DC motor control with ATmega32 Optocoupler and Darlington Pair

DC motors are commonly used in many applications, including robotics, automation, and various industrial applications. In order to control the speed and direction of these motors, a microcontroller is used to send signals to the motor. The ATmega32 microcontroller is a popular choice for controlling DC motors due to its versatility, reliability and performance.

One of the key challenges in controlling DC motors is ensuring that the signals sent to the motor are isolated from the microcontroller. This is necessary to prevent damage to the microcontroller due to the high voltages and currents that are present in the motor control circuit. One solution to this problem is to use an optocoupler, which is an electronic component that allows signals to be transmitted between two isolated circuits.

In addition to using an optocoupler, another component that is often used in DC motor control circuits is a Darlington Pair. A Darlington Pair is a type of transistor that is commonly used to amplify signals. When used in conjunction with an optocoupler, it can improve the performance of the motor control circuit.

To control the speed of a DC motor, a technique called Pulse Width Modulation (PWM) is often used. This involves sending pulses of varying widths to the motor, with the width of the pulses determining the speed of the motor. The ATmega32 microcontroller is capable of generating PWM signals, which can then be transmitted to the motor through the optocoupler and Darlington Pair.

In order to achieve optimal performance, it is important to carefully consider the design of the motor control circuit. This includes selecting the appropriate components, such as the optocoupler and Darlington Pair, as well as choosing the right values for the resistors and capacitors in the circuit.

When designing the motor control circuit, it is also important to consider the voltage and current requirements of the motor. The ATmega32 microcontroller has a maximum output current of 40 mA, so it is important to select a Darlington Pair with a high enough current rating to handle the requirements of the motor.

Here is how you can use optocoupler and darlington transistor to protect and improve performance of  DC motor control with ATmega32 microcontroller. The optocoupler used is 4N35 and the darlington pair is made up of 2N3904 transistors.

The following is the circuit diagram of interfacing DC motor with ATmega32A, 4N35 optocoupler, darlington pair.

DC motor ATmega32 optocoupler

In the above circuit diagram, the 4N35 optocoupler is used to isolate the DC motor and darlington pair. The pin 1 which is the anode of the 4N35 optocoupler is connected to the ATmega32 PD7(port D pin 7). The cathode of 4N35 optocoupler is connected to +5V. On the other side of the optocoupler. The collector C of 4N35 is connected to the +5V while the emitter E is connected to the base of the one of the darlington pair transistor. The darlington pair is connected in such a way that the emitter of the first 2N3904 transistor is connected to the base of the second 2N3904 transistor. The DC motor is connected as load at the collector of the first 2N3904 transistor. The diode across the dc motor acts as protection diode against sudden surge of current which might damage the microcontroller pin. Also a switch is connected to the pin PD2(Port D pin 2) of ATmega32. This switch turns the DC motor at different rate. The signal is sent from the Atmega32 to the optocoupler from the port PD7 pin. 

The program code is as follows.

#ifndef F_CPU
#define F_CPU 4000000UL

#include <avr/io.h>    
#include <util/delay.h>  

int main(void){
	DDRD |= (1<<PD7);
	DDRD &= ~(1<<PD2);
	PORTD |= (1<<PD2);

		if(PIND & (1<<PD2)){
			PORTD |= (1<<PD7);
			PORTD &= ~(1<<PD7);
			PORTD |= (1<<PD7);
			PORTD &= ~(1<<PD7);


In this DC motor control example we have used two general purpose transistor 2N3904 to make a  darlington pair transistor to drive the DC motor. Often due to rating of the DC motor dedicated DC motor driver such as L298N is used. Also here we have used the Atmega32 microcontroller. A simpler way of building such hardware to control dc motors is used Arduino. See the DC motor Speed control with Potentiometer and PWM using Arduino for this.

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