I'm in my highschool's engineering class, and we have been given Arduino uno's (power outputs are either 3.3v or 5v) but the LED's we have are rated for 2.2v, and we don't have resistors. How can I power an 2.2v LED constantly without frying it, or how can I set up a PWM to lower the voltage from 3.3v to 2.2.v?
You either need a current limiting resistor of some kind of constant current driver circuit for the LED
Is it too late for a refund of your tuition? That doesn't sound like much of an engineering class.
Quick get someone to buy a resistor assortment, or a few dozens of some nice values that will handle 99.44 percent of your current requirements.
LEDs are forgiving, but will not be happy with voltage and no current limiting.
haha, which might probably need… parts you don't have, like resistors.
a7
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Renegade idea but most LEDs will glow a little bit at low current so put two in series with a 3V3 powered Arduino or three on a 5V Arduino to split the voltage across them. 
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does it matter if multiple LEDS are in a row instead of separate connections to the LED from the same power output using a breadboard?
I don't understand what you are proposing. Can you hand draw a schematic and post a pic of it?
Well, I think the goal of your class was for you and your mates to figure it out. But, since you asked for help...
That´s the simplest idea in my opinion.
A second thought: if you´ve learned PWM, choose one of the pins that support it and set the pin to 44% of cycle (44% of 5V = 2.2V) using analogWrite. In other words, as 44% of 255 is 112,2,
analogWrite(pin,112);
Please inform your instructor that failure to use resistors with LEDs may destroy the Arduino, the LEDs, or both.
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Without a current-limiting resistor, the Arduino, LED or both can be destroyed.
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It might take a bit longer to destroy the LED, otherwise a non-starter.
In any circuit like that, driven PWM or not, there will be an inherent limit to the current passed through the LED.
Unfortunately this is likely to be comfortably in excess of the maximum the part can take.
a7
I can't recommend that. Even though a typical LED might be rated at 100 mA (1/10 Duty Cycle, 0.1 ms Pulse Width) maximum forward current, the Arduino output pin will probably not be too comfortable with it.
All right folks, @steelcrest I retreat my suggestion
HINT: A lead (graphite) pencil.
DON"T anyone say any more about it!
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It looks like you have them wired in parallel which would require current limit. I'm suggesting you wire them in series.
No puns but:
The graphite from a #2 pencil lead will give you somewhere between 5k ohms and 40k ohms of resistance per centimenter.
A typical LED requires about 10 mA - 15 mA current for the emisson of satisfactory level of light AND maintains about 2.2V - 2.5V across its anode-cathode terminals. If don't comply with these requirements, then there is a chnace for many things to go out-of-order.
You are getting a pulsed (On-Off-On.....) output which must be filtered to get the 2.2V DC level; unfortunately, the OP does not have capacitors (I guess!).
Without any additional external components, the LED will still be subjected to 3V3 if the Arduino can maintain that GPIO output under those conditions. Depending on the duty cycle of the PWM waveform, the time average current will be the duty cycle fraction of the current available from the GPIO pin. At 3v3, your LED will have a very low resistance and that current will likely be more than the maximum instantaneous current that the GPIO pin is specified to survive.
Since this is an engineering class, turn it into a teaching experience and study the Forward Voltage versus Forward Current curves from your LED data sheet to see how it might be possible to get a little light.
Use a blue LED?