Question for Electronics peeplezzzz

Great question.

https://en.wikipedia.org/wiki/Ohm's_...rcuit_analysis
OHM's Law

You would need a VOM meter instrument

Should be able to deduce it but the voltage rating should be the same for all the LED's it you want them to display right (I think?)
Wattage/Volts=Amps

all LEDs work on approx 2-4 V (depends on color) and you should put some resistors for voltage drop - to stabilize circuit (limit the current) so voltage should be higher. LED is semiconductor, not resistor like classic bulb. I found you nice text about LED voltage and how to create circuit. See the link down.
However, most of diodes will work ok if you put them on 3V battery and if you put them direct on higher than 6, they will die. if you put them parallel, you need resistor for each of them, voltage is same, but current is higher - if you have 12V supply (car battery), you can put them serial with one resistor (make calculation to design circuit) so it's cheaper and more simple - but if one is not working, all circuit is stopped
You can't measure the power supply - you need to calculate what you need by knowing your power supply voltage. Typical current for diodes is 20 mA so with that info you calculate which resistor do you need.

LED Basics; gaining an understanding of how to work with 3 and 5mm LEDs

here is wiki if you want to learn more about structure of diodes: https://en.wikipedia.org/wiki/Light-emitting_diode

Thanks, thanks and thanks!

Save time and just buy one already made...

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Cost-cutting LED's will soon be a big market in Quebec now that the transfer payments from Ontario have stopped.

The short answer is: you should use a resistor for each LED.

You can use a reasonable value resistor (like 500 ohms or 1k) in series with the power supply to find out the voltage of the LED (measure it directly across the two leads) but there is no real way to see how many milliamps it can take.

Different colour LEDs will often have different forward voltages, so running them in parallel (without their own individual resistor) will result in the LEDs with lower forward voltage dissipating more power. IOW, your red LEDs would be overly bright, possibly even to the point of failing, while your blue LEDs would be dim, maybe not even lighting at all.

One resistor for each LED.