![]() It also gives you better control over the two different currents, but it isn't free. Resistors are really cheap (unless you have super high requirements), and a resistor generally doesn't take up a lot of space. Is the cost significant? It really depends. Note down the deflection of multimeter, as shown below. Connect RED probe to anode, BLACK probe to cathode. Take a multimeter and put it in ohmmeter range. Mark the either anode and cathode of the diode under test by slightly bending its one terminal. Just be aware that sometimes the arrangement of parts does matter, and other times it doesn't. And the terminal connected to BLACK probe is cathode. Identify the anode and cathode terminals of the LED. Let us now see how to test an LED using a digital multimeter. Also, another method is using the surface structure wherein a flat surface indicates the cathode and other one is the anode. It's not unreasonable because a typical small LED's current is below the limit of being safe to drive directly from the MCU, but is slightly worry-some if you have a lot of other stuff going on. Longer one is anode and the shorter one is the cathode. It's unclear from the original question if the kit/tutorials you're using have this type of LED driving circuit. ![]() This is an extra part we would have to have in our design meaning extra cost and space. If we instead moved R1 above Q1 or even above D1, we would have to add an additional resistor between MCU output and Q1 to limit the base current. In the above schematic we are using the resistor R1 to both limit current flowing through the LED as well as limiting the base current (current from MCU output to R1). Simulate this circuit – Schematic created using CircuitLab By cleverly placing the resistor it can serve multiple functions at the same time, thus allowing you to reduce the part count and save some money/space. For example, sometimes people will use a BJT transistor to allow for higher current drives through the LED because micro controller pins are limited to low current outputs. However, there are designs for driving LED's using transistors where choosing where you place your resistor does matter. You have non-reactive components (ideally, which is pretty close to reality) thus any current flowing into the diode/resistor must flow out, thus current would be limited equally well by having the resistor on either the anode or cathode side. Usually as RedGrittyBrick said it doesn't matter.
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