NPN transistor with LEDs on emitter and collector side; Arduino with ULN2003

Thread Starter


Joined May 22, 2018
Hello Readers,

I am a mechanical engineer but overwhelmed by electronics and motivated to make some. Just a highly energetic beginner.
I have two circuit questions and attached two pics.

1. NPN_2LEDS. - 1 NPN transistor is connected in a simple way in picture. Collector and base is connected to 5v. Emitter is grounded.
The circuit works fine when an LED is connected only to either the emitter or to the collector end.
But when i add another LED (1LED @ emitter and 1 @ collector) , the behaviour is confusing to me.
Only the emitter LED burns while the collector LED is dark and the collector current is zero.
Why is the collector current zero?

2. Arduino with ULN2003 - Pin 13 from the arduino is connected to pin 1 of ULN2003 darlington transistor and its pin 16 connected to an LED.
Circuit works. I am delighted. But when i measure the voltage across pin 1 of darlington transitor and ground the voltage is 1.56V, when the arduino pin supplies 5V. I dont know why.
If i disconnect from the arduino and connect pin 1 of darlington transistor to 5v supply the measured voltage is 4.93 V (and makes sense).
The voltage applied at the other end of the LED is also 5V. Why this difference?

Thanks in advance for your answers.



Joined Jan 29, 2010
hi BC,
Welcome to AAC,
I assume the LED supply is 5v.?
What colour are the LED's.?
Clipped and converted your images
ESP1 26-May-18 13.20.gif ESP2 26-May-18 13.20.gif


Joined Aug 1, 2013
To an ME, an electronic circuit schematic looks like a new/strange/foreign language. BUT, you gotta learn at least the basics, because a schematic is *everything* to us. For example, this:
Collector and base is connected to 5v. Emitter is grounded.
means the transistor is connected as a dead short across the power source, and was destroyed in 5 milliseconds after power was applied. Obviously that didn't happen and your circuit has more parts than that, but text descriptions of circuits usually don't go well without long rambling sentences full of many precise adjectives. Please post a sketch of a schematic. A picture may be worth 1000 words, but a schematic is more like 10x.

We need a part number for the transistor to determine if it is connected correctly. There are several different pinouts for the TO-92 package.

Also, the values for each resistor; the color bands often do not come through correctly in photos. It looks like the a base resistor of 1K and two LED resistors of 22 ohms. If true, the 22 ohm parts should be 220 ohms.

Also, most small LEDs have a extended rim around the base of the plastic package, and part of the rim is flattened. The lead closest to the flattened part is the cathode of the diode. It is the only indicator of correct insertion.

Where do the red and black clip leads go, which buses are 5 V and which are GND, etc.

Last edited:


Joined Jan 29, 2010
Morning BC,
What colour are your actual LED's.?
Look at this image to get an indication of the actual voltage drop across an LED. [VF typ]
Spec Sheet.jpg


Joined Jan 29, 2010
hi Ben,
Consider the transistor case.
You have two white LED's in series which have a nominal working voltage Vfwd of 3v, so thats 6v total.
You have a 5v supply, so two white LED's in series will not work as you expect.
Do you follow OK.?

Thread Starter


Joined May 22, 2018
Hi eric,

I dont understand how the two LED's are in series (I am newbie.....). The LEDs are part of the same circuit, but the emitter LED is going to the ground and the collector is connected to +V.

Could you please explain?



Joined Jan 29, 2010
Basics, a White LED has a typical working voltage 0f 3.2v.
You can see from the diagram that with a120R in series from a 5V supply the LED has a current of ~15mA.

Consider connecting two White LED's in series, they would have forward voltage drop of 6.4v.
So when connected to a 5v supply, only a very small current would flow and the LED's would not light.
There are other problems with your circuit.

Do you follow that OK.?
ESP1 29-May-18 08.07.gif


Joined Nov 29, 2009
Are we missing something here? Looking at the picture of the OP's breadboard, the diode on the collector side isn't connected at all! He has the collector resistor in the adjacent hole. Thus current is simply flowing from +5V, through the forward-biased b-e junction of the transistor and through the 'emitter' LED. That's why only one is on.


Joined Jan 29, 2010
hi Phil,
At first glance it does look as though he has has skipped one hole, but closer examination shows the wire is actually bent back into the correct hole.
ESP1 01-Jun-18 13.17.gif
Thanks for the suggestion. I have posted two circuit diagrams of the problems that i explained above.

Fritzing is NOT a schematic. It is a waste of time. It is a step backwards for learning electronics.

Thread Starter


Joined May 22, 2018

I started learning by doing and thought that would be a good approach. How else would i learn?
I also seek the advice of how to learn it correctly?

My goal is to make electronic projects for my work and stay motivated.
Could you all please give me directions? That shall be wonderful.

Thanks for all your replies