[sdiy] flashing LED?? blinking led FLED (are they internally similar to a UJT?)

[Didrik Madheden]

My guess would be that the LED contains two chips, one special purpose chip
for flashing, and one for the actual LED element.

Which exact method they use is hard to know, but it could be anything from
some simple multivibrator to a digital circuit which in turn might have a
simple RC oscillator on board that feeds a divider. The latter might be
more likely since the divider circuit can work with an arbitrarily high
input frequency whereas a simple multivibrator would require a capacitor
that's directly used in the oscillator. This would require a larger value
cap which might need to be an external (third!) component instead of being
integrated to the die.

The circuit would clamp the supply voltage during the LED on period, but
that's fine as long as the circuit can work on the 2 V or whatever the LED
clamps the voltage to.

As an aside, I remember a story, which for the life of me I can't find now
online, of a color changing or flashing LED which when hooked up to an
oscilloscope showed a peculiar waveform. This turned out to be a Happy
Birthday or Jingle Bells or something like that. This was a multipurpose
chip that worked equally well as a LED driver as it did as a chip for a
musical greeting card!

As for your build, I suppose it doesn't really matter what produces the
oscillation, in terms of the circuit. As long as you get a periodical on
period where the device draws more current, the little bug will jump around
erratically, which was the whole point. It just so happened that the
flashing LED ticked all the boxes. It's cheap, available, happens to do the
right thing by mistake. And maybe most importantly it's interesting: it's
producing light in sync with the movement.

Here a second tangent. You can make an observation that you need a quite
high number of components if you're going for absolute minimalism. Even the
simplest 555 oscillator will require 3-4 external components. For contrast,
something like an ATTiny85 based Arduino board can be fairly small, can be
programmed directly over (software bit banged!) USB, and is very flexible
because it can be programmed to do almost anything in code. I would
personally consider something like that for the oscillator. This is also an
excellent learning opportunity for your son, because you can explore
different behaviors through code. Maybe the bug can make three short
movement and one big or something else interesting.

/Didrik

On Tue, 11 Jun 2024, 17:03 loss via Synth-diy, <synth-diy at synth-diy.org>
wrote:

> I am building a small motor based "BUG" robot with my son. One of the
> parts the project calls for i have never seen before..a flashing led or fLED
>
> The text says that if you use a regular led the motor will not work
> properly (it must somehow be using the FLED as a timer which allows current
> to flow from the storage capacitors
> to the motor. The text gives no further explanation but it does reference
> it as a timing element.  Later projects use a hard to obtain part called
> the 1381 trigger but i would rather use parts I have on hand.
>
>
>
> Obviously I could order an FLED but as the datasheets are very sparse I
> was wondering if anyone has any idea internally how these parts work. Where
> does the timing mechanism come from?
> I was wondering if even though they are a two leg device..that internally
> they may have similarities with PUT or UJTS. I have a whole bunch of PUTS
> around.
>
> thanks!
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