4 Push Button as Input and Led as Output with Pic Microcontroller

This is a simple tutorial/project on how to interface a transistor (2n2222), led and push button with Pic microcontroller. In the project i am going to switch on and off(blink) an led with the help of a push button. Since transistors are used at outputs we can drive heavy loads with the same circuit. Pic16f877 microcontroller is used in the project. Port-B of Pic16f877 is used as output port. Port-B of pic16f877 is an 8-bit port. 8 transistors are connected to the general purpose input/output pins (gpio's) of this port. Transistors bases are connected directly to the pins of port-b. Led's are connected to emitter side of transistor in series to a 560 ohm resistor. Collector side of each transistor is supplied an external 5 volt signal.
An external 20 Mhz crystal is used to supply clock to microcontroller. Crystal is attached to microcontroller in parallel to two 33 pf capacitors. Push button is attached to Port-D Pin#7. 1 K ohm resistor is attached in series to a push button, other leg of push button is attached to +5v signal. 

Push Button and Led with PIC16f877a microcontroller

Code is written using MPLAB-IDE and High Tech C compiler is used to compile code and generate Intel hex file of the code. First the necessary header file htc.h is included in the code. This header file is necessary to be included in every code that is written in MPLAB-IDE and uses High Tech C compiler to compile the code. If you didn't include this header file in your code the htc(high tech c compiler) compiler will be unable to recognize the code. This header file tells the htc compiler that this is his code and he has to compile it.

_XTAL_FREQ 20e6 statement is specifying the crystal frequency that is attached to the microcontroller. I am Using 20 Mhz external crystal with the controller so i specified 20e6(Equivalent to 20e6=20,000,000).

Statement #define Input RD7 is defining Port-D Pin#7 as Input. Now we can access Port-D Pin#7 with the name 'Input'. TRISB=0x00; Statement is declaring Port-B as Output. TRISD=0x80; Statement is declaring Port-D Pin#7 as Input. (0x80=1000 00000

Declaring Pic Microcontroller Port as Input/Output

Switching on and off(blink) an led logic is placed in while(1) loop. When ever we press the push button, Current passes through the button towards Port-D Pin#7 and microcontroller reads it as logic-1. As a result Port-B outputs 0 and transistors base receives no current due to which their is no current flow between collector and emitter and led becomes off. Now when ever button is released no current flows through the circuit and Port-D Pin#7 reads it as logic-0, this outputs 1 at Port-B and base of transistors receives some current. Since base receives some current so the channel is on now. Current start flowing through collector to emitter and led becomes on.

Push button to switch off led and release button to switch on led. You can also reverse the logic. Just change the statement if(Input==0) to if(Input==1). This will on led when Push button is pressed and off led when released. Pushing button and releasing it continuously will cause your led seem blinking.

Project Code

Download the project code from the links given at the bottom of the Post.

#include < htc.h>
#define _XTAL_FREQ 20e6
#define Input RD7
void main()
{

TRISB=0x00; //Port-B as Output
TRISD=0x80; //RD7 as input
while(1){

if(Input==0)
PORTB=0xFF;
else
PORTB=0x00;
}
}

C-like

Driving heavy Loads(using Transistor) with the same configuration given above

You can also drive heavy loads with the same circuit, just remove led and attach a heavy load like Toy dc motor. Use a transistor which can drive heavy loads. 2n2222 supports 40 volt and 800 mA between collector and emitter. You can drive a load which consumes 40 volt and 800 mA using the circuit given above.
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Some tutorials on driving heavy loads with microcontrollers are below.