Hey pimps...

Im having problem! I want to create a pulsing LED and cant for the life of me find a schematic on the net that will work. the two ive got have been rebuilt several times on breadboards and strip board and just dont work so the challenge is out! HELP!

Must run a single red led (im guessing 2.1v & 25mA... ish) with a frequency of about 0.5Hz for a full cycle.

Ive looked at simple triangle wave generators using Op Amps and capacitors but they are generally not powerful enough.

Could i simply run a triangle wave into a transistor to fade the current up and down? im not sure... Help would be greatly appreciated!

Thanks in advance


Mark

if you want slow fade look up a light house model circuit. if you want it to just blink u can get a blinking led.
fade is done either with current ramp or pwm with a 556 dual timer

Have you looked for Fading LED?

http://ourworld.compuserve.com/homep...5.htm#eyes.gif

EDIT

Yes they are powerful enough, up to 20mA on some, if you put a transistor on them they can be. If you want the parts (the op amp and the transistor) I can give them to you for free

You'd have to ship the parts to the UK, Nerdz

Quote: Originally Posted by Nerdz Have you looked for Fading LED? http://ourworld.compuserve.com/homep...5.htm#eyes.gif EDIT Yes they are powerful enough, up to 20mA on some, if you put a transistor on them they can be. If you want the parts (the op amp and the transistor) I can give them to you for free

Thanks for the kind offer there Nerdz! (+rep for the generosity) Greatly appreciated but ill save you the postage as i need to order a few things from Maplin anyways so i'll just stick these bits on the end of my order.

Ive seen that circuit before... but the version i was copying had the second op-amp with reversed inputs! DOH!

Thanks again!


Mark

I've actually been working with this circuit. There's a much more robust version for LEDs with a 12V source on the page at http://ourworld.compuserve.com/homep....htm#eyes2.gif . This is a very flexible circuit. One thing I've found is that the LEDs may not fade all the way to black...this can be adjusted by increasing the 36K resistor to something in the 40Ks. Remember to change the resistor on the back end of the transistor to account for just using the single red LED.

Good luck...if you have any trouble with Bill Bowden's circuits, he's a really nice guy. E-mail him, and you're almost guarenteed a helpful response.

Wondering about the same thing only with a sequence of about 100+ LEDS white and red.

You would need a couple more transistors in parallel with the output to do that.

Ive used the circuit and it does work. You can also use an electrolytic cap, but you just have to know where the negative lead goes. You can adjust either the cap or 47k resistor (the bottom most one) to alter the rate.

Just when you thought a thread had finished.... haha!

Ok... so going to be using this circuit

http://ourworld.compuserve.com/homep...5.htm#eyes.gif

Just a quicky... if using just one LED (20ma, 1.85v) would i need to change the series resistor?


Mark

yeah use ohms law (supply - 1.85) / .020 supply voltage minus led forward voltage divided by current draw gives you the resistor you need.

As always, I used 270ohms...

Quote: Originally Posted by markwalker84 Just when you thought a thread had finished.... haha! Ok... so going to be using this circuit http://ourworld.compuserve.com/homep...5.htm#eyes.gif Just a quicky... if using just one LED (20ma, 1.85v) would i need to change the series resistor? Mark

Were it me and I wanted optimal brightness (within' the limits you listed above of 1.85V @ 20mA) I would use around 180 ohm resistor if your source is 5V but since the circuit you linked is for a 9v supply you would want around 390ohm....

Hey there.

Incase anybody is actually still interested in the happenings of this circuit...

FINALLY got it working today! HELL YEAH!

Turns out i had to put a resistor inline with the base of the tranny. The Op-Amp chip was ramping the current, but it was always above the threshold hence the always on LED.

So got the thing pulsing but was a bit disappointed with the range. was very very dim and not going all the way to zero... after an hour or so of fiddling (with the circuit... ) i found my problem. Sharing a 9V source with the IC and the tranny was not working too well. Kept the 9v going to the IC and used a straight (and separate) 5v source to the tranny and VOILA! much better!

One last question for you all and then this thread can finally rest in piece...

The fade UP is much much quicker than the fade out.... the led lights up pretty quick but takes about 3 times as long to fade to black... any ideas why? ive swapped various combinations of resistors over but cant seem to figure out the charge / discharge paths for the Cap....

As always... any help is greatly appreciated!


Thanks in advance!


Mark

Hmm thats odd..

Are you using an Electrolytic Cap? Or a non polarized cap?

Polarized... tried it in both ways round, tried two identical in parallel, series, all ways ropund you can imagine... totally confused

http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html


Read that

Quote: Originally Posted by Nerdz http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html Read that

Interesting stuff there... I never knew there were SO many types of capacitor! lol


I think it could well have something to with the fact that - as the guy said in the article - electrolytic capacitors are "cheap" and "not very accurate"... i would still have hoped for a slightly more regular charge - discharge pattern but maybe just my bad luck? gonna get hold of some more tomorrow and try them out, also gonna mess around with all the resistors some more and see what i can do.

Really appreciate the help here! You def. deserve your "tech mastah" icon!


Muchos Gracias!


Mark

Dont forget electrolyitcs are polarized, meaning they go in a certain way. There is a black band that often denotes negative. Or there will be lines like this - - - - - going down the cap. In other cases it will be labled positive.

Electrolytics have an accuracy of around +/- 20%. They tend to dry out too, but it takes some where around..(well a long time..if you dont use them)