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Transistor combinations questions


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Hi there,

I'm from Malaysia, and I've learned quite a few stuff about electronics back when I was studying Form 5. Generally I'm very amazed and very interested in the mechanisms of the electronic circuits. So I decided to build one automatic switch based on a transistor, some resistors and one switch, and it works perfectly.

But now I want to build a single LED Flasher, and of course, searched the info thru google and the website as well, but I couldn't understand what's astable and bistable multivibrators, since these theories are applied into the blinking LED project as well. Is there a very simple explaniation about these and also some analogies?

And I've seen the single LED blinker circuit too - and find it strange that sometimes the (+) from the battery can be connected to the (-) side of the transistor (for an NPN ones). Which is correct?

Also, can I subsitute the 2N3906 and 2N3904 transistors to CS9012 and CS9013? Are they different? :D

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Astable means NO stable condition. That is, an astable multivibrator will never settle into a fixed state - it will always switch from one state to another and back, all on its own, without any external input. Lamp flashers are astable, needing no external input to switch from off to on to off to on etc, all on their own.

Bistable mean TWO stable states. A bistable multivibrator will settle into either of two states and stay there until unsettled by some external input - then they switch to the other state, until it is externally signalled to switch back again. They do nothing on their own. Static RAM is implemented using bistable multivibrators to store binary 1s and 0s, and keep them unchanged until told to change. Bistable multivibrators do not change state AT ALL unless externally signalled to do so.

Monostable means ONE stable state. These multivibrators settle into some state and stay there until signalled to change. When so triggered, they switch to the other (unstable) state, stay there for a while and return automatically to their previous, stable state, where they remain until the next external trigger signal. Sometimes monostable is called "one shot".

By 'state' we normally mean "on' or 'off'.

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Just a couple of small diagrams of how inverters can be used to make a bistable multivibrator. The resistors 'R' are present to avoid connecting a power rail directly to the output of a gate, which is bad.

The switches are pressed momentarily to force an input to a specific level (high or low depending on which switch), and cause the multivibrator to flip into one of its two stable states.



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Not much actually - when the switch is pressed, the pair of inverters will change state and settled into a condition in which the switch is no longer fighting the collector of the first transistor.

If the first transistor is switched fully on, though, it is effectively a short circuit from its collector to ground, and pressing the top switch (in the absence of R) will connect the +ve supply to the same collector. This will cause oodles of current to flow through the transistor, until the state change induced by pressing the switch propogates around to switch this transistor off again. So it's a momentary current surge.

The current pulse is very short, but large, undesired and possibly dangerous to the transistor none-the-less.

R simply makes sure that the transistor's collector is never subjected to the full clout of the +ve supply.

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