Easy transistor circuit won't work

Other Hardware General Electronics
1

I have a problem with a simple transistor circuit I made.
The transistor won't turn on my 2 IR leds.

int transistor = 6;

void setup(){
  pinMode(transistor, OUTPUT);
  
}
  
  void loop(){
    digitalWrite(transistor, HIGH);

  }

The resistors are placed parellel to split the current to reduce heat.
The 2 IR leds work perfect when I connect them without the TIP121 transistor.
I've also tested another TIP121 transistor, same problem.
It's strange that the transistor works when it is in use with a push button(the push button tells te Arduino to turn the transistor on).

It's a little time ago I since I worked with Arduino, so maybe it's something stupid :slight_smile:
I hope it's all clear for you guys.

2

You ONLY have resistors on your LEDS... where is the resistor on the base of of the transistor? (Yes, you need one)

I assume you want something like this:

IRLED.JPG

3

There is no resistor for the transistor.
I saw on internet some tutorials without.
But if you say I need one, which one would be good?

4

It looks to me that you are missing at least 2 things:

  1. A resistor between the Arduino output and the base of the TIP121, without this you are likely to burn out the Arduino pin; and

  2. A ground wire between the emitter of the TIP121 and the Arduino, unless the ground is connected via the power supply.

How do you know your IR LEDs are not working?

5

dc42:
It looks to me that you are missing at least 2 things:

  1. A resistor between the Arduino output and the base of the TIP121, without this you are likely to burn out the Arduino pin; and

  2. A ground wire between the emitter of the TIP121 and the Arduino, unless the ground is connected via the power supply.

How do you know your IR LEDs are not working?

  1. Which resistor would be good?
  2. The ground wire of the emitter goes to the GND of the power supply, is that a problem?
  3. They give a red glow, and I have an IR sensitive camera.
6

TiboJ:

  1. Which resistor would be good?

For a TIP121 it is not critical, 1K will do.

TiboJ:
2. The ground wire of the emitter goes to the GND of the power supply, is that a problem?

Not a problem, provided the power supply ground is also connected to Arduino ground.

I suggest you use a multimeter to check the voltage between base and emitter of the TIP121 when the LEDs are supposed to be on. It should be around 1.3V.

What is the voltage of the power supply, and what is the forward voltage drop of the IR LEDs? You have 2 in series, and together with the high saturation voltage of the TIP121, the total voltage you need may be more than 5V.

7

Not a problem, provided the power supply ground is also connected to Arduino ground.

The power supply isn't connected to the arduino gnd at this moment.

I suggest you use a multimeter to check the voltage between base and emitter of the TIP121 when the LEDs are supposed to be on. It should be around 1.3V.

Voltage between base and emitter is 0 volts, and between collector and emittor 10.6v.

What is the voltage of the power supply, and what is the forward voltage drop of the IR LEDs? You have 2 in series, and together with the high saturation voltage of the TIP121, the total voltage you need may be more than 5V.

Power supply is a 3s lipo(at this moment 12.4v) and the forward voltage of the IR leds is 1.5v.

edit: the 1k resistor did not make it work.

8

I saw on internet some tutorials without.

The Internet is not your friend.

edit: the 1k resistor did not make it work.

It should have...

9

pwillard:

edit: the 1k resistor did not make it work.

It should have...

But it didn't...I even tried another pin...
I'll try again.

The Internet is not your friend.

Actually, it helps me alot.
That's why I'm on the internet on this forum :slight_smile:

10

See my drawing in post #2... is that what you did?

11

pwillard:
See my drawing in post #2... is that what you did?

Yes, that's what I did.
Nice you drawed it :slight_smile:

edit: it still doesn't work.

12

It works when I build it...

So... what does that make you think about?

I used a E3055 instead of a darlington... but the difference it not so major it would not function. 12V is the source... My "normal" LEDS have 1K resistor.

Your's actually works out to 82 ohms.

13

I don't know what's wrong...
I'm searching for 2 hours now and still didn't find it...

14

Your's actually works out to 82 ohms.

Do you mean that I need a 82 ohm resistor for the transistor?
My total resistance for the leds is 82 ohms, if you mean that.

15

4x330 ohm resistors in parallel equates to around 82 ohms. Kinda low for 12V current limiting in the 20-50 mA range. What I mean is... they would burn out.

Also, 2 hours of searching... that seems suspect... what are your searching for?

FYI: I did finally swap in a TIP121 and it works the same.

16

pwillard:
2 hours of searching... that seems suspect... what are your searching for?

The problem...
I was looking for a solution on the internet, searching the problem in my circuit, searching tutorials to see if I'm doing something wrong,...
I will rebuild everything.

17

Well, the place that skipped using the base resistor on a NPN BipolarJunction device needs to be avoided.

18

I've just build it again, and still no luck.

Well, the place that skipped using the base resistor on a NPN BipolarJunction device needs to be avoided.

So how can I fix the circuit? Is my transistor broken?

4x330 ohm resistors in parallel equates to around 82 ohms. Kinda low for 12V current limiting in the 20-50 mA range. What I mean is... they would burn out.

They work on 140mA, and they didn't burn out, because they work if I connect them without the transistor

19

TiboJ:
The power supply isn't connected to the arduino gnd at this moment.

That's the problem then. You need to connect power supply ground (i.e. negative side of its output) to Arduino ground.

20

dc42:

TiboJ:
The power supply isn't connected to the arduino gnd at this moment.

That's the problem then. You need to connect power supply ground (i.e. negative side of its output) to Arduino ground.

Haha yes! It works now!
Thank you VERY much!!!!!

So, can anyone explain me why there is a resistor needed with a transistor? I never did it before...