Transistor 2N2222A wont accept base input from arduino

Question

I'm New in electronics. I read "Robot Building for Beginners" book and I'm trying to make a line follower circuit. Instead of LM393 and 2N2907A I'm using arduino nano and 2N2222A. Here's my code:

const int leftPin = 9;
const int rightPin = 2;

int leftSensor;
int rightSensor;

bool switchLine = HIGH;

void setup() {
  pinMode(11, INPUT_PULLUP);

  pinMode(rightPin,OUTPUT);
  pinMode(leftPin,OUTPUT);
  digitalWrite(rightPin,LOW);
  digitalWrite(leftPin,LOW);
}

void loop() {
  switchLine = digitalRead(11);

  leftSensor = analogRead(6);
  rightSensor = analogRead(5);

  if(leftSensor > rightSensor)
  {
    if(switchLine)
    {
      digitalWrite(rightPin,LOW);
      digitalWrite(leftPin,HIGH);
    } 
    else
    {
      digitalWrite(rightPin,HIGH);
      digitalWrite(leftPin,LOW);
    };
  }

  if(leftSensor < rightSensor)
  {
    if(switchLine) 
    {
      digitalWrite(rightPin,HIGH);
      digitalWrite(leftPin,LOW);
    } 
    else
    {
      digitalWrite(rightPin,LOW);
      digitalWrite(leftPin,HIGH);
    };
  }
  delay(20);
}

And here's my circuit (I did not draw other parts of the circuit because they work fine):

^{simulate this circuit – Schematic created using CircuitLab}

If I connect one LED without transistor to arduino output it works, if I connect base of transistor to positive terminal three LEDs light up, but when I connect base to arduino it does not work. Multimeter shows there is 5V on base of the transistor but no current.

Answer 1

what is the vf of the LED’s. if you want to turn on the transistor , it base should be around 0.7 V above emitter. that means the output of the MCU should be (0.7 + (3x Vf_LED)).

Answer 2

if I connect base of transistor to positive terminal three LEDs light up

Yes, that'll probably work despite the transistor being wired as an emitter follower.

but when I connect base to arduino it does not work. Multimeter shows there is 5V on base of the transistor but no current.

Because the transistor is wired as an emitter follower - the emitter "follows" the base voltage but about 0.7 volts lower. So with 5 volts on the base there will be about 4.3 volts at the emitter and each LED will share that voltage because they are in series.

That's approximately 1.43 volts across each LED and, unlesss they are infra red devices (requiring a lower forward voltage to illuminate) you won't see any glow.

The most appropriate way to drives LEDs in a situation like this is to put them in the collector like this: -

Now the BJT fully turns on to virtually a short circuit (maybe 100 mV across it) and there is almost the full 9 volts across the current limiting resistor and LEDs. It's usually the most appropriate way to simply drive motors too except i would consider using an N channel MOSFET instead of a BJT.