I have an older strand of christmas lights that has these blue LEDs that have a flat top, as opposed to the rounded top that most LEDs come with. I think aesthetically they would work great in a project I am doing with my son but I my initial attempts to use them have come up short and I realize I do not know how they work.
To me an LED is a diode which means that electricity should only flow through one way. Yet when I put my multimeter on these I get the same resistance value each direction. The other thing that is confusing is that the same cheepo multimeter is measuring 25 ohms. My other LEDs are a a couple orders of magnitude greater then that.
Is there such thing as an AC LED? Is it realistic to use old christmas light LEDs in arduino projects?
Feel free to point me at older posts, I suspect that I am the first person to struggle with this. Thanks!!
You can't take a meaningful measurement of the resistance of an LED as it varies with the current through it. You can measure the forward voltage for a particular current.
Put a 220 Ohm resistor in series with one LED and slowly increase the voltage across the LED - resistor and see what happens. If it doesn't light reverse the polarity and try again.
Also if you use a multimeter with a diode function the LED may dimly light when you have the probes on in a forward bias config. This is helpful to determine the + and - leads.
I guess those are small incandescent bulbs. Recently I have disassembled an old (90s) car radio. It included some LED-like bulbs drawing about 100mA @ 5V (I did not measure cold resistance). Maybe yours are similar.
