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wendallsan
New Member

1 Posts
Posted - May 14 2009 :  5:44:21 PM  Show Profile  Reply with Quote
Hi all,

I'm very new to electronics, and have bought a Radio Shack Electronics Learning Lab to play around with. One of the circuits in the book for the lab is a dual led flasher (schematic is attached to this post). I've got it built and working fine, I am just now curious as to how it really works-- my guess is that power flows through both transistors to start, as the base of both is grounded. But as power flows, the caps charge and then one or the other loads enough to shut down one or the other transistors, which then starts the oscillation between the two transistors. Is this what's going on, or is there some other explanation? I'm just starting to come to grips with transistors, and appreciate your help!
Edited by - wendallsan on May 14 2009 6:25:21 PM

audioguru
Nobel Prize Winner

Canada
4218 Posts
Posted - May 14 2009 :  7:29:27 PM  Show Profile  Reply with Quote
The polarity of the capacitors are backwards in the RadioShack transistor multivibrator circuit (look in Google, some are correct and some are wrong like this one).

The transistors and resistors are not exactly the same so when power is applied one transistor conducts and its capacitor turns off the other transistor. The capacitor charges slightly in reverse through the 100k base resistor and the transistor that was turned off turns on which turns off the transistor that was turned on.

The base end of a capacitor is forced above the supply voltage.

Download Attachment: transistor multivibrator-PNP.PNG
131.65 KB
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