Twin LED Flasher

yep. you could try replacing the capacitors.

 

Try running it off a 6V supply instead of 9V. The two-transistor astable and monostable multivibrators shouldn't really be run from more than about 6~7V because when one collector goes low, the charge in the connected capacitor forces the base of the other transistor negative, and if this negative voltage is more than about 7V the base-emitter junction of the other transistor is forced into zener breakdown and the transistor can be damaged.

If you want to run the circuit at 9V, just insert a diode in series with the base connection to each transistor - a 1N914/4148 is fine, with its cathode to the base. That will stop the base-emitter junctions from being reverse-biased.

When each LED turns OFF, the current through it doesn't fall to zero immediately. Current is needed to charge the capacitor connected to the collector of the driving transistor. This current flows into the base-emitter junction of the opposite transistor (in the forward direction this time). So leakage in the capacitors could cause the problem.

Other possible causes are LEDs that light detectably on extremely low forward currents, and possibly even light from the other LED being picked up and appearing to come from the OFF LED.

You can eliminate current flow in the LEDs by adding a resistor (e.g. 22k) in parallel with each LED, or (if this is more convenient), from the collector of the driving transistor to the positive rail (i.e.in parallel with the series combination of the LED and the 470Ω resistor).

 

Someone else will have to answer those physics questions I'm afraid. I learnt that stuff once but have never used it, and if you don't use it, you lose it!

I can explain the astable multivibrator in terms of voltages and currents (I think in terms of conventional current, not electron flow) if you think that will help. The circuit is very old and I'm sure you could find already written step-by-step descriptions of how it works with a quick Google of keywords like transistor astable multivibrator detailed description.

 

The reverse voltage is only small - around 0.5V. Not going to cause problems.

I'll check out the link.

 

I think you need to only concern yourself with electron holes in relation to semiconductors. Your just going to confuse yourself if you try to relate this to current and charge distribution inside a capacitor. It is far better at this stage to just concentrate on how the components work regarding conventional current flow. I myself don't feel comfortable with the notion of electron holes moving, I could never get to grips with it.
I myself use the notion of spaces or voids where another electron could jump in and jump out again. And this in turn releases a photon which is the energy that the circuit uses. I think it might be useful if we did a resource on this explaining how we all feel this works, many of the guys have a different approach, it could be quite interesting. I know Steve an Bob have a wealth of physics knowledge and it would be priceless getting their view on all this. I too can contribute but to a much lessor degree.

Adam

 

Your just like me I am never happy until I know how all this works. Some engineers are just happy knowing the basics. And dont get me wrong some of the worlds greatest know far more than me and have less physics experience. So it all depends on your take on all this. But you dont have to know the physics to be a great engineer.

Look at Kris, an outstanding engineer and he admits this sort of knowledge is long gone. Kris is proof without question you dont need this amount of insite to be good at what you do. For me its a sad love one that I struggle with on a daily basis. Life to me is a big puzzle with lots of missing parts that I will never find the answer to.

But in my quest I have aquired as some might seem useless information but for me it is fun. This is the same for electronics, which is my first love. Always trying something different which does not always work so really dont get so hung up about how it all works you will learn this in time if you want to.

Adam

 

Yes you do and the problem which is confusing is the conflict of electronics and physics. So there you are thinking you got this transistor stuff all sorted out. Current flows from collector to emitter dependant on applied base emitter voltage (NPN), yep got that. Then the physics tell you it goes the other way, bugger. Well it doesnt matter really. if you want to know it fine but you will get confused if you dont fully understand it. I work in both ways I find it easier for certain things. And then they go and chuck in negative voltages which really screws things up
Adam

 

Thanks for the compliment Adam I only wish it were true!

Well, conventional current is just like electron flow except in the opposite direction, so once you "know" one, you "know" the other. Conventional current fits better with the direction of the arrows in semiconductor symbols - current flows "in the direction of the arrow".

To me, it also makes sense for current to flow from the positive rail at the top of the circuit (nearly always the case), to the negative rail at the bottom of the circuit, and for current to flow through the collector load resistor, into the collector, and out the emitter. But I have thought in terms of conventional current flow ever since I started learning this stuff; maybe someone who had always used electron flow would find it more natural to think the opposite way.

Personally I don't consider the physics of the components I design with. I think of them as black boxes - black boxes whose behaviour I know quite a lot about, but still black boxes defined by that behaviour.

 

This is the start of my resource but I thought it fitting to post as a reply to this thread.

The world of Physics, shall we say can get a bit confusing at times, that’s probably an understatement. There are so many different opinions on what actually happens regarding current, energy, electric fields, magnetic fields and charge within a conductor.

Understanding the ins and outs of this is not necessary needed if only a basic understanding of electronic components, and conductors is needed. In most cases that is all that is needed. Many great designs exist without the designer needing to go into the physics of component operation and what happening in the wires down at the microscopic level.

I mean we can produce very reliable, complex designs without even realising that we are working the wrong way round to what actually happens. But we have crafted the skills over time to allow us to ignore this and still understand how to design circuits that actually operate in reverse to what we think.

What I am talking about is electrical current and it’s direction, when Ben Franklin made the decision on the direction he had a 50-50 chance of getting it right. Well he got it wrong. But by the time we worked all this out it was too late to do much about it so we invented the terms conventional current direction and actual current direction. Strange isn’t it?

We can even work out how to control components like BJTs even by using the incorrect terminology i.e. we say current control and not voltage control. How? Because all the physical properties of components have something in common, and that’s resistance, inductance and capacitance and the mixture of all these called impedance.

Every component has a total impedance it just depends on the percentage concentration of mixture of the three elements which determines what the component is and its uses. This is what makes a capacitor inductor and resistor different. So when we apply a voltage across a resistor for example we will produce a current in that component.

This current will produce a voltage difference between the two terminals of the resistor. So if we increase the applied voltage then the current will increase because of the increase in voltage between the two terminals of the component. So this makes sense because of the linked relationship between current voltage and resistance (Ohms Law) we can increase the voltage and have an increase in current but we can also reduce the resistance and have an increase in current also.

So if we take the BJT example in figure 1 because it is perfect for explaining how incorrect statements can still be used to predict reliably component behaviour.

Figure 1 BJT NPN collector/emitter current control example.
​

Three engineers are asked to modify the circuit in a way as to vary the current in R1. Then asked to explain the operation of what they have done. They are all told they can’t change the applied voltage, that is fixed and also the value of R1 itself is fixed. So they all say well that’s easy we need to adjust the value of P1 and because it’s a variable resistor we just turn it in either direction to vary the resistance. This will increase or decrease the current.

Engineer 1 says “ I confirm that I have reduced the base resistance of P1 to increase the collector current. So I confirm that this resistor change is controlling the collector current”

Engineer 2 says “ I confirm that I have reduced the base resistance of P1 to increase the collector current. So I confirm that the increase in current in P1 as a result from changing the value of P1 is controlling the collector current”

Engineer 3 says “ I confirm that I have reduced the resistance of P1 to increase the collector current. So I confirm that the increase in current in P1 results in a lower voltage drop across P1 as a result from changing the value of P1 which in turn increases the voltage at the base of P1 ”

Now only one of these is technically correct. All three methods used by the engineer is the same, changing the value of P1. But three different explanations.

Engineer 1 is only concerned with the changing value of P1 he is not concerned about the current or voltage. Engineer 2 is only concerned about current and nothing else. Engineer 3 is only concerned about the base voltage and knows that if this changes then the collector current will change.

But the one thing they all have in common is they all knew that by adjusting the value of P1 this would adjust the collector current. This is how we have accumulated so many different reasoning in electronics, but only one can be absolutely correct. But does it really matter as long as the results are the same, no not really. Unless to are teaching someone the correct way all this works then do what ever works for you.

Adam

 

Then the rabbi says, "Oy vey! Dis iss not a very funny joke!"

Seriously Adam I think you need to focus more on presenting information that will be useful to the reader. These may be interesting thoughts, but to write a useful resource, you really need to ask yourself, "what questions are visitors to Electronics Point likely to ask?" and provide answers to those questions. If you look at the other resources you should see what I mean.

 

Yeah valid point Kris, but this is just an example of how people assume the wrong thing and still get the same results. This happens all the time and this is why people don't look for the correct answer because they think they know it. Look at all the recent hoo-hah about BJTs.
But I get your point.
Cheers
Adam