I have a Nano, one Led, and a ULN2003 which I am using to switch the LEDs on and off with. I will be upgrading the single LED to four lasers, but even with one LED the Nano is overheating right around where the on board LEDs are.
What's happening?? Why is it overheating, and why on that spot??
I would include a schematic but I am on a public computer so don't have KiCAD or Eagle available. I'm gonna try an online schematic tool, but if anyone has any brain waves in the mean time I'd be grateful.
How are you powering the Nano ?
A hand drawn schematic is fine
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I am powering the Nano from the computers USB port.
Its overheating more or less where the VR is, which is odd considering its plugged into a PC.
I see no current limiting resistor in series with the led, and you are powering the led from D3. You should power the led from VCC directly. Your ULN2003 is then the switch that turns the led on/off.
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Ok, I just did that. But why is it overheating? I mean it gets so hot I can't touch it.
And the COM on the ULN2003 needs to be connected to VCC as well. and the current limiting resistor and yes that is a bit odd that the Regulator is throwing out all that heat.
well that explains it. no resistor in line with the led will draw to much current through the 3.3v regulator.
and as stated before 'COM' on the ULN2003 needs to be connected to the same VCC as any device connected to the output pins.
What for? Adding it didn't change the overheating, and it had already been switching the LED as per my program.
That isn't needed. I'm running a 3.2v LED off 3.3v....
OK ! as you like, but stop complaining about the temperature
That schematic looks how I use them without running into problems. Assuming you have some sort of current limiting resistor in series with your led. If not I suspect the led is asking too much of your overheating 3.3V regulator.
I must admit that I don't connect COM to VCC, unless I am driving inductive loads with the ULN2003. For just driving leds I ignore it.
Edit. Not enough coffee yet.
If the forward voltage drop is 3.2 and you are powering from 3.3 I think a test with a 5 ohm resistor can tell you if the led is the problem
Other idea: What if you power the led from VIN instead of 3V3 and use a 90 Ohm resistor?
That way you bypass the onboard voltage regulator in the nano.
5Volt - 3.2 Volt (forward voltage) = 1.8 Volt to drop at the resistor and you want 20 mA.
1.8V / 90 Ohm = 20 mA
The LED wasn't the issue. I unplugged the Nano and plugged it back in once it cooled down, and it isn't heating up again.
Use the +5V on pin 27 for the anode of the led with a resistor of 100Ω or 120Ω (or more) between +5V and led.
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Leds are current driven not power driven. The 3,2V is the power that stay over the led. But the typical forward current is much important. The ULN2003 Vce(sat) is at least 0,9V. 3,2V + 0,9V= 4,1V. For a white led is 3,3V power not enough.
Isn't an E12 resistor. E24 has 91Ω
Not using the current limiting resistor in your led connection is a common novice mistake. Regardless your LED voltage you MUST use the resistor if do not want to fry your arduino or power supply
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so just to get it clear, there is a voltage drop of 3.2v from the 3.3v supplied, that leaves you with 0.1v over 0R,
so if V = I * R that means that I = V / R = 0.1 / 0 which turns out to be really quite a lot 
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