The red flash

Not seeing the circuit I am picturing a possible charge pump effect taking place? Or, it could of just been a bad LED.

Jamie

_______ // | | .-->|--. | 555 3|---||--/\/\/--| |----. |___1___| | \\ | | | `--|

Could it be a bidirectional red/green device that they oscillate to display yellow occasionally?

I've seen LEDs glow (or should I say smoulder) a different colour when you drive them WAY over spec before they get hot and burn out.

Could be a short somewhere, preventing the limiting resistor from doing its job.

This acts like a charge pump voltage doubler. When pin 3 is low the cap charges at vcc (minus led dropout), when it goes high, the cap is connected in series to vcc with the right polarity, therefore one of the leds gets vcc+vcc-led_dropout, roughly 22V. That led likely failed because the limiting resistor value was too low; you either raise its value or (better) change the circuit. Try this one. When the 555 isn't inserted both leds should light.

Speaking of LED failure modes... I remember this time back in school when a classmate just had to figure out what would happen if you reverse polarized a green LED at 50V.

Yes, I had the presence of mind to turn my face away from it, but that piece of epoxy that shot away from the LED when it disintegrated stung when it hit my hand.

So for future reference, if you are going to demonstrate (or experiment with) failure modes of LEDs, take precautions. And I assume that the magic smoke (if any) given off isn't too healthy either.

/Teo.

Does this phenomenon occur only on 4/1 ?

Ed

Did it ever light up green?

In the old days, it was much easier to get scrap LEDs than good ones. Certainly it was cheaper. I always tested them with voltage, because I could never trust that they'd work, or be the right color. It wasn't uncommon to find that they'd barely light.

If you never lit this one up, then I'd say you either got a mixed up one, or it was never supposed to be green. Of course, that's harder to do these days when many LEDs are packaged in the same color as the actual light emission.

Michael

Most of the LEDs that I've been using recently (all SMTs) are clear, with the polarity markings in green, no matter the wavelength.

Self correction: it doesn't. I didn't pay attention to the leds being connected to ground, not vcc/2 or something else. Looks like part of my brain was thinking about H bridges and the other about operationals:) Anyway, while my proposed modification is correct, the reason why the led failed remains unknown. Maybe a defective part or an error somewhere.

It's not the "explosive nature of LEDs" that counts. It seems to be the epoxy. There was a time when power diodes weren't so small as the 1N4000= =20 series, and I had a bunch of fairly big epoxy encased power diodes. I=20 remember putting some of those in backwards and they seemed pretty deadly. In both cases, it was the fact that the actual component inside would heat things up enough and fast enough that the epoxy package would break=20 up and be propelled out.

Though I'm not sure if that was as spectacular as the time I put a=20 tantalum capacitor in backwards.

Michael

:) I hadn't considered the date

It was several weeks previous, and then there was an earthquake that destroyed and scattered the evidence.

yeah, it worked as intended on several occasions for about about 30 minutes total, then it switched sides.

I keep a CR2032 cell on my bench for testing leds and always use it to confirm the polarity and test the parts, the cell's 3V output and internal resistance make it well suited for testing LEDS. A CR2016 would be better sized but the 2032 was free, it came from a scrapped PC

I think I got it from futurlec last year some time