Re: Astable Multivibrator help!



MooseFET wrote:
On Jan 25, 5:50 am, Fred Bloggs <nos...@xxxxxxxxxx> wrote:


The circuit may be locked up with both transistors stuck in saturation.


Yes that sure looks like it;


In order to guarantee oscillation, you need to add a diode logic OR
function which provides base drive only if one OR the other of the
transistors is OFF, like shown below, a lock-up cannot occur because
there will be insufficient base drive.


There is a simpler circuit that does it.


Modified:

.
. Vbatt
. |
. [150]
. R1|
. .----------+----------.
. | |
. [330] [330]
. R2| R3|
. +------- -------+
. | | | |
. .---+ | | +---.
. | | [39k] [39k] | |
. +| \| R4| |R5 |/ |+
. === Q1 |-----+ +-----|Q2 ===
. 10u|C1 <| | | |> C2|10u
. | | 2N3904| | 2N3904| |
. '---|-------|-----' | |
. | '-------------|---'
. | |
. |LD1 LD2|
. --- ---
. \ / ~~ \ / ~~
. --- ---
. | |
. '----------+----------'
. |
. com


If you remove the positive feedback both sides will settle with just
about 2V on the collector of the transistors, depending on the HFE.
As a result both sides will have a gain well over unity. With the
positive feedback, it just takes a little noise in the transistors to
get things started.

I don't care for that one because the Hfe affects timing, once the initial turn on through the coupling capacitor is charged up, the ON transistor starts to go linear, and this affects the timing in an Hfe dependent and unpredictable way. The diode method is predictable. Speaking of which the OP needs to increase his timing caps to 22u for 1Hz on each LED.

.

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