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### Topic: Multivibrators (Sorry girls, nothing to get excited at here... :) ) (Read 1 time)previous topic - next topic

#### cjdelphi

##### Dec 03, 2012, 10:56 am

Providing the image is able to be seen above..

I've just built that circuit and now i have 2 LED's flashing, I'm still a little confused as to how this circuit works?

1. On applying voltage, at the start, do both caps start charging..
2. Resistor 2 and 4 are both connected to the Minus side of the capacitors yet connected to a positive + rail... huh?   why?

Don't tell me magic

#### Riva

#1
##### Dec 03, 2012, 11:04 amLast Edit: Dec 03, 2012, 11:06 am by Riva Reason: 1
Probably doing a google search for 'Flipflop circuit' will save thousands of words

Edit:
I already did it http://www.talkingelectronics.com/FreeProjects/5-Projects/Page16.html
I might seem like an idiot but remember it takes one to know one.
Forum user generally fall into three categories (the good, the bad & the a**hole) what category do you fit in?

#### cjdelphi

#2
##### Dec 03, 2012, 11:36 am
where do you think i got the picture from?

2. googling is not going answer my questions that i asked

#### KirAsh4

#3
##### Dec 03, 2012, 11:43 am
A transistor doesn't have a "minus" side.  It has a base, collector, and emitter.  Read up on how a transistor works and you'll understand why the resistors are connected the way they are.  And that link that Riva gave will explain in plenty detail how the circuit actually works, how the capacitors work and how they affect the circuit.  Google itself might not answer your question directly, however it will give you plenty of references of where you can find the answer.

#### Grumpy_Mike

#4
##### Dec 03, 2012, 11:43 am
1) a capacitor can only charge up if there is a voltage across it. Initially on switch on both sides of the capacitor are connected to +V so the can not charge.
2) the -ve end of the capacitor can not rise above 0.7V because it is on the base of a transistor, so it will always be more negitave than the other end.

#### cjdelphi

#5
##### Dec 03, 2012, 11:54 am
mike ahh ok that's helped me understand it more

cheers

#### retrolefty

#6
##### Dec 03, 2012, 11:55 amLast Edit: Dec 03, 2012, 12:05 pm by retrolefty Reason: 1
Quote
Don't tell me magic

I was told in the Air Force when we studied this circuit that it indeed was magic.

That is if your question really is: upon initial power up for this circuit which transistor will switch on first, Q1 or Q2? And will that always be the case upon each powering up event? Lets let the peanut gallery chew on that one for awhile.

Possible answers:

A:   Q1 always.
B:   Q2 always.
C:   It's always random if either Q1 or Q2 starts first.
D:   It can be either Q1 or Q2 but it will be the same transistor starting first for each specific circuit built.
E:   Some other magic will decide which transistor starts first.

#### cjdelphi

#7
##### Dec 03, 2012, 11:58 am

A transistor doesn't have a "minus" side.  It has a base, collector, and emitter.  Read up on how a transistor works and you'll understand why the resistors are connected the way they are.  And that link that Riva gave will explain in plenty detail how the circuit actually works, how the capacitors work and how they affect the circuit.  Google itself might not answer your question directly, however it will give you plenty of references of where you can find the answer.

transistors? i know how they work...

re read my question...

#### Grumpy_Mike

#8
##### Dec 03, 2012, 12:07 pm
The key to understanding a multi vibrator is to know that when one end of a capacitor changes the voltage it is at, the other end also changes by the same ammount.
So if you have 5V across a capacitor and you change the positave end to zero the the other end of the capacitor instantly goes to minus 5V.

This is the basis of a lot of circuits like negitave rail generators and voltage multipliers.

#### dhenry

#9
##### Dec 03, 2012, 12:28 pm
Quote
1. On applying voltage, at the start, do both caps start charging..

Both charge up, but at different rate. The one that gets charged up first flips the other transistor first.

Quote
2. Resistor 2 and 4 are both connected to the Minus side of the capacitors yet connected to a positive + rail... huh?   why?

Those capacitors see reverse polarities so the polarity marking is likely to indicate that they are larger electrolytics. They should be non-polarity capacitors.

#### MarkT

#10
##### Dec 03, 2012, 02:13 pmLast Edit: Dec 03, 2012, 02:22 pm by MarkT Reason: 1
If the base resistors are too small both transistors will turn on and stay on - make them 20 times larger in value than the collector
resistors and the circuit seems to start reliably (and will even run at down to about 0.8V supply!  My circuit seems to start up in the
same state every time, for what its worth - I wouldn't rely on this though unless using non-symmetric component values.

The capacitors see about the supply voltage across them for half the cycle, then a reverse Vbe (0.7V or so) for the other
half cycle - most electrolytics will tolerate this small reverse voltage but its not recommended.

The name for all oscillators of this sort is "relaxation oscillator" - there is a sudden change, then an RC (or RL) circuit discharges
or charges (relaxes...) and then suddenly a new flip in state happens.  Positive feedback causes the sudden change in state, which
saturates, then the relaxation phase before the next positive-feedback step can happen.  The classic 555 timer circuit is another
good example of a relaxation oscillator.

[edit:  just for completeness, I'm using 2N3904's, 4k7 on collectors, 100k on bases, 10uF non-polarized electrolytics.
Runs with a period of about 2.0s at 0.8V, 1.6s at 5V and the rapid switches have a 100ns rise time or so.]
[ I won't respond to messages, use the forum please ]

#### cjdelphi

#11
##### Dec 03, 2012, 02:21 pm

Quote
Don't tell me magic

I was told in the Air Force when we studied this circuit that it indeed was magic.

That is if your question really is: upon initial power up for this circuit which transistor will switch on first, Q1 or Q2? And will that always be the case upon each powering up event? Lets let the peanut gallery chew on that one for awhile.

Possible answers:

A:   Q1 always.
B:   Q2 always.
C:   It's always random if either Q1 or Q2 starts first.
D:   It can be either Q1 or Q2 but it will be the same transistor starting first for each specific circuit built.
E:   Some other magic will decide which transistor starts first.

I prefer the first answer given to you were in the air force - magic

#### cjdelphi

#12
##### Dec 03, 2012, 02:30 pm
yup, I'm merely trying to get my head around how it works... making them is simple, but how they work..

but this -5volt swing has got me curious, i need an oscilloscope - thanks all, tomorrow i'll probe this circuit with my multimeter and give this relaxation oscillator / multivibrator /  flip flop /astable osccilator / a good going over lol, i'll increase resistor and cap sizes and try and watch the voltages across the caps and bases of the transistor to see this in action, i built it, it worked i just have to slow it down..

p.s

if i see a yellow canary, it's a canary, if i see an orange canary, i don't start suddenly start calling the canary (even though it's a different colour) something else! (electronic engineers seem to do, 1 name is never enough)

#### Riva

#13
##### Dec 03, 2012, 02:54 pm
Quote
if i see a yellow canary, it's a canary, if i see an orange canary, i don't start suddenly start calling the canary (even though it's a different colour) something else! (electronic engineers seem to do, 1 name is never enough)

Then how would anyone else who never saw the canary know what colour it was  XD

The opposite is true with men/women and colours. To me green is green, red is red & blue is blue in almost all there shades but to the wife there are several different names  :~
I might seem like an idiot but remember it takes one to know one.
Forum user generally fall into three categories (the good, the bad & the a**hole) what category do you fit in?

#### dxw00d

#14
##### Dec 03, 2012, 03:09 pm
Quote
To me green is green, red is red & blue is blue

To me, green might be green, or red, or brown, and so might red. Blue might be blue, or purple, or green. Yellow might be yellow, or green, or orange. I hate resistor colour codes.

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