LEDs and me, our continued relationship and luminosity
January 13, 2010 10:32 AM   Subscribe

Sack of LED components: help me determine if they are worth a damn.

You know the deal: went into the electronics store as a newbie, came out a newbie with a sack of LED components made up mostly of 8 segment numbers. I bought them thinking at worst I could bolt them on top of something when I needed "dead computer" parts, but if I could make them light up, it'd be the coolest.

I'm sure there's a good page someplace out there in the haystack telling me everything I need to know about how to drive these but I've got no clue where to start. The nature of this sack makes me think maybe they're salvage and are just junk and can be thoughtfully disposed of...
posted by Ogre Lawless to Sports, Hobbies, & Recreation (9 answers total) 3 users marked this as a favorite
can you post a picture of one?
posted by token-ring at 10:44 AM on January 13, 2010

An Arduino + one of these articles would show you how.
posted by wongcorgi at 11:05 AM on January 13, 2010

Response by poster: T-R: how could I not for a uname like "token-ring"? I'll arrange to do so later today.

wongcorgi -- thanks: Arduino would be a lovely way to go as I could Do Something More Interesting with the resulting curcuit. For some reason I thought one needed a bit of restraint and resistance to make sure one didn't under/over power pieces and that this might require some familiarity with the unit I would not have.
posted by Ogre Lawless at 11:13 AM on January 13, 2010

Best answer: There are basically three things you need to know to make an LED light up.

LED stands for "light emitting diode", and "diode" is the name for an electrical component which only allows current to flow in one direction. If you wire the LED up backwards, it won't light. Unless you have applied a lot of voltage, this won't damage the LED; just switch the leads around.

An LED won't light up below a certain minimum voltage, which is determined by the chemistry of the LED itself, which is closely related to its color. For an orange or red LED, as used in most seven-segment components, this will be about 1.8 volts. (For green, blue or white LEDs, this will be more like 3.6 volts.)

Above this threshold, more voltage means more current. Too much current wears your LED out too fast. You can use a resistor to limit current; simply attach the resistor to either one of the LED's pins, then connect the appropriate power lead to the other end of the resistor, so that the power enters or leaves the LED through the resistor. There is an excellent online calculator page that will help you figure out what size resistor to use, depending on how many LEDs you are driving and what your battery voltage is.

Digit displays like the ones you have are basically eight independent LEDs embedded in one package. The package will have one pin for each LED, plus one shared pin. Some packages share the negative pin, some share the positive pin; you'll have to try it and see which works.

You still need current-limiting resistors for such a package, and this time it does matter which end you connect the resistor to. You can't connect the resistor to the shared pin, because each LED needs its own current limiter. Instead, connect one resistor to each of the individual segment pins.
posted by Mars Saxman at 11:48 AM on January 13, 2010 [1 favorite]

Best answer: All Electronics publishes this PDF guide for hooking up LEDs. It's simple and should get you going.

For 8 segment LEDs, they are either common cathode or common annode. I would make a test rig with the appropriate current limiting resistor, fix a power lead on one pin and then touch all the others. Doing this will make one of three things happen:
  1. Nothing - either the part is dead, you found a No Connect pin on the fixed lead or the power leads are backwards. Move the fixed lead and try again or switch the polarity of the fixed lead
  2. Exactly 1 segment lights - this means that the fixed lead is on the segment control pin and where it lights is your common annode/cathode. Fix on the common annode/cathode and move the other lead (don't swap polarity!) and you'll have all the rest of the segments
  3. All the segments light up in turn - hooray! You got it one!
You might be able to run the LEDs on 3V with no resistor. For a typical 2V forward bias, you'd need a 50 ohm resistor, which isn't much at all.

The worst case in testing this way is that you'll let the magic smoke out. Oh well. My brothers and used to turn LEDs into tracer rounds by running 22V through them.

A decent enough guide is here and another here. I like doing this kind of hacking with a Basic Stamp 2 module.
posted by plinth at 11:51 AM on January 13, 2010 [1 favorite]

Oh, and if you want to hook up many segments at once, consider the MAX7219 multiplexer.
posted by plinth at 11:56 AM on January 13, 2010

Response by poster: token-ring: the collection
posted by Ogre Lawless at 12:48 PM on January 13, 2010

Response by poster: Thanks, Mars Saxman and plinth -- great answers both.
posted by Ogre Lawless at 12:50 PM on January 13, 2010

Google is your friend here in that you will be amazed what all you can find a spec sheet for out there. The hard part is figuring out which numbers matter and which don't.
posted by Kid Charlemagne at 4:34 PM on January 13, 2010

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