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Transistor Astable Oscillator Help Please

Discussion in 'General Electronics Discussion' started by Enigma, Aug 15, 2012.

  1. Enigma

    Enigma

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    Jul 15, 2012
    Im having trouble getting any form of this type of oscillator working. Even a circuit I put together on 'Multisim'doesnt work and Im completly baffled as to why??
    This circuit>> http://www.mediafire.com/view/?leybj0peqdrbq4b was built using the schematic on this link >> http://www.circuitstune.com/2012/01/astable-multivibrator-2-led-flashing.html .
    As can be seen it is to the exact spec of the schematic but it doesnt oscillate. Instead the led's remain ON.
    Ive gone over it a few times and ive been trying to get one of these things to work since Saturday night but no luck. :confused:
    I feel I am doing something wrong because any circuit ive tried, even on Multisim hasnt worked even if they are exact but I was hoping that maybe someone can point me in a direction where I can put two and two together myself.
    Thanks Guys :)
     
  2. GreenGiant

    GreenGiant

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    5
    Feb 9, 2012
    are the two transistors both the same?
    if they are, from the angle that you took the picture it appears that you have the pins on at least one of them connected incorrectly.

    are the capacitors polarized I cant tell?

    you also have something connected to the +/- rails connected incorrectly, following convention the red rail would be positive, which means that your LEDs are connected correctly, but your transistors are not grounded, and your resistors are grounded when they should be on the +voltage rail
    if you are using blue as positive then the resistors and transistors are connected correctly but the LED's are not
     
  3. MrEE

    MrEE

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    Apr 13, 2012
    Why does this circuit have a 1k in parallel with the led? I would always use a series limiting resistor with an LED. I'd remove the 1k resistors and try a 220 ohm to 470 ohm resistor in series with each led. this allows you a wider range for the supply voltage.

    I also see something fishy with the hookup just as GreenGiant suggested.
     
  4. duke37

    duke37

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    Jan 9, 2011
    Have you got the capacitors and transistors connected the right way?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You need to place the LEDs in series with the resistor going to the collector of each transistor.

    As best that I can make out, you have the LEDs going from the collector to ground.

    In this case, yes, they will always be alight. They will also be pulling the collector down to a low voltage which will make it harder (or impossible) for the device to oscillate.

    If it is oscillating, you might see some variation in the brightness of the LEDs.

    Connect them up correctly and it should work (presuming all your other connections are correct)

    edit: I've just looked at the circuit diagram, and both you and it are wrong (in different ways). As noted in an earlier post, the LEDs are shown in parallel with the collector resistor, they should be in series, and you have them connected in a different manner again. Once you connect the LEDs correctly, you may find that the circuit needs a slightly higher voltage (maybe 6V) to run correctly
     
    Last edited: Aug 15, 2012
  6. Enigma

    Enigma

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    Jul 15, 2012
    Guys!! Thanks to your help I've found the answer!
    After reading your reply's over and over and scratching my head since last night, today at the workshop something occurred to me.
    If you look at this assembly guide on this link >>http://www.play-hookey.com/digital/experiments/rtl_astable.html I originally built the circuit by following this guide before trying the component values on the other link that I copied to my first post. This guide is using the top rails opposite to how I would set up my rails so ground on this guide is the lower rail.. So in effect I had the resistors and grounding ties to the emittter the wrong way around!!
    Hope that makes sense but anyway its working now so I can study the circuit!! What a relief!
    Is there any golden lesson I should be learning from this experience?

    p.s Also, I noticed that the transistors get quite hot at 3.7V..is that normal? Its on 1.9V now and they are cool.
     
  7. Enigma

    Enigma

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    Jul 15, 2012
    GreenGiant you was actually bang on!
    Thanks so much ;)
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    With respect to multisim: This astable multivibrator works only because real components are not ideal. Due to tolerances of the components the circuit develops an asymmetrical behaviour and starts to oscillate.
    You can simulate the circuit by either
    1) introducing an asymmetry (e.g. use slightly different base resistors) or
    2) set the initial conditions for the simulation such that Q1 is definitely off (Vbe=0V) and Q2 is definitely on (Vbe=0.6V). I don't know how to do this in multisim, but it should be possible.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's because there's no resistors in series with the LEDs.

    The LEDs are also quite likely to suffer an early death.

    Try doing as I suggested. Put the LEDs in series with the collector resistors, NOT in parallel with them.
     
  10. duke37

    duke37

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    Jan 9, 2011
    The 1k resistors in the collector circuit do nothing but increase battery drain and heat the transistors.
    Do as *steve* says and put a resisitor in series with the LED, 220 ohm would be about right.
     
  11. Enigma

    Enigma

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    Jul 15, 2012
    Thanks
    Harald: thanks but im not sure how to do that in multisim either?
    Steve: thanks again, I have tried to put the collector resistors in series with the leds but the leds just stayed lit so I put a new pair of 220Ohm resistors in series with the leds leaving the 1k collector resistors in place and it works fine now and the transistors dont heat up at 3.7V anymore..
    This is all great but I dont know WHY this happens so this is what I need to work out now . Also I noticed that if I take a collector resistor and put it in series with an led during oscillation it keeps oscillating but the other led, as it was, turns off and when I then do the same as I did to the first, without turning the power off, they both just stay lit and no oscillation?? Why I dont know but it seems pretty strange but Im new to this so theres obviously a good answer why..
    duke37: I dont know, with my knowldge, why the1k's would increase batt drain and heat the transistors? Could you point me in the right direction maybe so I can learn why this happens please?
    Thanks Again Guys ;)
     
  12. duke37

    duke37

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    Jan 9, 2011
    The multivibrator consists of two amplifiers, crossed coupled with a pair of capacitors. The base current is supplied by the 5k resistors and the collector current would normally be supplied by the 1k resistors.

    In your case, you have placed a LED in the collector circuit which will do the job (with a series resistor) of the 1k resistor. Thus adding another resistor just adds another ~5mA drain on the battery. More current going through the transistor means more heat.

    Your original circuit had no way of defining the collector current and would be very sensitive to the applied voltage. Adding a series resistor means that the current does not vary a wildly as the voltage is varied. There is a thread on how to drive LEDs properly.
     
  13. john monks

    john monks

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    Mar 9, 2012
    Are you really locked into 3.7 volt supply?
    You may have to juggle your 5K resistor up or down a little.
    And you might try a 9 volt supply but before you do make sure you have a resistor is in series with the LEDs.
     
  14. Enigma

    Enigma

    21
    0
    Jul 15, 2012
    Duke37: Thanks, I have rearranged the circuit as you have advised and it works perfectly well. This is it: http://www.mediafire.com/view/?z9288t4aicj4vlt

    I think I need some clarity on this sentance from you: Thus adding another resistor just adds another ~5mA drain on the battery. More current going through the transistor means more heat.
    Does adding more resistors increase the current in a circuit? Wouldnt it decrease the current? Maybe Im a bit lost but Im im not sure?
    Also what did you mean by: Your original circuit had no way of defining the collector current and would be very sensitive to the applied voltage. Adding a series resistor means that the current does not vary as wildly as the voltage is varied. I must admit that Im totally in the dark about that sentance lol but I want to learn what you mean so is there anything you could advise me to study and get this in my head for good please.

    I noticed that when I turn the Voltage down the circuit Oscillates faster.. Is this because the Caps charge to a lower voltage hence a quicker cycle?

    john monks: Thanks, Im just using 3.7V as the circuit I copied was using this Voltage.

    Thank You Guys ;)
     
  15. duke37

    duke37

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    Jan 9, 2011
    If you have a resistor in series with another resistor, the total resistance will be higher and the current will be lower.
    If you have a rsistor in parallel with onother, then extra current will flow through the additional resistor.

    A led needs a certain voltage to turn it on, depending on the type. Say this voltage is 2V, if you have a supply of 1.9V you will get very little current. If you have a supply of 2.1V then you have 0.1V to drop in the circuit resistances. If these are low, the current will be high and the power lost will show itself as heat.
    If you have a power supply of 4V then you need to drop 2V. A resistor of 100 ohm would drop this when passing 20mA which is about what a led needs to be bright.

    The variation in speed is due many different factors including non-perfect components. I have not worked out the effect of the led voltage or the voltage drops in the transistors. This is not going to be a very accurate or stable oscillator. If you want this, you can go to a crystal controlled oscillator, with a divider to give the frequency you want. A simple crystal oscillator can have an accuracy of better than one part per million. Accurate oscillators always use stabilised power supplies and often temperature control.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    There's a number of questions here. Firstly with it not working when the LEDs are in series with the 1k resistors. This is probably because the voltage drop across the LED leaves insufficient voltage for the circuit to work. In addition, because the base resistors are then tied to a higher voltage, it may well force both transistors to be on, hence both leds light up. You may recall that I suggested you might need a higher voltage power supply to make it work like this.

    The second question is why does it work with 220R resistor in series with the LED and a 1k in parallel. Well, this allows the collector to be pulled right up to +3.7V when the transistor is off. This is important (or at least a significant voltage swing) to allow the capacitor to turn off the other transistor.

    Why don't the transistors get hot? Longer, but possibly more important question.

    I presume these are red LEDs. That means they will have about 1.7V across them at almost any current. They are placed in series with a transistor which acts as a switch.

    What do you think happens when you have a 3.7V power supply, and a device that will drop 1.7V switched across the power supply? Well that extra 2V has to go somewhere.

    In normal operation, as the transistor is turned on more and more, the voltage across it falls, so it doesn't dissipate much power (measured by the voltage across it multiplied by the current through it).

    In this case, the more the transistor is turned on, the higher the current that flows through it, but the voltage across it remains almost exactly the same. The power it dissipates gets quite high (and incidentally the LED gets very bright and quite possibly quite warm too).

    When you place a resistor in series with the LED, the voltage across the resistor rises as the current through it rises. So, as the transistor turns on, the voltage across the transistor falls, the voltage across the resistor rises, the current is limited to a reasonable value, and no-one gets hurt (oops, nothing gets warm or hot).

    This is actually a VERY important point because far too many people make this mistake with LEDs, causing problems down the track.

    In the projects section there is a sticky thread about LEDs that goes into more details explaining why this is so and how to get around it. In your case, the series resistor is the right thing.
     
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