Ghetto Programming: Getting started with AVR microprocessors on the cheap.

Step 6 Make some Blinkenlights

An integral part of the Ghetto Development Environment is the plug-in LED.

The AVR chips can all source a fair amount of juice (50mA or more), which is enough to burn out your standard red LED, so it's a good idea to put protective resistors inline with your LEDs. Enter the Ghetto Blinkenlight.

To make them, simply solder a smallish (150 ohm) resistor to the negative lead of an LED, then you can just plug it straight into 5v and it won't burn out. (5 - 1.4) v / 150ohm = 24mA, which is just about right. Make a few -- I made eight. We only use one here.

Comments

[tissit]

Lead length is obviously a useless technique since leads get clipped. Round leds have a flat edge too, but that's useless if your leds are not round. Quite reliable way to find the polarity is to peer through the plastic. The leg that attached to the large piece of metal is negative. - -> |7. <- + I've seen that fail once, though, and with SMD that goes out the window anyway. So, one method remains: Just try. Put a suitable series resistor in and try power both ways and see which works. Or use a meter which does nearly the same thing. Or a curve tracer... :)

[Kociubinska]

The easiest way to find a polarity of a LED is to connect two identical LEDs in antiparallel and use a single series resistor. The diode that lights is connected right. The other one will not destroyed.

[geeklord]

Or a multimeter...

[sysadmn]

Stoopit newbie question - is the negative lead of the LED the longer or shorter one? Does it matter which lead you use if you are consistent? I haven't done hardware in 25 years! It's great to get started again.

[Koil_1]

the ground is usually the the one attached to the larger piece in the LED. That is if the lens is clear enough to see the actual parts inside. I've noticed that not all LEDs have a flat spot on the rim. Further more some are square which kinda defeats the flat spot thing. Seriously though, it might be a noob question but it wasn't stupid. The only stupid question is the one you don't ask. Don't be so hard on yourself OK.

[The Real Elliot]

When they come out of the box, longer is positive. Something I just learned recently -- the negative has a slight flattening on the round disc at the bottom of the plastic lens. Useful after you've trimmed the leads. My convention is to put the resistor on the negative end, but it doesn't really matter as long as the resistor's in the circuit somewhere. And you can really just try it out. If there's light, it's right.

[BlackHatCracker]

I have found it easier to place the resistor on the negative. That way when I am doing something, then I dont screw things up on it... lol.. Thank god I live close to a Radio Shack, and worked there for 8 yrs. I have messed up a few times and blown a few of them out myself. Although true, it really don't matter too much on which side, just keep it the same on all. If at all possible too when you do this, use red or black heat shink tube on them. Makes it a lot easier when it comes to remembering what side is what.

[andrew101]

yep. also if you have non-diffused leds if you look inside at the innards of the led there are 2 plates of metal. the smaller plate of metal is positive.