user68942: I'm connecting an LED strip that takes 12V input to an Arduino Uno R3's 5V VCC using a B547 transistor. 
Here's my circuit:
Base -> 1K ohm resistor -> DO13
Collector -> -ve of LED strip
Emitter -> GND
VCC 5V -> +ve of LED strip

Nothing happens when I try to turn on the LED strip. However if I remove the 1K resistor and turn on the LED, Arduino's built in LED gets dimmer but the LED strip does not turn on.

What am I doing wrong? 
TIA!

user4143: Ok. First off, if you have a 12V LED strip then you have to power it from 12V. Powering the Arduino from 12V and then powering the LED strip from the 5V pin of the Arduino is not powering the LED strip from 12V, it's powering the Arduino from 12V and the LED strip from 5V.
The LED strip's anode (+) must be connected to the + of the 12V supply or, if it's only a very short LED strip then you can use VIN on the Arduino.
LED strips typically have a number of LEDs (eg 3) in series with a resistor to make one "segment", and many "segments" in parallel. This gives a total forward voltage of typically 9V or more - so you must provide at least that to get the LEDs to light.
Secondly the BC547 is not a good transistor to use for switching an LED strip.  It has a maximum continuous collector current of 100mA. That means that you can only have a maximum of 100mA going through your LED strip before the transistor blows.
For reference, 100mA is the equivalent of 5 "normal" (5mm, of the kind you'd use in a breadboard) LEDs at full current and brightness. A typical LED strip (depending on length) can use many hundreds or thousands of mA.
Really you should be using an N-channel MOSFET that is rated for at least as much current as your LED strip requires to run. For safety, and to reduce heat dissipation, you ideally want it to be many times more powerful than the LED strip needs.
A common choice is the IRF530, which is rated at 10A with a switching voltage (the "threshold" voltage) that is low enough for a 5V Arduino to control (there are more suitable ones, but these seem to be the "go-to" component for many Arduino users - they're adequate for most uses).

simulate this circuit – Schematic created using CircuitLab

68942: <p>I'm connecting an LED strip that takes 12V input to an <code>Arduino Uno R3</code>'s <code>5V</code> VCC using a <code>B547</code> transistor. <br>
Here's my circuit:</p>
<pre><code>Base -&gt; 1K ohm resistor -&gt; DO13
Collector -&gt; -ve of LED strip
Emitter -&gt; GND
VCC 5V -&gt; +ve of LED strip
</code></pre>
<p>Nothing happens when I try to turn on the LED strip. However if I remove the 1K resistor and turn on the LED, Arduino's built in LED gets dimmer but the LED strip does not turn on.
<br>
What am I doing wrong? <br>
TIA!</p>
 4143: <p>Ok. First off, if you have a 12V LED strip then you have to power it from 12V. Powering the Arduino from 12V and then powering the LED strip from the 5V pin of the Arduino is <em>not</em> powering the LED strip from 12V, it's powering the Arduino from 12V and the LED strip from 5V.</p>
<p>The LED strip's anode (+) must be connected to the + of the 12V supply or, if it's only a very short LED strip then you can use VIN on the Arduino.</p>
<p>LED strips typically have a number of LEDs (eg 3) in series with a resistor to make one &quot;segment&quot;, and many &quot;segments&quot; in parallel. This gives a total forward voltage of typically 9V or more - so you must provide <em>at least</em> that to get the LEDs to light.</p>
<p>Secondly the BC547 is not a good transistor to use for switching an LED strip.  It has a maximum continuous collector current of 100mA. That means that you can only have a maximum of 100mA going through your LED strip before the transistor blows.</p>
<p>For reference, 100mA is the equivalent of 5 &quot;normal&quot; (5mm, of the kind you'd use in a breadboard) LEDs at full current and brightness. A typical LED strip (depending on length) can use many hundreds or thousands of mA.</p>
<p>Really you should be using an N-channel MOSFET that is rated for <em>at least</em> as much current as your LED strip requires to run. For safety, and to reduce heat dissipation, you ideally want it to be many times more powerful than the LED strip needs.</p>
<p>A common choice is the IRF530, which is rated at 10A with a switching voltage (the &quot;threshold&quot; voltage) that is low enough for a 5V Arduino to control (there are more suitable ones, but these seem to be the &quot;go-to&quot; component for many Arduino users - they're adequate for most uses).</p>

<p><img src="https://i.stack.imgur.com/hSkWC.png" alt="schematic" /></p>

<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fhSkWC.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>
