# Multivibrators

Discussion in 'General Electronics Chat' started by richard3194, May 20, 2014.

1. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2

So, without having really studied multivibrators, I was somewhat taken aback the other day from coming to an understanding that a S R latch is a form of multivibrator.

I believe a regular motor starter circuit, where pressing the start button latches power to the motor and the stop button resets to the initial condition is a S R latch circuit.

That would make this motor circuit a multivibrator circuit.

Because I read "Any digital circuit employing feedback is called a multivibrator." I always assumed the word "multivibrator", referred to a circuit that was a kind of oscillator. Seems it does not have to be.

2. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
a set reset flip flop is also called a monostable multivibrator. an astable multivibrator is a type of oscilator.

3. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
But, isn't a set/reset arrangement a bi-stable circuit, stable in two states.

4. ### AnalogKid Distinguished Member

Aug 1, 2013
4,523
1,247
Yes, a SR is bistable. When Abraham and Bloch coined the term "multivibrator", they were describing a squarewave oscillator. The "multi" part was there because of all the harmonics a squarewave produces. Anything with two cross-coupled amplifying devices meets the original intent. Personally, I would not use the term to describe a logic latch since there is no timing element as there is in a monostable or astable.

OTOH, in the old GE transistor manual they start with the traditional astable circuit and morph it into both a set-reset and a toggle FF.

ak

Last edited: May 20, 2014
absf likes this.
5. ### crutschow Expert

Mar 14, 2008
13,000
3,229
I agree with AK. Commonly the term multivibrator is used to mean a circuit with a timing element and I use the term only in that context. Thus a monostable multivibrator generates a single pulse when triggered, and an astable multivibrator generates a free running pulse train or square-wave. In each case the frequency/pulse parameters are determined by the circuit timing elements. Therefore a latch or FF of any sort without a timing element would not be included in this definition.

Technically the definition may be broader than that, but I believe those definitions are somewhat archaic and now typically used only in an academic setting and not in industry.

absf likes this.
6. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
Like I say, I've read "Any digital circuit employing feedback is called a multivibrator." Maybe it should be "Any digital circuit employing a timing element is called a multivibrator."

What should be the modern definition of a mutivibrator?

This is my attempt "Any circuit that outputs a square or rectangular wave is a multivibrator whether an oscillator or not".

This might include a regular on/off switch though operated by hand. No problem though if the switch is electrically operated by automation (timing arrangement).

Last edited: May 22, 2014
7. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
Let's just look at this: If I wire up a circuit with a manual switch, I can mimic a mutivibrator by manually toggling the switch. But, that cannot be a multivibrator circuit and a definition might need to make that plain.

8. ### MrChips Moderator

Oct 2, 2009
12,432
3,360
Try Wikipedia.

9. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
monostable multivibrators have been around a long time, as relay circuits in some of the first computers, tubes in newer ones, transistors in still newer, and now integrated circuits. used as individual bit storeage, latches, and in counters, shift registers, and more. also make the dial telephone work.

10. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Even if that statement were true, which it is not, that does not mean the converse is true either.

That is not all multivibrators are digital circuits.

alfacliff and others have made various useful statements, since the term is often mixed up or qualified with other terms.

For instance monostable, bistable and astable can be applied to circuits that are not multivibrators, but are often taken as synonymous with them.

The clue is in the two parts of the word.

Multi as in more than one

vibrator as in oscillator.

The combination of more than one oscillator with suitable cross coupling will yield one of the many types of multivibrator.

The oscillators are usually of the non linear, often relaxation types.
It is not true to say that the output from a multivibrator must be a square, rectangular or pulse waveform. That depends upon where the output is taken from.

11. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
The real issue isn't what historical definitions are or have been for the term "multivibrator", it's what the modern definition should be. Specifically - does it include a flip flop/bistable?

Some think it does not because it's simply a latch circuit. But go to Wikipedia article and you will see: "In the bistable mutivibrator..."

12. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
I'm seeking a definition that tells me exactly what a mutivibrator is.

I would have said it's something to do solely with the output of the circuit.

Apparently it need not be an oscillator.

13. ### crutschow Expert

Mar 14, 2008
13,000
3,229
Wikipedia describes three types of multivibrators: Astable, Monostable, and Bistable. I can live with that.

djsfantasi likes this.
14. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
Perhaps we are in a situation where we cannot define a multivibrator, but we know one when we see one. Not very scientific, but that's where we are, I think.

15. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
Is this an accurate definition?:

"An electronic circuit used to implement a variety of simple two-state systems."

Maybe getting there, but perhaps there could be a clearer definition. The above seems a bit vague.

It must, apparently include the flip-flop circuit/non oscillator circuits.

It may have to be a definition that excludes a regular motor control start (latch)-stop (reset) circuit (not sure). And exclude a simple on/off manually-operated switch.

Last edited: May 22, 2014
16. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Can't see this as anything useful.

The lightswitch on my wall is definitely not a multivibrator, but conforms to your definition.

What are you really trying to achieve here?

17. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
Take these two definition quoted:

"Any digital circuit employing feedback is called a multivibrator."

"An electronic circuit used to implement a variety of simple two-state systems...."

Both taken from articles on the internet and both criticized as not explaining what a multivibrator is.

So, I'm challenging anyone who thinks they know, to come up with a definition that accurately defines a mutivibrator.

Seems difficult to come up with a definition without including circuits that you would reject as multivibrators, or one which will include circuits which are not oscillators.

Last edited: May 22, 2014
18. ### MrChips Moderator

Oct 2, 2009
12,432
3,360
Don't attempt to redefine a word that has been in common usage simply to suit your understanding.

Other examples are "latch", "RAM".

A multivibrator is a simple two-state system otherwise known as a flip-flop.
There are three kinds of multivibrators:

1) monostable
2) bistable
3) astable

19. ### richard3194 Thread Starter Member

Oct 18, 2011
65
2
Actually that's quite good because it immediately made me see that it's basically about flip flop. And there are 3 kinds of flip flop as per the above.

I was not perceiving astable and the monostable as a flavour of flip flop for some reason.

Of course a definition should exclude things like the regular motor start/stop circuit or the manual on/off switch.

20. ### MrChips Moderator

Oct 2, 2009
12,432
3,360
Actually, flip-flops fall into a separate category of device which also has its varieties:

1) R-S
2) D-type
3) T-type
4) J-K
5) master-slave