Electronics > Beginners
How to wire this setup?
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Youkai:
OK so I switched to NPN transistors and the circuit works now; at least the Red, Green, and Blue light up correctly when the Arduino tells them to.

New problem though. The LED are SUPER dim. Also they are the same brightness whether the 12v supply is plugged in or not. The Arduino is using USB from the computer for power right now. So I guess that for whatever reason only the Arduino voltage is getting through.

After a short web search I found this but I don't really understand it. https://electronics.stackexchange.com/questions/152960/bc547-bc548-with-5v-at-base-cant-control-12v-at-collector-emitter

Can someone explain what is going on? Will a transistor not work? Is it just wired wrong? I'm using the wiring schematic from my earlier image: https://www.eevblog.com/forum/beginners/how-to-wire-this-setup/msg1531745/#msg1531745
james_s:
If you're using an NPN transistor, you need to set it up as a low side switch with the emitter grounded and the load between +12V and the collector. If you try to switch the high side with the collector connected to +12V what you will get is an emitter follower, the voltage on the emitter can never be higher than the voltage on the base. The circuit drawing you have looks correct for an NPN transistor.
Youkai:
Ok thanks for that james_s. Still having weird issues though. Specifically the LED is super dim.

It's wired like my diagram but for the sake of re-iteration it is working like this:

* +12V > LED common Annode > LED Red pin > Transistor Collector
* Arduino pin > 1k resistor > Transistor Base
* Transistor Emitter > Ground (the ground of the 12V power supply)
Using this setup the switching works correctly but the LED are VERY dim. What is wrong? Is it possible the arduino just isn't supplying enough current to fully turn on the transistor?
james_s:
Possible, how much current does the LED string draw? What's the gain of your transistor? A 1k base resistor is going to result in around 4mA base current, which with a typical small transistor should give you up to around 400mA.
Youkai:
According to the specifications section of my LED strip (https://www.superbrightleds.com/moreinfo/flexible-led-strip-lights-color-changing/rgb-led-strip-light-flexible-custom-length-led-tape-light-with-9-smds-per-ft-3-chip-smd-led-5050/3502/) it draws 113mA per foot and I'm using only one section (3 led) which is well under a foot.

What is the "symbol" for gain? This post farther up has the specs from the back of the package. I'm using the one on the far left. https://www.eevblog.com/forum/beginners/how-to-wire-this-setup/msg1531484/#msg1531484 If gain is "hFE" then it's 200.

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