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circuit idea: // note -> times I specify in further text are symbolic

 IDEA:
-cap gets charged to full and is not currently connected to power supply
-it starts to discharge throught paralel LED + resistor over next 2 seconds
-when cap has too low voltage or capacitance to run an LED
transistors base reads LOW from LED and using a NOT gate, CAP gets connected to the
oower supply

here is a pic: enter image description here

TRICKY PART
  i want the last step (battery charging the capacitor)
  to be 0.2 seonds long no matter what, how could I do it?

 WHAT I DO HAVE :
 I have all basic elements for breadboard, timer NE555 chip, a RaspberryPI

Sidequestion:
When capacitor is empty and transistors base reads low at ONE POINT
is there any way I could make that point variable
ex. Transistor reading LOW when CAP is 1/2 charged or 3/4 ?

tyvm:))

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  • \$\begingroup\$ why do you want to do this? for wider voltage variations you cannot use transistor instead you have to use opamp comparator. \$\endgroup\$ – yogece Nov 4 '14 at 3:37
  • \$\begingroup\$ I don't realy see the usecase of your circuit. What is the Idea behind it? What do you want to use it for? You may use a monoflop to charge the transistor for a fix time. \$\endgroup\$ – Botnic Nov 4 '14 at 10:33
  • \$\begingroup\$ is monoflop basically Oneshot (circuit)? i want the event to happen periodicaly. IDEA: when traffic light is not operating, there is yellow blinking light. I want to make LED bright then getting slowly dimmer, but at one point I want LED to get max brightness again, then repeat cycle \$\endgroup\$ – Not_Important Nov 4 '14 at 20:38
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A little while ago I was trying to think of a way to replicate the basic functionality of a LM3909 (now obsolete). I wanted a short flash, followed by a long-ish delay (about 5-10 seconds) and I ended up with a circuit which looked something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

I was also trying to waste as little energy as I could get away with, so I thought that using the charge in the cap to light the LED was a much better idea than just dumping it (although the way the LM3909 could switch the cap to be in series with its supply was even better).

In your case you want the timing reversed, so this requires a few small changes (a diode and a larger cap).

schematic

simulate this circuit

Unfortunately this circuit doesn't really give you much control over the voltage levels at which it switches between charging and discharging. You could get some variation by driving the 555's Control pin to something other than its default 2/3 Vcc, but that would also affect the timing of the circuit.

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