Stumped about how this switching circuits system works

I have stumbled upon this circuit from an online electronics course (, chapter "Dual Flash Lights"):

The behaviour of this system is that both LEDs switch on and off intermittently. It's quite hard to understand how/why exactly it behaves this way, so I recreated it in the CircuitLab:

The resistor/condensator values are copied from the schematics, with one exception: C1 is changed to 9μF because the model got stuck if both capacitors were exactly equal (in real life the capacitors have minuscule differences that cause the asymmetry).

Unfortunately, this did not bring the light I was hoping for. The simulation reflects current through the LEDs properly, as it happens in reality. However, the voltage level at the transistor bases casually drops to minus 9, although the ground is set to the battery minus. Also, it is not clear from where the capacitors draw the current to charge. It can't be through the 47kΩ resistors because that would violate the capacitor polarity. Can the charging really happen through the transistor base, is there really any serious current flow? I'm confused.

by Passiday
January 13, 2019

When Q2 is turn on, the C2 is parallel to Q1(C2+ connect to Q1-E, C2- connect to Q1-B)

by IsaacWang
February 20, 2020

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