What is the easiest way to get LEDs to blink?

No_6 · Post by **No_6** » Mon Mar 02, 2009 1:41 am

Just need to get the running lights of my model to blink. I'm using 3mm LEDs. Unfortunately, I don't know the specs on them. The one thing I do know is that the red and the green are of different intensities, and if I try to run them in parallel from AAA batteries, the green doesn't go on, but the red does. I also have a small resistor on the + lead (the resistors came with the LEDs)

photoguy · Post by **photoguy** » Mon Mar 02, 2009 9:16 am

SCC-7107 USS Atlantis wrote:Personally

The small PCB

The blinking LEDs suffer two drawbacks

1) Lower lifespan - you'll need to replace it sooner
2) Non-adjustable blink rate - you're stuck at what the factory set up as the rate

With the PCB route, you can customize the blink rate and the LED's will last longer

As an example

This schematic yields this result

I just test built one of these for my Defiant, and it looks pretty good. As far as your green LED not lighting, try switching out that resistor for a lower value one. (Check the packaging - Radio Shack has a stripe code on the back of each package, and they are standardized.)

You may need to bump up the current. I haven't built a ton of lighting circuits, but the ones I have, have worked very well on 2-AA, or 3 1/2V wall warts.

No_6 · Post by **No_6** » Mon Mar 02, 2009 4:15 pm

Cool! Thanks!

This wiring thing is much more, er, involved than I thought.

Sparky · Post by **Sparky** » Mon Mar 02, 2009 4:26 pm

If you get newer LED's you should be able to find Red and Green LEDs that want the same current. Once you have that they will look much more close in intensities. Put the 2 in series and add the correct resistor so that they are getting the same current and correct voltage.

You have an existing circuit right?

jwrjr · Post by **jwrjr** » Mon Mar 02, 2009 11:49 pm

The reason why you get different results with red and green leds is in the way they are made, specifically what is called "forward voltage". In English, "forward voltage" is the voltage you have to apply to a LED (any diode, really) to make it turn on. For a red led this is generally 1.75 volts. For a green (or blue, or white) this will be around 3.5 volts. Three volts will make a red led turn on brightly. On the other hand, three volts will make a green led turn on dimly, if at all.
In this lies how to calculate the led resistor. Take the supply voltage, subtract the forward voltage for the led (see above), and divide the result by the max current for the led (usually 25ma). This gives you the minimum value for the resistor. Anything higher is OK. The led just will not be as bright. Sometimes this is desirable.
So if you use the same voltage and the same resistors for red and green leds, the green will not have as much current flowing through it and not be as bright. And if the supply voltage is low enough, you can hace a situation where the red led shines and the green led does not even turn on.

No_6 · Post by **No_6** » Tue Mar 03, 2009 9:02 pm

photoguy wrote:
SCC-7107 USS Atlantis wrote:Personally

The small PCB

The blinking LEDs suffer two drawbacks

1) Lower lifespan - you'll need to replace it sooner
2) Non-adjustable blink rate - you're stuck at what the factory set up as the rate

With the PCB route, you can customize the blink rate and the LED's will last longer

As an example

This schematic yields this result

...the only problem is that I don't understand wiring diagrams at all. I do much better with photos.

Thrusterhead Jones · Post by **Thrusterhead Jones** » Wed Mar 04, 2009 1:21 am

You (or somebody else) mentioned red and green LEDs of the same current rating. This doesn't mean that they will be the same brightness level. The way the human eye works green will appear brighter than red- this is why emergency vehicles and traffic worker's vests are now being produced in a high vizibility green rather than the more standardized red or orange.

If you're going to put in LEDs in future models, invest in a cheap LED tester. It should give you a better idea on how the LED performs. Asia Engineer has them on EBay for $6.99 with free shipping, and outragiously cheap prices on standard sized/shaped LEDs in all popular colors- but you gotta buy a whole lot of them.

Hope this helps.

Sparky · Post by **Sparky** » Wed Mar 04, 2009 11:39 am

Maybe that is also why the green led's have a lower mcd rating compared to the red from the same line up (I did a quick search on digikey)? The marked advantage to making sure they are the same current is that you can put them in series rather than in parallel.

The take away note, is get red and green LEDs from the same product line, and make sure it is a newer product line, they do the job of making sure the LEDs are matched.

Lord Darth Beavis · Post by **Lord Darth Beavis** » Wed Mar 04, 2009 10:22 pm

Flip a switch?

WHAT?!?!?!?