Why this DC motor won´t power up with these AA batteries?

Using Arduino Motors, Mechanics, Power and CNC
#1

Hi there,
I´m working on my first arduino project. It´s a bluetooth controlled toy car. The car isn´t a toy, I´ve bought the parts and ensembled them. I´m using arduino Mega 2560, and an Adafruit Motor shield.

The problem is that after a lot of reading, watching tutorials, and asking here in the forum, I´ve managed to get a working car. Just that: When I connect the car to the (motorshield ensembled) arduino, and the arduino to the computer using the usb port, I get wheels moving. But as it needs some extra power, I´ve attached to them the three AA batteries that came with the car that I´ve ensembled.

My problem is that when I use the batteries, the car won´t move. The arduino and motor shield leds will light up, but the wheels won´t move at all.
Why could that be?

Any help will be very much appreciated!

I´m attaching an image with the schematics, and the code that I´m using.
THANKS!!
Rosamunda

#include <AFMotor.h>
AF_DCMotor motor1(1);
AF_DCMotor motor2(2);
int SwichState=0; //estado inicial del swich

void setup() {
  pinMode(31,INPUT); //tomamos data del botón
  pinMode(51,OUTPUT); //preparamos la luz para encenderla al activar el motor
}
void loop() {
SwichState=digitalRead(31);
if(SwichState== HIGH){
motor1.setSpeed(1000);
motor1.run(FORWARD);
motor2.setSpeed(1000);
motor2.run(FORWARD);

digitalWrite(51,HIGH);
}else {
digitalWrite(51,LOW);
motor1.setSpeed(0);
motor1.run(RELEASE);
motor2.setSpeed(0);
motor2.run(RELEASE);
}
}

arduino troubleshoot.png
arduino troubleshoot.png1024×700 46.2 KB

#2

Put a voltmeter on the motor pins. Is any voltage reaching them? Are you sure your switch 31 is working as you expect?

I would put a Serial print after "if(SwichState== HIGH){" to verify that this code is actually reached.

But as it needs some extra power

3 AA batteries = less that 4.5v minus the voltage drop in the motor controller. You've got less power not more.

#3

I suggest you either get a decent mosfet based motor controller with a low voltage drop (unlike the one you have at the moment), or try powering it from four 1.2V NiMH AA cells to increase both the voltage and the current capacity.

#4

Four 1.2V NiMH AA cells equals 4.8v. 5-6 cells will probably be needed. A low voltage drop mosfet controller is of course a good choice.

#5

Thanks for your replies!!!
I think I´ll have to buy a decent mosfat after all :slight_smile:

But what intrigues me here is that if I eliminate the arduino from the picture, and connect both DC motors directely and only to those 3 AA batteries, the car runs perfectly well, I mean with enough power to get it moving.

Why it loses power with the arduino?

#6

Rosamunda:
Why it loses power with the arduino?

Because the Adafruit motor shield is based the L293D, on an old bipolar motor control chip that has a high voltage drop (typically 2.6V @ 1A load). A mosfet-based controller like this Motor Driver 2.5A MC33926 - ROB-11080 - SparkFun Electronics or this Pololu - TB6612FNG Dual Motor Driver Carrier will work much better for driving low-voltage motors. Note that the minimum supply voltage for these motor drivers is 5V and 4.5V respectively.

#7

Thanks for your reply!
(I´ll have to go and have one of these. I´m on holiday now, out of my country, and I´ve bought these stuff to get acquainted with the arduino while on it).

#8