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I am trying to build a simple LED blinker as a nightlight for my son's bedroom, based on this circuit.

Diagram showing a 3-led blinker using astable multivibrators

But my knowledge of electronics is basic at best. This circuit has a 10k pot that was supposed to control the blinking speed, but for some reason I don't understand it has to go to 0 to start the blinking. After that, I can turn it to the maximum value to get the slow steady fading I wanted.

But I don't want to have to turn a potentiometer manually every time the circuit starts. I'm sure there's a way to make this happen automatically using a capacitor somewhere, but as I said I'm not good enough to figure that out by myself.

Edit to add: Tried changing one of the 10k resistors (I actually put a potentiometer there to try several different resistances) but didn't work. I also tried changing one of the capacitors by a 33uF one, and then a 470uF one. The circuit simply doesn't start if I don't turn VR1 to 0.

Edit 2: I didn't have access to the CS9013 transistors, so I used a S9014 instead( the datasheets seemed similar enough for me) Also, for testing purposes I'm using a 9V power source instead of a battery.

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2 Answers 2

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You can use a capacitor in parallel with the pot (literally around the two leads) to maximize initial current flow. This should retard the timing just enough to unbalance the circuit at the start. The capacitor acts as a short when first turned on, which simulates the pot being set to zero. After some time the capacitor will charge and hold the voltage at the level of the pot.

Bonus: if you have access to a 555 timer, you can implement a more reliable blinking circuit. https://www.instructables.com/id/Flashing-LED-using-555-Timer/

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    \$\begingroup\$ I do have a TON of 555s actually 9mistakenly bought them in bulk). I was right now trying to use one of them to start the circuit, but I'll use the capacitor because I'd rather keep the footprint small. It worked! \$\endgroup\$
    – darthstark
    Commented May 11, 2020 at 19:15
  • \$\begingroup\$ Glad it worked! What value did you use for the capacitor? @darthstark \$\endgroup\$
    – nate
    Commented May 11, 2020 at 19:24
  • \$\begingroup\$ 470uF. I tried to go lower but it wouldn't delay enough for the transistors to start up. It takes a few seconds to discharge after deactivating the circuit, so reactivating it right away doesn't work, but that's OK. \$\endgroup\$
    – darthstark
    Commented May 12, 2020 at 20:56
  • \$\begingroup\$ That's about what I expected, and the delay between turning off and on may be solved with using a DPDT switch that connects a low resistor to the capacitor to discharge it much faster than the pot along with turning the entire system on/off. \$\endgroup\$
    – nate
    Commented May 13, 2020 at 0:27
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Change 1 of those lower 10K resistors to say 9.1K, you need to unbalance the circuit so 1 transistor preferentially trips before the others, it will change the timing of that light slightly, but will make it more reliable at start up,

with a low noise voltage source like a 9V, its possible there is not enough difference between the 3 transistors to cause a consistent start up.

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  • \$\begingroup\$ Tried that but it didn't work... Also tried changing one of the capacitors to unbalance the circuit but it also didn't work. \$\endgroup\$
    – darthstark
    Commented May 11, 2020 at 18:36

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