Problem with astable oscillator

[GeirA]

I'm a beginner in electronics, and build small curcuits on a breadboard to understand the basics.

Now I have built this astable oscillator:

http://www.circuitsgallery.com/2012/03/astable-multivibrator-using-transistors.html

I have replaced the 2 1uF with 10uF since it was the only ones i got.

But - my left led is on for aprox 5 seconds, while the right one is on for about half a second.  I have checked the components are correct, except for the leds - which have no markings.

They should be on for aprox the same time??


Sorry for my bad english!

[KTP]

Do you happen to have an oscilloscope?  The circuit looks reasonable to me, depending on the components you used and if you followed the diagram.

[bilko]

Try swapping the capacitors over and see if it makes any difference. If it does the caps are not the same value, if it doesn't change, try swapping the resistors. Because the circuit is symmetrical, you should be able to find the problem without a meter or scope.

[bradleytron]

Just a suggestion:
To enrich your experience and study of these circuits you may consider downloading the LT spice simulator from Linear Technologies, its free and very user friendly.

[efron]

Hi GeirA,

Make sure your capacitors are OK and that they are in correct polarity (use of electrolytic capacitor).

Make sure your bipolar transistor are correctly installed (it is easy to mislead the pins).

Even if the circuit should work in principle, I would suggest to put the LEDs on the collector side. I don't like too much the LEDs on the emitter because they are put in the base-emitter path for the commutation of transistors (even when the transistor is still not saturated, there is base-emitter current through the LED, but not enough to be seen - this is not proper).

What is your Vcc voltage?
What is your LED characteristics? I mean 3mm or 5mm and colour.

[6502nop]

Like this (efron's suggestion to move LEDs added)...

Read the notes in the PNG, and re-check the values of the resistors. My guess is that one of those 1Meggers isn't a 1Megger, or it got swapped with one of the 1Ks.
If you swap the 1Megs with, say, a couple of 470Ks, then the blink rate should bump up to about 2.5 seconds, and the Yellow-Violet-Yellow bands won't be confused with the Brown-Black-Green/Brown-Black-Red ones (I'm R-G color blind, so I'm used to this happening!).

nop

[efron]

Hi,

It is even better this way (see picture).

Always keep the capacitors connected DIRECTLY to the BJT collectors.

I've also found another bug. In the original circuit (in this post and also in the referenced web page) , the capacitor polarity is inverted ! When using electrolytic capacitors and NPN transistors, the positive lead MUST be on the collector side. Otherwise the behaviour cannot be guaranteed.

This is because, at any time, the collector lead will always be more positive than the base lead. When the transistor quickly commutates to saturation (almost shortcut), the collector lead goes to almost 0V. At that instant, as the voltage difference in the capacitor can not drastically change, the negative lead of the capacitor will go under 0V (this is what makes the other transistor be OFF during the discharging phase of that capacitor).

For this to work properly, keep the + lead of the capacitor straight connected to the transistor collector.

[6502nop]

Oops! My bad. Good catch efron...

Let's try that again.

nop

[6502nop]

{Sigh}
Where do I begin?

1) The circuit isn't mine, it was a modified version of one provided by the OP, GeirA. If you scroll back up, you'll see that GeirA was playing with said circuit, and had a problem/question with it, which we hoped was answered.

2) I provided this new circuit after efron pointed out some problems, to help out GeirA, as folks on this forum are want to do. That's how we roll.

3) Efron then pointed out that I had made a couple mistakes in my modification, since corrected. I may plan on running all my schematics by efron in the future, as he's got the eyes of a hawk.

4) We are now just waiting to hear back from GeirA as to what he found wrong, and how it was corrected. At no point was it asked if there was a better solution than two transistors, two caps, four resistors, and two LEDs (ten parts) - it was only asked what may be going wrong with the circuit provided.

5) Your suggestion isn't what anyone here would call "easier". The parts count is the same (10) - but the 555 will only operate from 3-15V, whereas the transistor circuit will easily run from 1.5-40V using off the shelf components. Both circuits will need to be tweaked for desired function. The footprint/layout will be about the same (two TO-92s can easily fit in a DIP-8). There is no advantage of the 555 over the transistors in this case.

6) GeirA mentions that he is a beginner, and doesn't have many parts stocked (the reason he's using 10uF instead of 1uF). A 555 circuit won't help him one damn bit if he doesn't have any 555s on hand. Also, it won't help GeirA with the R/C time calculations, as the 555 operates differently with respect to timing (1/3-2/3 voltage rails on the tank circuit for the comparators).

Lastly, Electroalek, when it comes to electronics, beginners should take baby steps. GeirA is learning transistors, caps, LEDs, and our friend the R/C time constant - just like he should - before tackling the harder stuff. Your suggestion would be the equivalent of someone posting a question about how a 2-stroke engine works, and someone like you tells him to abandon the 2-stroker and go right on to a Rolls-Royce V12. You're forgetting that there is a lot to learn about that 2-stroke that'll be handy to know when tackling that V12.

Baby steps.

nop