Boom circuit

[EyeLoop]

Hi techies,
I have a vain personal project that my feeble electronics skills can't crack and being myself a pretty lame autodidact (yes there are such beings still trying to achieve stuff anyway) I humbly ask this community which of you have an insight on : how to make a flash of light that would be fired by a push button switch then slowly fade out. (for the context: the idea is to make a physical message box that lit like a volcanic eruption when a message is put in).
I get that the light fading can be achieved with a discharging capacitor and that a clever play of transistors would be involved in the directing of the accumulated charge but my pitiful fiddeling just reached the point of frustration. I'm using a simulation app called "EveryCircuit" and at this point, I just can't tell if my design is s**t or if the app can't emulate the behavior.
I tried many things and I'm just gonna upload the give-up-version as a mind-fixer for you. You'll find it soooo easy to debunk but please if you're determined to answer, do focus on the how to make it work, I already know this is a wheeless car.
I'm interested in leads, designs and scales. Like I said, I have an education to get.
here's the thing:

the idea is to let the capacitor on the right module (almost) fully discharge on button push. When the capacitor is discharged, the circuit should return to its original state and let the capacitor recharge for the next push and stay charged until so.

thanks a bunch to whoever takes this seriously.

Regards

And if you find my prose weird or inadequate, please excuse my french, just doing my best with your language.

 

[dr pepper]

If I get your explanation you can do that with a resistor, large capacitor, switch & Led.

 

[throbscottle]

You don't have component designations on your schematic, but I can see some things wrong with it.
Transistor on bottom left is PNP, yet it's emitter is connected to 0v. It will not conduct this way.
Transistor at far right has it's base and emitter connected together. It will not conduct this way.
LED is in series with the emitter of the transistor at far right. This will provide negative feedback to the transistor, which will tend to counteract the effect you want.

I get the impression that you want to press the button once, and the effect continues after releasing the button. The circuit you show won't achieve that because there is nothing to keep the transistor at far right conducting after you release the button.
There is no current limiting resistor for the LED. This not only protects the LED but also in this circuit would be the means to control the time over which the capacitor discharges - the time over which the glow decays.

I think your friend here is the humble 555 timer. There are hundreds, if not thousands, of example circuits you can find online using this chip. I suggest you investigate 2 possible approaches:
1) set up the 555 as a monostable. Set the pulse width to be somewhat longer than you want the glow to last. (Maybe twice as long? I'm not sure) Connect a resistor (lets guess 47k) to the 555's "discharge" pin, and connect the base of a transistor, lets say 2n2222, to the other end of the resistor. Connect the emitter of the transistor to 0v, and connect the LED in series with it's limiting resistor to the collector of the transistor. Connect V+ to power the LED. I think this will do what you want, but it is only in my head so it might not work or might need changing somewhat.

2) set up the 555 as a bistable. Normally you have a "set" and "reset" switch. Connect the output so it charges a capacitor via your LED and it's current limiting resistor. Instead of having the "set" switch, connect the capacitor to the 555's trigger input. This way, as the capacitor charges, the LED will light fully and then dim as you require, and when the voltage on the capacitor reaches 2/3 of the supply voltage (one hopes the LED is dim enough by then) the timer will be set again, halting the process. So the circuit is triggered by pressing the "reset" switch. Unfortunately with this arrangement you can't use the "discharge" pin to discharge the capacitor as it might seem because it follows the output. You could add a transistor to do the job though. Again this is off the top of my head and I only think it will work, and really needs designing properly.

 

[EyeLoop]

If I get your explanation you can do that with a resistor, large capacitor, switch & Led.

Thank you for your interest,
I suppose that summarizes the whole well except maybe for the switch. A switch requires a manual switching off. An important feature of this circuit is that it lets the capacitor discharge fully (or just under some low threshold) on a punctual impulsion (a push button emulates well the behavior that I'm expecting from the mechanical frame that will integrate this circuit) before starting reload the capacitor full to await the next push (that might come days later, so a metastable state would be welcome). My problem is : how to make a capacitor-charging module that flips off when the capacitor discharge into the LED then self flips on when the capacitor is exhausted. Any known look-alike circuit coming to mind perhaps?

 

[alec_t]

My problem is : how to make a capacitor-charging module that flips off when the capacitor discharge into the LED then self flips on when the capacitor is exhausted. Any known look-alike circuit coming to mind perhaps?

Yes: a monostable circuit, as suggested above.