Multivibrator amplification

Discussion in 'The Projects Forum' started by giovanni64, Aug 24, 2014.

  1. giovanni64

    Thread Starter New Member

    Aug 24, 2014
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    Hi,

    I would like to amplify a complementary symmetrical multivibrator to receive about 10 watt with a voltage between 12 to 14 volts. However, I don't know how to do it.
    I thought to replace transistors T3 and T4 with Darlingtons. Do the values of resistors R10 and R11 have to be changed?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Me either. I have no idea what you are asking.

    What is your amplifier's power supply, its input, and what is your desired output?
     
  3. giovanni64

    Thread Starter New Member

    Aug 24, 2014
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    The unit is a power multivibrator that oscillates at a frequency above the human hearing range - but clearly audible to the canine ear. The correct frequency can be set with P1.
    R7, R9, C2 and D1 are required to start up the oscillator by passing a short current pulse into the base of T1. The only disadvantage of this type of circuit is that power must be applied instantaneously. It will not work if the supply voltage rises slowly to its final value. However, if the unit is connected to a car battery, depressing S1 will reliably start the multivib. The minimum loudspeaker impedance is 4 Ohms. On a 12...14 V supply the unit will deliver 5 W into this impedance. Alternatively, on a 40 V supply it will deliver 25 W into 8 Ohm.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I would replace the whole circuit with a 555 (to reliably generate the desired frequency under all battery conditions) and a common 12v >10W amplifier IC, preferably a class-D type that will give great battery life.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Excellent advice. Both circuits can be purchased pre-built inexpensively, on e-bay for instance. You just have to be sure of the upper frequency range for the amplifier, to be sure it can handle doggy hearing.
     
  6. giovanni64

    Thread Starter New Member

    Aug 24, 2014
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    Thanks for your advice THE_RB but would it be possible to show me how to do this circuit with the 555 and a 10W amplifier. I'm not that good in electronics.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Search for "astable multivibrator" circuits using the 555. You'll find endless examples as well as calculators to help you choose your timing components. (I think you'll want a timing capacitor in the the 0.01µF range.) There are good 555 references in this site alone.

    You can feed the output of the 555 to the amplifier module thru a coupling capacitor just as if it was an audio signal. I believe for the high frequency you want, another 0.01µF capacitor would be fine for that task. An inline inductor could also be used to round the wave shape. We can talk about that when you get farther along.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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  9. giovanni64

    Thread Starter New Member

    Aug 24, 2014
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    Thanks a lot so far wayneh and THE_RB but I'm still looking around to find the right thing. I would like to have an adjustable frequency at least between 20 and 30kHz.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    The 555 is very capable of providing those frequencies. Another choice is an op-amp square-wave generator, but you would need a "fancy" op-amp to accommodate those frequencies. Most generic op-amps would be too slow.

    There are apps for smart phones that can generate waves of various shapes (square, sawtooth, sine) throughout the audible range. I don't know if any smartphone can put out 30kHz, but it would not surprise me. My point is, smartphones make very handy signal generators if you already have one in your pocket anyway.
     
  11. giovanni64

    Thread Starter New Member

    Aug 24, 2014
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    Now I've found a 555 calculator. The frequency changes between 20 and 31kHz when Resistor R2 is changed between 2 Kohm and 0.8 Kohm. R1 is 3 Kohm, R2 - 2 Kohm and C is 0.01uF.
    http://www.circuitsgallery.com/2012/10/555-astable-calculator.html
    I would like to change the frequency with a potentiometer.
    However, I don't know how and where to feed the 555 with the class-D amplifier.
    The piezo speaker I'm going to use has the following data as shown on the image.
     
    Last edited: Aug 29, 2014
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