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single transistor oscillator

Discussion in 'Electronic Design' started by Vivek N, Oct 24, 2013.

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  1. Vivek N

    Vivek N Guest

    I tried building a single transistor LED flasher like this :

    I didn't have the recommended 2n2222 - I tried with a BC548, 2n2369 and 2n3055 - None of them work.

    I'm using a trimpot set to 2 kiloohms and a 470 microfarad capacitor. Instead of a 100 ohm resistor I put 3 superbright LEDs in series.

    I get 1.2 volts across the LED and 8.6 volts across the capacitor. My supply is exactly 12.8 volts

    Does this required negative resistance effect only work with specific transistors?
  2. mike

    mike Guest

    Did you try to build the circuit as designed???
    You can't take any diagram and substitute EVERYTHING
    and expect it to work.
    Make it work with ONE led and the proper values first.
    You don't have enough voltage.
    Add up 3x the led forward voltage plus the avalanche break down of
    the transistor.

    If you wanna build one for fun, have fun.
    This is an un-repeatable design.

    The answer to your question is YES.
    The effect varies considerably between transistors and
    among instances of the same type.

    The transistors on your list have such loose specs that you
    can sweep the floor at the plant making any transistor
    and they'll meet that spec. The one you happen to have may
    be only loosely related to any other of that type. The effect
    you need is not specified and can be anything.

    And there exists a simpler circuit that's guaranteed to work.
  3. Phil Allison

    Phil Allison Guest

    "Vivek N"

    ** From the article:

    " The 2N2222 NPN transistor seems to work reliably in this circuit. Other
    transistors may be more temperamental, and others might not work at all. "

    ** Well, that will sure as hell stop it working.

    ..... Phil
  4. The property of the transistors that this oscillator depends on is well
    known and described too but as its use is way outside the scope of the
    specifications of any known transistor you can never design a reliable
    circuit with it.

    Though as you obviously has no knowledge of the electronics involved, you'd
    better start with a circuit as near as possible to the published design. As
    you have no 2N2222 available, another general purpose smal signal may do.
    Suppose this way you will find a transistor that works for you. Only then
    you can try to change LEDs and find out that three superbright ones in
    series does not work and never will.

    If you really want a one transistor oscillator you'd better go for a 2N2646.
    That's a uni junction transistor and you can find numerous examples using
    google. As I have no datasheet of it inside my head you'll have to find it
    yourself to find out how many super brights you can use in series with it on
    a 12V power supply.

    As there are single (NPN or PNP) transistor oscillator circuits FAIK there's
    no one really suitable for blinking a LED. A simple two transistor bistable
    is much better (and cheaper).

    petrus bitbyter
  5. Guest

    You know I may have put off some people asking how to do something wrongly, I said I don't care as long as it works. That had to do with a crossover cable and internet sharing, not destroying silicon.

    I would say that as designed, that circuit might work half the time. Secondly, it is definitely not good for the transistor. It will likely fail soon. Of course modern manufacturers don't care, but I would never build something like that.
  6. Oops. Bistable should be astable.

    petrus bitbyter
  7. Massoud

    Massoud Guest

    Look closer, the emitter is back biased ( which is a zener by nature,
    wide hysteresis and about 5 volts) and collector is forward biased.
    Reversed of normal use. You have a rc ramp voltage and as you see the
    flash, it is the discharging of the capacitor. Although cap voltage
    doesn't go to zero.


    --- news:// - complaints: ---
  8. Jasen Betts

    Jasen Betts Guest

    something like it it should work with most transistors.

    having some BC548 on hand I built it on solderless breadboard with a
    red LED 100 ohm and 3.9K resistors and a used 470uF capacitor, it ran
    at about 4Hz.

    according to my oscilloscope the negative resistance zone of my BC548
    is only 100mV wide. it starts at about about 8.2V so you need to have
    8.2V across the transistor which with a 12V supply leaves 3.8v for the
    the input resistor, the LED and the LED series resistor

    three LEDs in series is going to blow that budget, two will be pushing
    it, so find a 100 ohm resistor

    your BC548 may be different to mine. but about 12V seems like a good
    starting point for a supply, if it doesn't light the led increase the
    voltage until it starts.

    I'm seeing some wierds harmonic oscillations with this circuit,
    but I can't be sure if it's caused by my powersupply, mains pickup,
    or something else, all my stored 12V lead-acid batteries are dead,
    and I don't feel like taping 8 AAs together.

    I'll score some used, but still functional UPS batteries on tuesday,
    and maybe do some more experiments.
  9. Guest

    No- you can make it work with any transistor. The circuit should work with the BC548. The problem is your 2K resistor is allowing too much current through the transistor in steady state for it to shut off. With a 12.8V supplyand 8.6V across the capacitor, that makes for 12.8-8.6=4.2V across the 2kR for 4.2/2k= 2.1mA, which is too much. As a first cut, I would replace the 2k with a 20kR. Dunno anything about your superbrights so not going to speculate, but putting a 100R in series with them can't hurt. This should make it flash oscillate.
  10. Guest

    Load line of the collector circuit doesn't intersect the Ie/Vec curve below Ie breakdown sustaining current...
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