I have built two parallel H-bridges from "Design 1" on this page as you can see in the attached schematic.

However when I apply the external power (VCC1 in the center of the diagram) the two motors do not spin.

Using my multimeter, I can see that the entire circuit is working: the motor is getting a 'forward' and 'backward' current as the Arduino sets the pins high. The multimeter is reading +/- 5.54V with the motor replaced by the multimeter. (Across the motor this drops to 0.13V)

So then I measure the current by putting my multimeter both in series with the motors and replacing the motors. There's only 40mA available! Looking around the web I found the suggestion to reduce the 220Ω resistors at R2, R3, R5 and R6. So I switched in (randomly) 27Ω resistors and the output current is still only 40mA.

What else should I try to bring up the current enough to drive the motors?

Edit: So my uneducated guess is that it's likely that the limitation is coming from the transistors. The Ns are BC549 and the Ps are BC559. The datasheets list "low current" as a feature, but I figured that was the collector.

Edit 2: The motor is this one which requires 70mA when running (and presumably more during startup)

Schematic of two H-bridges controlled by Arduino

Edit 3: Code driving the Arduino:

// Pin setup
int pinLeftMotorFwd   = 5;
int pinLeftMotorBck   = 6;
int pinRightMotorFwd  = 10;
int pinRightMotorBck  = 11;

// Directions for the {left, right} motors to achieve the four directions
int FORWARD[]   = {  1,  1 };
int BACKWARD[]  = { -1, -1 };
int RIGHT[]     = { -1,  1 };
int LEFT[]      = {  1, -1 };

void setup() {

    pinMode(pinLeftMotorFwd,  OUTPUT);
    pinMode(pinLeftMotorBck,  OUTPUT);
    pinMode(pinRightMotorFwd, OUTPUT);
    pinMode(pinRightMotorBck, OUTPUT);

void loop() {
    go( FORWARD, 10 );
    go( RIGHT,    3 );
    go( FORWARD, 10 );
    go( LEFT,     6 );  

// Move the bot in a particular direction from the array above for a given number of seconds
void go ( int dir[], int dur ){
    leftMotor(  dir[0] );
    rightMotor( dir[1] ); 
    delay( dur * 1000 );

// Sill both motors (OK, so stop both motors. But stop is a keyword in C)
void still(){
    leftMotor(  0 );
    rightMotor( 0 );

// Set the left motor to Backward (-1), Forward (1), or Stop (0)
void leftMotor( int dir ){
    // Pull both pins low
    digitalWrite(pinLeftMotorFwd, LOW );
    Serial.println("pinLeftMotorFwd LOW");
    digitalWrite(pinLeftMotorBck, LOW );
    Serial.println("pinLeftMotorBck LOW");

    // If we're going forward, pull the Fwd pin high
    if( dir == 1 ){
        digitalWrite(pinLeftMotorFwd, HIGH);
        Serial.println("pinLeftMotorFwd HIGH");

    // otherwise, pull the Bck pin high
    else if( dir == -1 ) {
        digitalWrite(pinLeftMotorBck, HIGH);
        Serial.println("pinLeftMotorBck HIGH");

// Set the right motor to Backward (-1), Forward (1), or Stop (0)
void rightMotor( int dir ){
    // Pull both pins low
    digitalWrite(pinRightMotorFwd, LOW );
    Serial.println("pinRightMotorFwd LOW");
    digitalWrite(pinRightMotorBck, LOW );
    Serial.println("pinRightMotorBck LOW");

    // If we're going forward, pull the Fwd pin high
    if( dir == 1 ){
        digitalWrite(pinRightMotorFwd, HIGH);
        Serial.println("pinRightMotorFwd HIGH");

    // otherwise, pull the Bck pin high
    else if( dir == -1 ) {
        digitalWrite(pinRightMotorBck, HIGH);
        Serial.println("pinRightMotorBck HIGH");
  • 1
    Design 1 would not have been my first choice. – Ignacio Vazquez-Abrams Dec 28 '14 at 7:09
  • What is the VCC1's power source. Is it a battery? By the way, 220 Ohm is pretty low value. That will result in 22mA of current (at 5v) going into the base. With a gain of 270 that would result in a collector current of around 6A. I'd try using increasing the resistors to 1 kOhm to 5 kOhm. – Gerben Dec 28 '14 at 18:53
  • @Gerben: Ahh .. I see I may have messed up. I reduced the resistance. I'll try increasing it before I switch to a different circuit per Mark Williams below – RickMeasham Dec 29 '14 at 1:39

Design 1 wouldn't be my first thought on how to do this either! The transistors aren't a great choice, but are cheap & common.

The layout here is more conventional, and you can physically link 1-2, & 3-4 to connect to your arduino without the risk of short circuits.

I suspect your code may be setting both lines high together - the only advantage I can see to the posted circuit is that this does not necessarily cause smoke (but may well do as you describe). This is why posting the code is recommended!

  • Thanks for the suggestion. The transistors were on-hand. Ultimately once I understand the concepts from "first principles" / discrete components, then I'd just replace the whole lot with either a motor driver circuit or a dedicated H-bridge. The table at the bottom of the circuit you suggest indicates I need to switch pins between VCC/GND/Disconnected. How can linking 1-2 attached to a pin and 3-4 attached to another pin give me full control? (Code isn't setting both high, but I'll attach it anyway) – RickMeasham Dec 28 '14 at 22:43
  • 1
    The point of the H-Bridge is that the transistors work in pairs - so you would have 1&2 or 3&4 on together, but never 1&3 or 2&4 - if you go back to 'Design 1' you will observe that a single input has been put through an inverter stage to ensure this doesn't happen - your re-design replaces that with software - OK 'til a bug sets up a short.. So with the standard arrangement on that link, the truth table at the bottom (disregard the not-connected options) shows you the combinations. You have off/brake/forward/backward and possibly PWM as well.. – Mark Williams Dec 28 '14 at 23:51
  • No diodes at home, so I built it on circuit.io: 123d.circuits.io/circuits/511807-h-bridge where I had the exact same problem as my original circuit. Until I realised the NPNs were in backwards. Then everything came good. Going to check my original circuit tomorrow and see if I did the same there before I rebuild. – RickMeasham Jan 2 '15 at 11:28
  • Turns out my actual problem was having the transistors around the wrong way. I couldn't get my circuit to work so I built yours and got exactly the same thing. The above circuits.io result gave me a clue so I played with them and got them around the right way. Turns out that the side that appears flat in Fritzing is the round side and vice-versa. – RickMeasham Jan 13 '15 at 11:42

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