[Solved] 2 trasitors as Darlington pair

Hi all

I am trying to switch a 12V relay. I am using 2N 3904 NPN transistors (T1 and T2). Power source is 12V batt. Base on T1 is from output pin on Arduino UNO, 5V and connected line 10.
I have the connections like in the attachment. I have tried different values for resistors, but I do not get more than 4.46V or hardly any change from the emitters. I have also tried straight 12V to collector of T1. In fact, emitter on T2 is lower than that on T1 and I need 12V.
I also get .01V on emitter 2 when the transistor should be off.

Where am I going wrong. I am new to this and would really appreciate help.

Now, first off I don't know what kind of relay you're planning to switch... but you really shouldn't need several transitors to do so.

That said, there are bigger issues at hand. I think first of all, you need to understand transistors. They're not switches. You control an NPN by making current flow from the base to the emitter. It will start conducting when you apply approximately 0.7 volts to the base (and here is the important bit) measured against the emitter. Where are your emitters at right now?

Do yourself a favour and read though something like this
https://www.myodesie.com/wiki/index/returnEntry/id/3063
before continuing.

Your picture shows two transistors with some random resistors of too high a value to do anything much.

Find the pinout diagram and identify which transistor pins are emitter, base and collector.

Emitter goes to ground, collector to relay, other side of relay coil goes to 12V. A diode is also connected across the relay coil with cathode (stripe) side to 12V.

Base of transistor goes via a 470 ohm resistor to Arduino control pin. Pin must be set as an output and commanded HIGH to turn on the relay.

For a 2N3904, the relay must have a coil resistance no less than 100 ohms.

Hi,
find attached a circuit diagram of how you can use a single NPN transistor to drive a relay.

Tom... :slight_smile:

Hey, hey, he's doing it again!

What drawing program is that?

I just wish you would not use the "Attachments and other options" function. :astonished:

That comes from www.expresspcb.com

(I prefer attachments instead of links to websites that are blocked to corporate users, such as dropbox, imagur, etc).

Hi,
ExpressPCB, it has no funny download companions when you load it.
It has export to clipboard facility.
Its free, a PCB company provides it free, no locked out or restricted functions, the only thing is its PCB editor does not produce a GERBER file when you design a PCB.

Tom....... :slight_smile:
PS; tell me if dropbox works.

I personally get this:

"ACCESS DENIED" in big red letters.

Thanks for all the response.
I now know more than I did yesterday
Attached is the new schematic. I altered the bits on the breadboard and it works like a charm. Special thanks to Paul_B as I followed his instructions after which I had no need to try any of the other suggestions.
I will mark this thread as Resolved.

Schematic PNP.jpg