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By Frank gomez
I have a project, I have no idea if I am in the right place to ask for help.
The circuit I am trying to run has a 4.5V source (3 AAA batteries), a 5 v. led(5mm) and a resistor. I have no idea what the resistor value is, I have seen some plans that call for 220 ohms.

What is I hookup 100 of these LEDs to this circuit in parallel of course, ? Do I need 100 resistors? or only one would do it?
Best practice is that each LED gets its own resistor. LED forward voltages and resistor values are all variable within their tolerance bounds. If everything goes through one resistor, the LEDs will see different currents and sometimes bad things happen. A few LEDs in series with a resistor is usually without risk. 100 LEDs in parallel all in series with the same resistor isn't recommended.

To calculate the proper resistor for your LED, you need to know the supply voltage, the forward voltage of the LED, and the target current. Remember that V = I x R and you subtract the forward voltage from the supply voltage.

As an example, if you have a 5 V supply, an LED with a forward voltage of 1.5 V, and you're targeting a current of 20 mA, the math works like this: (5V - 1.5 V) / 0.05 A = 70 Ohms. For 5% resistors, you would round up to the nearest standard value which in 75 Ohms.
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By spoggles
How bright does the display need to be?

You can use a 'ultra bright' white led and obtain good illumination with very low current.

I have powered one with 3.3v using a 10k limiting resistor and obtained good visibility.

At this rate, you should be able to light 100 devices with less than 20mA.