8 channel quiz buzzer circuit using 8051 microcontroller

Question

Project: 8 channel quiz buzzer circuit using 8051 microcontroller from this site.

For the first candidate who presses their button, their number will show on the 7-segment display and the buzzer will make a sound.I am using peizo-buzzer in the circuit.

After making all the connections, the display is working but the buzzer is not making any sound. Buzzer delay time in code in 1ms. When we give direct 5V supply to the buzzer it works, but not in the circuit.

Please give me some solution for this.

#include<reg51.h>

#define SEGMENT P2 // PORT2 to Segments of 7-Segment Display
#define SWITCH P1  // Input Switches (buttons) to PORT1

sbit buzz=P3^0; // Buzzer
sbit rst=P3^3; // Reset Switch (Reset the display) - not the microcontroller
sbit digit=P3^7; // 7-Segment Display Common Pin (to enable)


void delay (int); // Delay function

int x=0,y,z;
unsigned char ch[]={0xc0,0xf9,0xa4,0xb0,0x99,0x92,0x82,0xf8,0x80,0x98}; // Hexadecimal values from 0 to 9.

void delay (int d)
{
    unsigned char i;
    for(;d>0;d--)
    {
        for(i=250;i>0;i--);
        for(i=248;i>0;i--);
    }
}

void main()
{
    SWITCH=0xff;
    SEGMENT=0xff;
    digit=1;
    buzz=0;
    rst=1;

    while(1)
    {
        while(SWITCH==0xff); // wait until any button is pressed.

        while (SWITCH==0xfe) // Button 1 is pressed.
        {
            SEGMENT=ch[1];
            buzz=1;
            delay(1000); // Activate buzzer for 1 second.
            buzz=0;
            while(rst!=0); // display the digit until the reset is pressed.
        }

        while (SWITCH==0xfd) // Button 2 is pressed.
        {
            SEGMENT=ch[2];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        while (SWITCH==0xfb) // Button 3 is pressed.
        {
            SEGMENT=ch[3];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        while (SWITCH==0xf7) // Button 4 is pressed.
        {
            SEGMENT=ch[4];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        while (SWITCH==0xef) // Button 5 is pressed.
        {
            SEGMENT=ch[5];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        while (SWITCH==0xdf) // Button 6 is pressed.
        {
            SEGMENT=ch[6];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        while (SWITCH==0xbf) // Button 7 is pressed.
        {
            SEGMENT=ch[7];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        while (SWITCH==0x7f) // Button 8 is pressed.
        {
            SEGMENT=ch[8];
            buzz=1;
            delay(1000);
            buzz=0;
            while(rst!=0);
        }

        SEGMENT=0xff;
        rst=1;

    }
}

Answer 1

The 8051 and clones typically use an I/O configuration that is described as "pseudo-bidirectional". They have active pull-down but quasi-passive pull-up (there's a transistor that is turned on briefly to improve the rise time). This means that they can only source a tiny amount of continuous current to an external device. The datasheet (DC Characteristics, page 10) shows that V_OH drops to 2.4V at just 60 µA of current.

This is not enough current to drive your NPN transistor. Instead, try putting a logic-level N-channel MOSFET there.

---

Note also that your other NPN (the one attached to pin 17 — you really do need to use reference designators in your schematics!) should actually be a PNP. This requires inverting the logic in your code that drives pin 17.

Answer 2

You don't say what type of device the buzzer is (electromechanical, piezo, etc.) or how much current it requires to operate, but I don't think either of those is the problem.

One millisecond is too short a time for the buzzer to wake up and make enough noise to be heard. Increase the on-time to 1 second to verify that the code is working, then adjust to taste.

BTW, what type of buzzer is it? Datasheet or link?