How to Do Electronics Project: Flashing LED Kit

[Hyperspace2]

Hello People .

I am first year electronics engineering student .

Recently I wanted to try hand in electonics projects.

And I am planning to do flashing led electonic kit given over net. I couldn't understand how its's really working. I have no idea. People help me . Advance thanks.


Also do you people know the internet site , where the project is given and working principle is also given.

 

[sophiecentaur]

The circuit is called an astable multivabrator. That means it will keep hopping from one state to the other. (Q1 on, Q2 off) - (Q1 off, Q2 on) and back again.
How it works?
Briefly, when Q1 turns 'on', the base of Q2 will be pulled down so Q2 is off and Q2 collector will go high. But C1 will gradually charge up, through R3 etc.. Eventually, the voltage on the base of Q2 will be high enough to switch Q2 on, which will turn off Q1 as the base is pulled down via C2.
Once Q1 is 'off' and Q2 is 'on', the voltage on Q1's collector will go up. R4 etc. will start to pull the base volts up until it turns on again, at which time, Q2 turns off and so on and so on.
The circuit cannot rest in anyone state.
Note, if you raise the supply volts from zero, very slowly, it is possible to make both Q1 and Q2 turn on at the same time and the circuit will not oscillate. That's a fundamental flaw in the design but it can be avoided by making sure that these conditions don't occur. Alternatively, you can 'give it a kick' by grounding one of the bases and off it will go.
You can do the same sort of thing using 'cross-coupled' logic gates.

 

[Hyperspace2]

Briefly, when Q1 turns 'on', the base of Q2 will be pulled down so Q2 is off ..

Thanks for reply sir
But I don't understand why Q1 turns on ( I guess it's base is alo pull down by C2)
Sorry for my ignorance.

 

[sophiecentaur]

Good questions.
You have to assume that, at switch-on, one transistor happens to turn on first. This is actually very likely because the times for capacitors of slightly different values to charge via resistors of also different values will nearly always be slightly different. Any slight assymetry will ensure that oscillations start up because one will 'get there' first. [Edit There is, initially, positive feedback which will exaggerate any imbalance until one transistor is hard on and the other is off. That situation collapses one the capacitors discharge]

The time for each switch over is governed by the 'time constant' for the Capacitor - Resistor networks. The variable resistors are put where they are, in series with fixed value resistors to provide suitable ranges of R which adjusts the time constants over the required range. It is common to use a fixed value of resistor in series with a variable one to avoid what could happen if the variable resistor were set to zero - a short circuit.

 

[sophiecentaur]

Hyperspace
Did you just edit out one of your questions? I could have sworn you asked about the variable resistors! :)

 

[Hyperspace2]

Hyperspace
Did you just edit out one of your questions? I could have sworn you asked about the variable resistors! :)

Yes you are true. I did it because I wanted to go step by step and learn. That all. So
please tell me about the the question above

 

[sophiecentaur]

So I have answered your next question?
In the last para of my last answer :)