequence for a transistor would go like this.

1. Set D2 High and read D1 and D3. If D1 is High, BE junction conducts, otherwise not. If D3 is High, BC junction conducts, otherwise not.
2. Set D1 High and read D2. If D2 is High, EB junction conducts, otherwise not.
3. Set D3 High and read D2. If D2 is High, CB junction conducts, otherwise not.

Now, if only the BE and BC junctions conduct, the transistor is of NPN type and is working fine. And, if only the EB and CB junctions conduct, the transistor is still normal but the transistor type is PNP. All other cases (like EB and BE both conduct, or BC and CB both not conducting, etc.) indicate the transistor is not good.

Circuit Diagram and Description

The circuit diagram for this project is pretty simple. It has two push button switches for inputs, named Select and Detail. Pressing the Select button allows to choose between the diode and transistor testings, and the Detail button is active only in case of transistor testing and displays the details of the test results like the transistor type and both the junction status. The three legs of a test transistor (E, B, and C) are grounded through 1 K resistors, and the two PN junctions will be tested through RA0, RA1, and RA2 port pins of a PIC16F688 microcontroller. The testing of a diode requires only two pins, and will use the E and C pins. That’s why they have alternate labels, D1 and D2, in the circuit diagram.

Circuit setup on the breadboard

Software

The firmware for this project is developed with MikroC compiler. While programming, you must be very careful about the direction settings of the three test pins (RA0, RA1, and RA2) because they change frequently while running the testing algorithm. Before setting any test pin to High, you must make sure that the other two pins are defined as input pins so that there won’t be any voltage conflict or short circuit between the port pins.

/*

Project: Diode and Transistor Tester

Internal Oscillator @ 4MHz, MCLR Enabled, PWRT Enabled, WDT OFF

Copyright @ Rajendra Bhatt

November 9, 2010

*/

// LCD module connections

sbit LCD_RS at RC4_bit;

sbit LCD_EN at RC5_bit;

sbit LCD_D4 at RC0_bit;

sbit LCD_D5 at RC1_bit;

sbit LCD_D6 at RC2_bit;

sbit LCD_D7 at RC3_bit;

sbit LCD_RS_Direction at TRISC4_bit;

sbit LCD_EN_Direction at TRISC5_bit;

sbit LCD_D4_Direction at TRISC0_bit;

sbit LCD_D5_Direction at TRISC1_bit;

sbit LCD_D6_Direction at TRISC2_bit;

sbit LCD_D7_Direction at TRISC3_bit;

// End LCD module connections

sbit TestPin1 at RA0_bit;

sbit TestPin2 at RA1_bit;

sbit TestPin3 at RA2_bit;

sbit Detail at RA4_bit;

sbit SelectButton at RA5_bit;

// Define Messages

char message1[] = “Diode Tester”;

char message2[] = “BJT Tester”;

char message3[] = “Result:”;

char message4[] = “Short”;

char message5[] = “Open “;

char message6[] = “Good “;

char message7[] = “BJT is”;

char *type = “xxx”;

char *BE_Info = “xxxxx”;

char *BC_Info = “xxxxx”;

unsigned int select, test1, test2, update_select, detail_select;

unsigned int BE_Junc, BC_Junc, EB_Junc, CB_Junc;

void debounce_delay(void){

Delay_ms(200);

}

void main() {

ANSEL = 0b00000000; //All I/O pins are configured as digital

CMCON0 = 0×07 ; // Disbale comparators

PORTC = 0;

PORTA = 0;

TRISC = 0b00000000; // PORTC All Outputs

TRISA = 0b00111000; // PORTA All Outputs, Except RA3 (I/P only)

Lcd_Init();                      // Initialize LCD

Lcd_Cmd(_LCD_CLEAR);             // CLEAR display

Lcd_Cmd(_LCD_CURSOR_OFF);        // Cursor off

Lcd_Out(1,2,message1);           // Write message1 in 1st row

select = 0;

test1 = 0;

test2 = 0;

update_select = 1;

detail_select = 0;

do {

if(!SelectButton){

debounce_delay();

update_select = 1;

switch (select) {

case 0 : select=1;

break;

case 1 : select=0;

break;

} //case end

}

if(select == 0){   // Diode Tester

if(update_select){

Lcd_Cmd(_LCD_CLEAR);

Lcd_Out(1,2,message1);

Lcd_Out(2,2,message3);

update_select=0;

}

TRISA = 0b00110100; // RA0 O/P, RA2 I/P

TestPin1 = 1;

test1 = TestPin3 ; // Read I/P at RA2

TestPin1 = 0;

TRISA = 0b00110001; // RA0 I/P, RA2 O/P

TestPin3 = 1;

test2 = TestPin1;

TestPin3 = 0;

if((test1==1) && (test2 ==1)){

Lcd_Out(2,10,message4);

}

if((test1==1) && (test2 ==0)){

Lcd_Out(2,10,message6);

}

if((test1==0) && (test2 ==1)){

Lcd_Out(2,10,message6);

}

if((test1==0) && (test2 ==0)){

Lcd_Out(2,10,message5);

}

}  // End if(select == 0)

if(select && !detail_select){     // Transistor Tester

if(update_select){

Lcd_Cmd(_LCD_CLEAR);

Lcd_Out(1,2,message2);

update_select = 0;

}

// Test for BE and BC Junctions of NPN

TRISA = 0b00110101; // RA0, RA2 I/P, RA1 O/P

TestPin2 = 1;

BE_Junc = TestPin1 ; // Read I/P at RA0

BC_Junc = TestPin3;   // Read I/P at RA2

TestPin2 = 0;

// Test for EB and CB Junctions of PNP

TRISA = 0b00110110; // RA0 O/P, RA1/RA2 I/P

TestPin1 = 1;

EB_Junc = TestPin2;

TestPin1 = 0;

TRISA = 0b00110011; // RA0 O/P, RA1/RA2 I/P

TestPin3 = 1;

CB_Junc = TestPin2;

TestPin3 = 0;

if(BE_Junc && BC_Junc && !EB_Junc && !CB_Junc){

Lcd_Out(2,2,message3);

Lcd_Out(2,10,message6);

type = “NPN”;

BE_info = “Good “;

BC_info = “Good “;

}

else

if(!BE_Junc && !BC_Junc && EB_Junc && CB_Junc){

Lcd_Out(2,2,message3);

Lcd_Out(2,10,message6);

type = “PNP”;

BE_info = “Good “;

BC_info = “Good “;

}

else {

Lcd_Out(2,2,message3);

Lcd_Out(2,10,”Bad “);

type = “Bad”;

}

}

if(select && !Detail){

debounce_delay();

switch (detail_select) {

case 0 : detail_select=1;

break;

case 1 : detail_select=0;

break;

} //case end

update_select = 1;

}

if(detail_select && update_select){

// Test for BE Junction open

if(!BE_Junc && !EB_Junc){

BE_info = “Open “;

}

// Test for BC Junction open

if(!BC_Junc && !CB_Junc){

BC_info = “Open “;

}

// Test for BE Junction short

if(BE_Junc && EB_Junc){

BE_info = “Short”;

}

// Test for BC Junction short

if(BC_Junc && CB_Junc){

BC_info = “Short”;

}

Lcd_Cmd(_LCD_CLEAR);

Lcd_Out(1,1,”Type:”);

Lcd_Out(1,7,type);

Lcd_Out(2,1,”BE:”);

Lcd_Out(2,4,BE_info);

Lcd_Out(2,9,”BC:”);

Lcd_Out(2,12,BC_info);

update_select = 0;

}       // End if (detail_select)

} while(1);

}

Download Source Code and HEX file

Output

Here are some of the pictures  I took of my diode and BJT tester while testing various input conditions.

Rest of the pictures