richardvanraay:

I am supplying 12v to vcc and taking 5v from the Uno.

Uno VCC, the common circuit voltage of the board is 5V.

I think that you feed the power jack 12V, does the board get warm in that corner? Older boards will with 12V, better to get a buck converter and feed the 5V output straight to board 5V and GND. New board has a converter instead of a regulator, is 90+% efficient so doesn't get warm, or barely so.

one circuit, two 3-3.4v. (3 of these independently wired)

one circuit, two 1.8-2.4v

one circuit, two 2-2.2v

All 20mA leds

So for the first; R=(5-3.2)/0.02=90

That is 90 Ohms?

Have I done the maths correctly?

What does that mean?

I really appreciate your help.

Thanks

Richard

A led is a light-emitting **diode**. Diodes let current flow one way through but not the other (until you push hard enough then you went for smoke and got it) and there's a voltage cost as if ground is raised, to get current flow. A led makes light then, as much as the current allowed, there is tiny resistance in the led junction.

The formula R = V / A is resitance = (volts - forward volts) / amps. We know volts, foward volts and desired amps (20 mA) and those tell us what resistance is required. You show Vf for your leds varies so take the cases that need the most resistance, forward volts is subtracted so take the smallest values, 3V, 1.8V and 2V.

And you have these leds wired not in series, one after the other but side by side in parallel?

In parallel the forward voltages do not add.

Resistance for led 1, 3-3.4Vf is (5V - 3V) / .02 = 100. Since the low value is used, 100 or so ohm resistor is good.

Resistance for led 2, 1.8-2.4Vf is (5V - 1.8V) / .02 = 160. Since the low value is used, 160 or so ohm resistor is good.

Resistance for led 3, 2-2.2Vf is (5V - 2V) / .02 = 150. Since the low value is used, 150 or so ohm resistor is good.

Wired in series, 5V could not light 2 of led 1 at all but 12V could do 1 of each with (12-3-1.8-2)/.02=260, 270 ohm resistor is the next standard size. The voltage going to ground is 5.2V, okay for Arduino pin at 20mA.

You need to study beginner electronics, maybe some physics. Are you out of school?