Driving LEDs with ULN2003 + PWM - do I still need resistors for the LEDs ?

Hi,

I want do drive 8 LEDs (3.6V, 50mA) with Arduino and ULN2003.
Do I need to use sesistors to step down the voltage from 5 to 3.6V or is it ok when I make it using PWM?

If I need resistors, do I need one resistor per LED or can I use one resistor for all 8 LEDs?

Thanks for help!

Do I need to use sesistors to step down the voltage from 5 to 3.6V or is it ok when I make it using PWM?

Yes you need resistors and no the resistors do not "step down the voltage".

do I need one resistor per LED

Yes.

The fact you are using PWM does not affect the peak current.

See:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

I could very well be wrong here but Resistors do in fact drop voltage in a series circuit.

They are most often referred to as Current Limiting resistors because in essence they are being used to set the current flow through the circuit. In a series circuit, current flow is constant. So if I know I have to limit my current to 50mA in a 5V circuit for example, I have to choose a resistor that will do that. Since an LED will typically give you the operating voltage and current, what we are basically doing is determing the voltage drop needed by the resistor to provide the 3.3v (for example) needed by the LED.

A Real World example:

Using Ohm's law, R = V/I. We know the voltage required by the LED is 1.7V we know the current required by the LED is 30mA and we are using a 5V source. So to determine what size resistor to use we determine how much voltage the resistor must drop to give us 30mA therefore:

Vresistor = (Vsource - Vled) this is the voltage left over for the resistor.

R = Vresistor / I

Using a 5V source with a 1.7V LED at 30mA gives us:

R = (5 - 1.7)/.030 = 110ohm resistor needed.

So Resistors do in fact drop voltage in order to limit current.

Someone please correct me if I am wrong. It has been MANY years since I learned basic series and parallel circuits.

I could very well be wrong here but Resistors do in fact drop voltage in a series circuit.

Yes they do.
What they don't do is "step down the voltage".

It is important to use words that the rest of the people in electronics use.

Step down is used as the term to convert one voltage to another, a resistor does not do that. Things that step down are transformers (for AC) and switching regulators. If you start off using the wrong words it will lead to confusion.