Designing around Output pin limitations

Question

I'm new to Arduino and am working on a project with my son. We are using the LillyPad USB board (schematic) to drive a bunch of LEDs (LED Datasheet). Our project is based loosely off of the 'Turn Signal Biking Jacket'.

Our project calls for 7-11 LEDs in parallel on a single circuit. In the 'Turn Signal' project the LEDs are driven off of a pair of output pins (1 HIGH and 1 LOW). In a circuit that looks like this...

^{simulate this circuit – Schematic created using CircuitLab}

Is this ideal? Would it make more sense to connect the LED-chain to Vcc and have the return line connected to a single output set to LOW (to turn on the lights)?

If we were to have 3-4 of these LED-chains on at once, do I have to be concerned with total draw on the microcontroller?

Help? Thoughts?

Answer 1

If you know the forward voltage of the LEDs you can easily do some calculations.

For a typical red LED the forward voltage is around 2V. Other colours vary. So we can work out that, at 5V supply, each LED with its own 150R resistor, would take:

     V    5-2
I = --- = --- = 20mA
     R    150

With 11 LEDs in parallel on at once that would be 0.02 * 11 = 220mA.

That is far more than the 25mA an IO pin is able to supply, so you will damage the Arduino doing it like that.

Instead you will need to switch each chain of LEDs with a transistor of some form. I'd choose a logic-level N-channel MOSFET as a "low-side" switch:

^{simulate this circuit – Schematic created using CircuitLab}

Now your Arduino doesn't care about the current - all it sees is the MOSFET.

If your LED forward voltages are less than 2.5V you can also chain them together to halve the current:

^{simulate this circuit}

Since the forward voltage is now double you use a smaller resistance to set the current (R=V/I, where V is 5-2Vf).