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### Topic: Multivibrators (Sorry girls, nothing to get excited at here... :) ) (Read 4303 times)previous topic - next topic

#### Grumpy_Mike

#15
##### Dec 03, 2012, 04:48 pm
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I wouldn't rely on this though unless using non-symmetric component values.

It is impossible to have anything other than non-symmetrical component values. All components even if marked with the same value have a tolerance. Even when you try and match components you only acheave a match to a certain number of places. Even if you acheave the impossibility of exactly the same component values, transistors with the same gain, and so on. They are physically diffrent things in physically different places. There willbe differences.

#### retrolefty

#16
##### Dec 03, 2012, 05:33 pmLast Edit: Dec 03, 2012, 05:40 pm by retrolefty Reason: 1

Quote
I wouldn't rely on this though unless using non-symmetric component values.

It is impossible to have anything other than non-symmetrical component values. All components even if marked with the same value have a tolerance. Even when you try and match components you only acheave a match to a certain number of places. Even if you acheave the impossibility of exactly the same component values, transistors with the same gain, and so on. They are physically diffrent things in physically different places. There willbe differences.

That's pretty much what they said in the Air Force, (plus they mentioned random circuit noise) would cause a preferential treatment such that the starting state on initial power up would always be the same, set or reset. But they also said don't worry about it too much, the circuit will oscillate, that's why it's called a astable multivibrator circuit.

From wikipedia:

Quote
Initial power-up
When the circuit is first powered up, neither transistor will be switched on. However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on. This will quickly put the circuit into one of the above states, and oscillation will ensue. In practice, oscillation always occurs for practical values of R and C.

Lefty

#### Grumpy_Mike

#17
##### Dec 03, 2012, 06:11 pm
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plus they mentioned random circuit noise

Yes forgot that bit.

The name for all oscillators of this sort is "relaxation oscillator" -

No a relaxation oscillator only has one active component that is in two states, this is a cross coupled bistable oscillator.

The multivibrator was the first circuit I worked out how it worked. I built a very large one on a wooden board with switches and knobs so I could alter the on and off times independently. I also had relays on the two outputs and used them to control disco lights back in 1964. A few years later I hooked it up to a telephone uniselector and was able to produce a long complex sequence for five lights of 3 amps each. The sparks flew.

#### retrolefty

#18
##### Dec 03, 2012, 06:19 pm
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this is a cross coupled bistable oscillator.

I don't think so, It's called a astable mutivibrator circuit, and oscillates continously. A bistable multivibrator circuit does not have cross coupled capacitors and requires external signal pulses to place the circuit into it's set or reset state. A bistable can latch last position, a astable is freerunning.

http://en.wikipedia.org/wiki/Multivibrator#Astable_multivibrator

http://en.wikipedia.org/wiki/Multivibrator#Bistable_multivibrator_circuit

#### Grumpy_Mike

#19
##### Dec 03, 2012, 06:23 pm
Different side of the pond definitions here I think.

#### retrolefty

#20
##### Dec 03, 2012, 06:27 pm

Different side of the pond definitions here I think.

That's hard to believe, British citation requested.

The word Bistable strongly implies the circuit is stable in either the set or reset condition, it does not change state other then from external signals applied to the circuit. Astable would clearly mean it's not stable in either state and is why it's a free running oscillator requiring no external control signals to operate?

Lefty

#### pwillard

#21
##### Dec 03, 2012, 06:29 pm
The first multivibrator circuit I created on a breadboard was based on the 7404.  I later built the 2 transistor version. I always thought was backwards... but the simplicity of the 7404 circuit made understanding the transistor circuit easier.  I really wish I could find a book I had in 1978 that described discrete transistor astable multivibrators. monostable multivibrators, bistable multivibrators and Schmitt Triggers.

#### AWOL

#22
##### Dec 03, 2012, 06:29 pm
Quote
I hate resistor colour codes.
me too - I was recently diagnosed with colour-blindness.
It came like a bolt from the green.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.
I speak for myself, not Arduino.

#### retrolefty

#23
##### Dec 03, 2012, 06:33 pm

The first multivibrator circuit I created on a breadboard was based on the 7404.  I later built the 2 transistor version. I always thought was backwards... but the simplicity of the 7404 circuit made understanding the transistor circuit easier.  I really wish I could find a book I had in 1978 that described discrete transistor astable multivibrators. monostable multivibrators, bistable multivibrators and Schmitt Triggers.

Well the wikipedia article on multivibrators seems pretty good to me.

http://en.wikipedia.org/wiki/Multivibrator

Lefty

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