A question for Marc re: pre-wired LEDs

Hi Marc, and everyone on Wearable Electronics-

A while ago you mentioned getting some pre-wired LED's on Amazon. If I recall they already have a resistor wired in as well? How many of them have you strung together, and how many would 9 volts power? I'd like to experiment with them but I'm not sure where to start and I'm a little intimidated to 'just do it'.

Thanks in advance for the help.



  • Hi Catherine,

    If you look at the product description, you can see that the ones Marc linked do have a resistor already. As far as figuring out how many you can power, you'll need to do some math. First, several of the questions/answers and reviews mention that they were able to power the LEDs with 9V just fine, although the description calls for 12V, so you're good there. Then the product description also says the LEDs need 20mA to light up (that's what forward current is anyway). So you can estimate that that's how much each LED will need to stay lit up. If you Google you can find this about a 9V battery:

    "A standard 9V battery has about 400-600 mAh capacity. In the most basic terms, these batteries can supply about 500 milliamps for one hour before being "dead"."

    So the answer is going to depend on how long you want to run the LEDs. You could power 500/20 = 25 LEDs for one hour from a single standard 9 volt battery. If you want to power them longer you'd need to use fewer or use an additional battery. They should be connected in parallel, btw.

    If you have a power supply instead of a battery, you'll need to look at how many amps it can put out in order to make this calculation.

    Just a tip: if you're ordering from Amazon, their Q&A section plus reviews tend to have a lot of good info of this sort. Of course Adafruit and SparkFun will have actual tutorials, which is even better, but you can still usually find out a lot of things just by seeing what people who already bought the product have to say.

    Anyway I hope this helps! I'm no electricity expert, but I've put together a couple different projects now and had to do these sorts of calculations... You can usually get by with the product description, some basic math, and an Ohm's Law calculator.

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