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I was trying to build a circuit that if switched on would slowly (in span of of 1~5s) light up around 20 LEDs - progressively, not all at once.

To achieve that i used one capacitor to initiate the "build up" effect (and discharge when switched off) and then used NPN transistor's emitter as a base input for next transistor, as shown on diagram below.

Build-up circuit diagram

It works but with each consecutive "part" the light is dimmer and fades out completely after 3rd LED. Which leads me to suspect that power on base of transistor is getting weaker - but it shouldn't require much to let power through.

Transistors i have used are BC337-25.

Resistors vary, i was experimenting with different ones but R1 is 1.5k Ohm, R2 is 100k Ohm, resistors between emitters and LEDs are 220 Ohm, and one between emitters and bases are 1k Ohm

As for Voltages, both inputs are currently 3.3V, but final version will be charged off one 3.9V 18650 battery.

The question is: is it possible to achieve that gradually lighting up effect (similar to "percentage charged") with this design, and if yes what is the problem with my design?

Note that while I only have some understanding of a topic I did my research around the Internet was unable to find good enough resources that I could apply to my problem.

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An LM3914 will replace the transistors and resistors driving the LEDs. Each one has 10 output and two can be joined for 20 outputs.

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Drawing the schematic in the conventional way will generally make it more clear what's going on.

  • Voltage decreased from the top of the schematic to the bottom. That way current flows from top to bottom.
  • Circuit reads from left to right. In your case C1 controls the rest of the circuit so it makes sense to put that on the left.
  • Draw transistors the "right way" up.
  • Use GND symbols rather than earth as the earth symbol implies a connection to mains earth (and the planet Earth) which probably isn't what you intend in this case.
  • GND symbols should point to the ground. Never sideways or upwards.
  • Your battery symbol has no connection on the right. It should be connected to GND.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The OP's schematic redrawn.

Now it becomes clear that each transistor's base voltage reduces by the base-emitter voltage drop of the previous transistor. This will be a 0.7 V drop per stage.

I can't see an easy way to do what you want with this approach.

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