user31105: Sorry for having (again) a beginners question, but I was wondering something.
If I would want to drive like 20-30 LEDs from an Arduino (Uno), and I want to use an external adapter (12V, 1A), going into the Arduino, in what way I should drive the LEDs? 
Problems I see:

I cannot connect too many LEDs per pin (max 20mA per pin)
I cannot connect too many LEDs per 'group' (I have to find out which pins are connected to what groups)
I cannot connect too many LEDS in total because of the max. 200 mA per VCC/GND (there are 2 or 3 GNDs, so that should be ok)
Using a relay/transistor etc, but how do I get the power? (there are also only 2 VCCs for 200 mA max)

So my basic question: how can I use the 1A from my adapter? 

user4143: The simplest thing you can do is group the LEDs together into chains that almost add up to the voltage of the power supply.
If you have LEDs with a 2V forward voltage (typical red LEDs) you could have up to say 5 per chain.
Each chain, since it only has 20mA or so maximum, can be driven with a simple small transistor.

simulate this circuit – Schematic created using CircuitLab
Connect the 12V to either VIN or the barrel jack, and also drive the LED chain through a suitable resistor (R=V/I = (12 - (Num LEDs * Forward Voltage)) / 0.02). You can have as many chains as your supply can provide current for.

31105: <p>Sorry for having (again) a beginners question, but I was wondering something.</p>

<p>If I would want to drive like 20-30 LEDs from an Arduino (Uno), and I want to use an external adapter (12V, 1A), going into the Arduino, in what way I should drive the LEDs? </p>

<p>Problems I see:</p>

<ul>
<li>I cannot connect too many LEDs per pin (max 20mA per pin)</li>
<li>I cannot connect too many LEDs per 'group' (I have to find out which pins are connected to what groups)</li>
<li>I cannot connect too many LEDS in total because of the max. 200 mA per VCC/GND (there are 2 or 3 GNDs, so that should be ok)</li>
<li>Using a relay/transistor etc, but how do I get the power? (there are also only 2 VCCs for 200 mA max)</li>
</ul>

<p>So my basic question: how can I use the 1A from my adapter? </p>
 4143: <p>The simplest thing you can do is group the LEDs together into chains that <em>almost</em> add up to the voltage of the power supply.</p>

<p>If you have LEDs with a 2V forward voltage (typical red LEDs) you could have up to say 5 per chain.</p>

<p>Each chain, since it only has 20mA or so maximum, can be driven with a simple small transistor.</p>



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



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

<p>Connect the 12V to either VIN or the barrel jack, and also drive the LED chain through a suitable resistor (R=V/I = (12 - (Num LEDs * Forward Voltage)) / 0.02). You can have as many chains as your supply can provide current for.</p>
