I'd like to I build a simple circuit with an LED to flash periodically like a firefly / lightning bug.

The timing part doesn't seem too difficult - just have to get the values right for the capacitor and resistor - instructions show the concept:

But would also like it to fade instead of just pulse - that is, the LED would come on slowly and fade out over about 2-3 seconds.

Not opposed to using an IC or 555 timer but was wondering if the whole thing could be done with just transistors, capacitors, and resistors.

  1. How can I adjust the circuit to fade the LED on and off when it is triggered?

  2. Also, without an IC are there any ways to add some degree of randomness to the circuit's timing?


closed as unclear what you're asking by Olin Lathrop, Leon Heller, Voltage Spike, uint128_t, Wesley Lee Mar 13 '17 at 23:57

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    \$\begingroup\$ We communicate circuits here with schematics. Cartoons need not apply. \$\endgroup\$ – Olin Lathrop Mar 13 '17 at 15:08
  • \$\begingroup\$ Since any IC or 555 is just composed of transistors, yes it can be done without ICs, but it's probably a lot easier with an IC. The easiest way to do what you're after is in software though. A $10 mini Arduino would get all this done in very short order. \$\endgroup\$ – horta Mar 13 '17 at 16:01
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    \$\begingroup\$ Please redraw the circuit with the schematic editor tool, these are hard to look at. \$\endgroup\$ – Voltage Spike Mar 13 '17 at 16:22

It's a pity the LM3909 was discontinued as it provided a vastly superior means to achieve what this circuit does, not least because it was capable of flashing a 2V LED from a 1.5V battery (and make it last for years), whereas this circuit requires an almost preposterous 12V supply to do the same. Nonetheless, it's interesting in that it makes use of a transistor characteristic that rarely gets used in practical designs.

Modifying it to add the features you want, however, is not going to be at all easy without an IC because of the obscure way it operates. You could add the fade up and down using an integrator, which would be trivial with an op-amp, although given long enough you could implement it in discrete components, but the random element is much harder. All electrical components generate a small amount of thermal noise, which you could amplify to the level where you could sample it to control the delay of a monostable; once the delay expires it would trigger the LED pulse, sample a new noise figure and trigger itself again with the new delay - but with discrete components this is going to be quite a task.

Or you could admit defeat with that route and use the smallest and cheapest microcontroller you can find, such as a PIC10F200, and use an output pin to drive the LED. By turning the LED on and off quickly you can control its brightness and you can implement a pseudo-random sequence to provide a variable delay. All you would need would be a 3V battery, the PIC (which my usual supplier sells for £0.34), the LED and a resistor - and it would be a great learning exercise.


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