Simple circuit (How is it working)?

[BRetonDP]

Hello everyone!
Even though I think I understand how they work, I don't have a lot of experience with transistors (let alone PNP). This little circuit is supposed to make the two LED's blink in an alternating pattern, but I can't get my head around 'why' this is happening. Would someone be kind enough to briefly explain the current flow in this the schematic to me?

The circuit belongs to a MAKE Magazine video.

[Kappes Buur]

Perhaps with the circuit redrawn it makes more sense:

[FrankBuss]

It's an astable multivibrator. Good explanation on Wikipedia: http://en.wikipedia.org/wiki/Multivibrator#Astable_multivibrator The concept is the same with PNP and NPN transistors.

[ludzinc]

There's a small caveat to applying standard circuit analysis here.

If you assume all the parts are ideal, when you apply power, NOTHING happens.  Indeed if you try to simulate it in spice, it will fail to oscillate.

In real life there are small differences that will allow one side to charge slightly faster than the other and the circuit will soon oscillate. 

[EEVblog]

Here you go:
http://www.talkingelectronics.com/projects/BasicElectronics-1A/BasicElectronics-1A_Page5.html
Same thing but using NPN's.

[GK]

In real life there are small differences that will allow one side to charge slightly faster than the other and the circuit will soon oscillate.

This circuit is prone to lock-up (both transistors staying on) if the supply voltage initially ramps up slow enough.

And a common design error is to not realise that the b-e junctions of the transistors will be alternately reverse biased by almost the full supply voltage when the circuit is oscillating. Most small-signal transistors can tollerate -5V Vb-e max. Zenering the b-e junctions in the long term causes irreversable damage (hfe degredation) to the transistors. The simplest fix is to put a 1N914 in series with each emitter.

 

[BRetonDP]

Thank you for your responses

While I understand the concept, I'm having problems, as FrankBuss stated, understanding why this circuit doesn't saturate both transistors and always stays on. That's why I wanted to know the current flow. Is it only because of a physical characteristic of the circuit, then? No deeper science?

[BRetonDP]

And a common design error is to not realise that the b-e junctions of the transistors will be alternately reverse biased by almost the full supply voltage when the circuit is oscillating.

Also, what does it mean to be reverse biased?