I don't understand if this is a series or parallel circuit.
2\$\begingroup\$ Are you asking if the resistor and LED are in series? Draw a circuit diagram to be able to decide ;) \$\endgroup\$– Andrew MortonMar 19 at 13:39
1\$\begingroup\$ Both, or neither, depending on what you mean. Are you looking at concepts of "power supply" and "load", or just "LED" and "resistor"? \$\endgroup\$– JustmeMar 19 at 13:46
\$\begingroup\$ just the "LED" and "resistor". \$\endgroup\$– ilytothemoonandbackMar 19 at 13:49
\$\begingroup\$ Take a look at this water analogy of a series vs parallel electronics.stackexchange.com/questions/511363/… \$\endgroup\$– G36Mar 19 at 14:01
3\$\begingroup\$ Please draw a schematic. \$\endgroup\$– winnyMar 19 at 15:18
Your circuit looks like this, from what I can tell:
simulate this circuit – Schematic created using CircuitLab
If R1 and D1 were in parallel, the circuit (which would not work properly) would look like this:
If you are including the Arduino in your description of "being in parallel or series", then the answer is a little less intuitive. However, you may consider the output of the Arduino to be a voltage source, like a battery. If I redraw the system in that context, where the Arduino is acting like a battery, powering whatever is connected to its output, then we have a "loop" arrangement resembling this:
Everything's still connected end to end, so it's still all a "series" of components, with even the Arduino forming a part of that series-connected path.
Some may argue that this last picture isn't really representative of what's actually happening. The power source isn't really inside the arduino, it's outside. If the LED is lit, because the D output has a high (+5V) potential, then the following representation is more accurate, because it includes the Arduino's power supply too:
While that last schematic is only representative of what's going on inside the Arduino, the actual circuitry consisting of transistors and other things, it is essentially truthful. The Arduino is indeed "switching" a current path from the battery positive to the Dx output.
Even there you can see that the battery, the switch inside the Arduino, the resistor and the LED all form a loop of components connected end to end, a configuration we call "connected in series".
It's pretty clear that the resistor and LED are in series, but technically, the entire system could be described as consisting of series-connected elements.
That's not something we'd usually say, because in the very first schematic above, you could say that the resistor and LED pair form a series connected pair, but that pair is in parallel with the Arduino, between its Dx and ground terminals. So while I've just gone to great lengths to explain why everything's in series, you can understand why your question is a little ambiguous, not very easy to answer, and this answer is ripe for people to disagree with me.
As far as I can see they are in series. They have a single point in common, the junction of the LED and the resistor and the same current flows from the Arduino, through each in turn and back to ground on the Arduino. If they had been in parallel the resistor would be connected to either side of the LED and their respective currents would be independent, the voltage across each would be the same.
The LED and resistor is a series circuit, and why it is because the LED is in series with the resistor, instead of being in parallel, from the point of view from the wires connected to resistor and LED. So equal amount of current must flow through all components in series.
Another way of thinking is that if the LED was parallel to resistor, the LED would burn up from overcurrent and the resistor would just be useless load that heats up and it will not limit the LED current.
If two, two terminal devices connect at a single point, and nothing else connects there, they are in series. If they connect at both ends they are in parallel. A long string of devices can be connected in series provided all the connections just have two devices meeting. Similarly, any number of devices can be connected in parallel providing each device connects to both ends.
If you want these definitions so you can decide where to apply a memorized formula, you're on the wrong track. In a series connection the same current flows through each device so the voltage drops add. In a parallel connection each device sees the same voltage so the device currents add. That's all you have to know.