I know there are some similar questions out there, but I'm really new to electronics and unfortunately they didn't provide me help. I want to control a tricolor LED strip (being able to dim each of RGB channels individually) with the Raspi.

EDIT This is the LED strip I'm using (german) http://www.conrad.de/ce/de/product/457534/X4-Life-LED-Streifen-RGB-Rot-Gruen-Blau I don't use the power connector, instead I exposed the 4 cables and worked with them directly.

EDIT2 I couldn't find a detailed spec, it's an end consumer product that comes with a controller and remote control ready to plug in. I figured out by measuring that 12V is full brightness and attaching a 12V battery confirms that. As far as voltage/brightness behavior goes, I can adjust everything in software, so that's not a problem for now.

I have software-based PWM for 3 gpio pins set up and everything so far works fine. The problem starts when dealing with the circuitry.

The LED strip has 4 connectors, one for +, 3 for RGB respectively (-). Each channel requires 12V to shine in full brightness. To start, I try to control the R channel, using a 12V battery as the power supply and a 4.5V battery to emulate the Pi (I'm afraid of connecting it directly for now, since I'm unexperienced, and I wanna make sure it works before actually connecting it). The R channel draws about 330mA when attached to 12V. I have 2 types of transistors lying around: BC728 (PNP) and BC116 (NPN).

This is what i tried: I crafted a 12V battery by chaining 8 1.5V batteries. I connected the + from the 12V to the + of the 4.5V to have a common ground, which is attached to the + of the LED strip. The - of the 12V goes to collector of the BC728 (PNP), the emitter is attached to the R channel. When I use the - of the 4.5v battery to touch the basis, the red LEDs should light, but they don't.

However, when I remove the LED strip and use a small light bulb, it dims lightly.

If I replace the 4.5V with a 9V battery I can even make the red LEDs of the strip shine, but not as bright as they should.

As it turns out, the voltage at the lamp/LEDs is considerably less than 12V. I tried adding a resistor between the basis and the 4.5V/9V battery, but it just makes matters worse (lamp/LEDs go darker).

My questions are: - What am I doing wrong? - If I get the circuitry right, how do I replace the 4.5V battery with the pi? After all then I only get 3.3V and there ground is negative, not positive. (And I have to choose positive ground, because of the LED strip, right?)

Thank you!

  • The question has no relevance to the Raspberry Pi. There are hundreds of different LED strips. They have different methods of control and power. You will need to identify the LED strip you are using and link to the detailed specifications for that strip. – joan Oct 25 '14 at 14:43
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    Thanks, I updated this. But it does have relevance to the RasPi, cause after all I wanna use it to control the LEDs. Do you think I should resubmit the question in electrical engineering? – user1841833 Oct 25 '14 at 14:53
  • It looks like a standard analogue strip, i.e. all LEDs have the same RGB setting at any instance in time (as opposed to digital where individual LEDs are driven). I do not know the best forum for the question. – joan Oct 25 '14 at 15:10
  • How about rewiring everything back together and watching the output signals from the controller when you switch to various colours? What kind of diagnostic equipment do you have at your disposal ? Could be the occasion to have fun with one of these: de.farnell.com/bitscope/bitscope-micro/… – Arnaud Meuret Oct 27 '14 at 11:36

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