Hi

I have a circuit diagram consisting of LEDs, capacitors and transistors as shown in the image below:

I built this and the LEDs successfully flash between each other. I am struggling to understand what exactly is going on here though... I understand the basics of transistors and capacitors, but don't understand what is happening here to cause the flashing.
Is anyone able to explain this in a simple way?
Also, based on this, is it possible to make a version of this with a single transistor and capacitor that flashes just one LED (if I omit one LED and connect a capacitor to 5v through a 220R resistor, this works, but surely I would be able to remove a capacitor and a transistor too)?
Thanks

 

hi K1,
Check this link out, come back with any questions.
http://www.learnabout-electronics.org/Oscillators/osc41.php

E

 

That link is very confusing - I'm not sure where to begin... Is there a variant of the above that can be built with a single transistor, capacitor and resistors that works on 5V? Starting simpler would be best for me!

There are a few circuits that do incorporate the above, however, they all require a 12V supply. I only have 5V available.

Thanks

 

hi,
There is this type of single transistor.
http://www.learningaboutelectronics.com/Articles/Relaxation-oscillator-circuit-with-a-transistor.php

 

Thanks; I've seen this one, but it only works at 12V. Why doesn't this work at 5V when the circuit shown in my first post does? (capacitor and transistor substituted)

 

It works by using the transistors Base Emitter reverse breakdown voltage.
The breakdown voltage needs to be greater than 5v for it to breakdown.

 

hi,
Look thru these images for 5v Astables.
https://www.google.co.uk/search?q=5...W_m_PYAhVRZlAKHcHmCcwQsAQIJg&biw=1176&bih=610

 

It works by using the transistors Base Emitter reverse breakdown voltage.
The breakdown voltage needs to be greater than 5v for it to breakdown.

Does this apply to the PN2222? I just built the circuit on that page and gave it 12V, however, the LED stays on constantly... I used a 220R resistor in series with the 100uF capacitor...

 

Applies to most transistors.

Look at post #7 link.

 

Thanks for that - that's what I have built initially, but couldn't understand how it worked. It doesn't look like there are any simple explanations of how this operates anywhere!

With regards to post #4, the idea makes sense, although the circuit doesn't work when built - the LED stays on (I replaced the transistor in their diagram with a PN2222 and the capacitor with a 100uF one...)

 

OK,
Look thru these videos'
https://www.google.co.uk/search?q=a...x-b&gfe_rd=cr&dcr=0&ei=Pd1pWu70Aa3L8gef_I6ABQ

 

Thanks for that and your assistance throughout. I will check these out.

In general, this seems to be a poorly explained topic - a guide that expresses these concepts simply would be great!

Cheers

 

If you stuck, come back, I we can go step by step.
There are loads of examples on the web you could learn from.
E

 

Look at the original two-transistor circuit and you will notice that it is symmetrical. So we have a see-saw type circuit that is called an astable oscillator. When one transistor is ON, the other transistor is OFF.

There is another way of drawing that circuit which might help you to understand what is going on.


The circuit consists of two identical common emitter transistor stages cascaded. Each common emitter stage is an inverter, i.e. the output at Q1 collector is 180° out of phase from the input at the base of Q1. The output from the first stage is coupled to the second stage via a coupling capacitor C1. Since the second stage is also an inverter, the output of the second stage Q2 is now back in phase with the signal at the base of the first transistor Q1. The output of Q2 is connected back to the base of Q1 via coupling capacitor C2. This creates a positive feedback loop.

With positive feedback in an amplifier, the output would hit either one of the power rails and stay there. Now, if we introduce a time delay in the feedback loop, the two transistors will flip-flop back and forth, hence creating an astable circuit. The time delay is created by the C1-R3 and C2-R1 coupling between the two stages.

Hope this helps.

 

Thanks for that MrChips. Set out like that makes it easier to "read".

I understand how this circuit works (yours without the capacitors):



However, I can't see how adding those would make each one flash on/off. I thought as the capacitors charged, the current they passed dropped off to zero...

 

As shown, both transistors would be conducting and both LEDs would be lit.
This is where the capacitors come into play. On first POWER ON, both capacitors are discharged and will start charging through its pull up resistor R1 or R3. Now, no two components are identical and hence one capacitor will charge faster than the other. Hence one transistor (let's assume Q1) is going to start conducting before the other. The slow transistor circuit Q2 will be pushed harder into the OFF state while the first transistor Q1 is turned ON.

Now C1 starts to charge up and switches Q2 ON. This pulls the base of Q1 to a low voltage and turns Q1 OFF.

The cycle repeats indefinitely.

The answer you are seeking is, there are very few one-transistor circuits that will self oscillate. These are:

1. Relaxation oscillator
2. Uni-junction transistor
3. Circuit with feedback such as Joule Thief or RF oscillator circuits.

 

A long time ago I wrote this post about this astable generator.
https://forum.allaboutcircuits.com/threads/astable-multivibrator-how-does-it-work.32164/#post-200183

Additional these posts should also help
https://forum.allaboutcircuits.com/...-circuit-how-does-it-work.122103/#post-979641
https://forum.allaboutcircuits.com/...jt-astable-multivibrators.115553/#post-902316
They try to describe how the capacitor behaves in this type of circuits.

 

A long time ago I wrote this post about this astable generator.
https://forum.allaboutcircuits.com/threads/astable-multivibrator-how-does-it-work.32164/#post-200183

Additional these posts should also help
https://forum.allaboutcircuits.com/...-circuit-how-does-it-work.122103/#post-979641
https://forum.allaboutcircuits.com/...jt-astable-multivibrators.115553/#post-902316
They try to describe how the capacitor behaves in this type of circuits.

Thanks for sharing this - I missed this initially! I will have a read through this before posting back.

 

Does this apply to the PN2222? I just built the circuit on that page and gave it 12V, however, the LED stays on constantly... I used a 220R resistor in series with the 100uF capacitor...

Both the resistor and the capacitor are way too small. Your time constant is R x C = 220 x .0001 = 0.022, or about 45 Hz. Your LED might be flashing too quickly to see.

In the original circuit the time constant is 1000 x .0033 = 3.3, or one flash every 3 seconds.

ak

 

See