specific LED behaviour: how can I achieve this?

[Dubious Drewski]

If I wanted an LED to behave exactly like this:
**broken link removed**
What's the simplest circuit I could build that would do this?

Note: the light is never completely off; it glows at medium brightness with a timed pulse of brightness.

(This thread is an extention of **broken link removed** this one here. I think it fits here.)

 

[tycheng]

Maybe you can try this circuit.......
Use the variable resistor to control the brightness of light!!

 

[tycheng]

please don't forget to press the help button if it works.......

 

[on1aag]

Hi Dubious Drewski,

With a nickname like your's you gonna get a dubious circuit.

It doesn't look like much but it works!
Although it doesn't seem to work with any transistor, I tested
a bunch of BC548's, 2N2222's, 2n3904's and a few other's, some
did work, other's didn't. It has something to do with the internal
structure of the transistor.
So you probably have to try a few to find a sort that works.
Use a 100 µF to 470 µF capacitor to start with, otherwise it takes
too long for the capacitor to charge.
I used a low current led but you can use any led you want but you
might need to lower the value of the 10 k resistor. If you want to
use a led with a higher forward voltage drop you might have to
increase the supply voltage a little. The transistor needs about
8,75 volt across it's terminals to start tunneling.
Although the circuit is fairly simple, for long time delays and
wide pulses the 4700 µF capacitor gets a bit bulky.

on1aag.

 

[Dubious Drewski]

Fantastic! Thank you, guys. I'm going to go figure out what those symbols on those diagrams refer to, then go buy those parts and wire them up and see what happens.

Electronics is a surprisingly engaging hobby.

 

[tycheng]

Does the BC548 a PNP transistor?
the symbol of this transistor seems wrong?

Added after 3 minutes:

sorry, it should be NPN.....
and the arrow of the transistor should points downward

Added after 8 minutes:

But why there is no base bias in this circuit?
Is BC548 a special kind of transistor?

 

[tycheng]

Hi! guys,

I think on1aag is right. The transistor of his circuit was used in breakdown region as the picture shows. There is no oscillator needed in his circuit. This is quite smart. But is there any reliability problem?

Added after 3 minutes:

the picture

 

[impakt]

Hello,
what is the role of the 4700uF capacitor in this circuit?