If what you want is to control individual LED's, then you might still be able to use an N-Chan MOSFET, but your control signal will need to be at least Vthresh above the source voltage to turn the LEDs on. Simulate this circuit – Schematic created using CircuitLab Here's an example circuit, ignoring LED current limiting schemes for now. If not – switch them and try again.If all you need is to turn all the LEDs on/off using PWM, you can attach the N-chan MOSFET between the common cathode and ground. If the diode lights up then you’ve successfully matched the positive probe with the anode and the negative probe with the cathode. Just turn the multimeter to its diode setting (it should be identified with a diode-shaped symbol) and touch each probe of the multimeter to the diode’s terminals. If you’re still uncertain you can just use a multimeter to test which end of the diode is its anode and which – its cathode.The pin near the flat edge should be the cathode. If the pins have been trimmed and equal in length, look at the edges on the diode’s outer casing – one should be flat and in line with the diode and the other should be extending a bit.In LEDs, the LED anode-cathode differentiation is done via the length of each pin at the end of each diode – the longer pin is usually the anode and the shorter pin – the cathode.
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