[SOLVED] How can I find the right R and C values for an astable multivibrator circuit?

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nsguiamano

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I'm currently using this circuit as my basis except that I'd have to use at least 10 LEDs.

It works fine up to 8 LEDs, but once I use 10, the light gets MUCH dimmer and it doesn't blink alternately anymore.

How do I fix this without changing the number of my components and source value?
 

The problem isn't about R and C values, just LED voltage drop.

Use two strings of LEDs in parallel, preferably with separate series resistors instead of R3 and R4.
 
FvM said:
The problem isn't about R and C values, just LED voltage drop.

Use two strings of LEDs in parallel, preferably with separate series resistors instead of R3 and R4.
Click to expand...


Oh wow! I didn't think of that. Many thanks!
 

Look on the datasheet of the transistors.
The 2N3904 transistors have a maximum allowed collector current of only 200mA and they perform poorly above 50mA. For an output of 50mA its base current should be 5ma but the 10k base resistors provide only 8.3V/10k= 0.83mA.

Ordinary LEDs are bright at 20mA and burn out over 30mA.

The circuit has 100 ohms in series with each LED to limit the current in a 2V red LED to 70mA which will burn it out (except the transistors do not have enough base current to turn on properly).

C1 was shown connected backwards.

I fixed the circuit:
 

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