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Radio Shack LED with built in Resistor — Parallax Forums

Radio Shack LED with built in Resistor

qxoticqxotic Posts: 47
edited 2012-08-28 12:02 in BASIC Stamp
I am going to use some LED's from Radio Shack. For example the red 276-270. They have a built in resistor but it does not say anywhere on the package or their website what the R value is. They are rated 12V typ and 12mA typ, 20 mA max.

If I am using a basic stamp BS2 or BS2pe pin to turn these on and off, I assume since they are typically 12V 12mA that I would not need to add any extra resistors... but I rarely act on assumption so wanted to be sure I am safe controlling them with a stamp pin without any additional resistors?

Better safe than sorry.

Thank you in advance for the input.



  • ercoerco Posts: 19,895
    edited 2012-08-28 08:45
    Certainly OK to try at 5V, they will be dimmer than 12V. Polarity matters, obviously: red is +, black is -. The internal resistor is chosen to yield a 20 mA draw at 12V, so it's approximately (12V-1.8V(LED fwd voltage))/.020=510 ohms, more likely 470 or 560 ohms. So at 5V, it will only draw about 6 mA. Before buying a lot of those $2 units, consider buying a bulk pack of 5mm red LEDs and 220 ohm resistors. Brighter & better.
  • qxoticqxotic Posts: 47
    edited 2012-08-28 09:08
    Thanks erco, I don't remember how long I've had these.. but since I already have them wanted to use them. Would definitely go the other route if I needed more. Thank you for sharing the formula. I was guessing on the decimal placement.. and had forgotten to subtract the fwd voltage. Much more peace of mind to be sure.

    Thank you and have a great day.
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-08-28 12:02
    You can get 5 volt LEDs from several suppliers. They're much easier to use for prototyping than LEDs with separate resistors. I have a bunch, both 12 volt and 5 volt.

    I looked on eBay recently and couldn't find any, but 12 volt LEDs were pretty common (popular for automobile use). Here are some at Mouser made by Lumex, they're the cheapest. Avago, Chicago Miniature, and Kingbright also make them.
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