Capacitor Not Fully Charging

Discussion in 'Homework Help' started by ISeeLED, Jul 27, 2011.

  1. ISeeLED

    Thread Starter New Member

    Jul 27, 2011
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    Hi,

    I have a problem which I posted on another forum, but it's been a few days and I have had no replies so I thought you guys might be able to help.

    I'm coming back to electronics after a few years and have been going through the book "Make: Electronics". I have made the following circuit:

    [​IMG]

    The gray part at the top works fine, the LED blinks on and off periodically as the capacitor charges up and discharges.

    The black part at the bottom does not though. The buzzer does not sound at all and no current exits the cathode of the PUT. I checked the voltage at the anode and it seems that the capacitor only charges up to 2.5V then stops, but the voltage at the gate is 2.75V, so I believe the anode voltage is not high enough to allow current through the PUT.

    I checked the capacitor in a circuit by itself and it does fully charge, but not in the circuit with the PUT.

    I also checked the circuit with the capacitor from the gray part of the circuit diagram (2.2uF as opposed to 0.0047uF) and the buzzer does sound.

    Can anyone point out any mistakes I have made or tell me why the buzzer does not sound when the smaller capacitor is used?

    Thanks
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Maybe you use wrong type of buzzer.
    Maybe you bought buzzer with generator, but for C=4.7nF you need buzzer without generator (piezo beeper disk).
    But that's only my guess.
     
    Last edited: Jul 27, 2011
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  3. ISeeLED

    Thread Starter New Member

    Jul 27, 2011
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    This is the buzzer I am using: http://www.maplin.co.uk/miniature-buzzer-3213?ordercode=FL39N.

    I was under the impression it doesn't matter what kind of capacitor is used as long as the voltage and current are the correct values for the buzzer, and I don't believe the buzzer is getting any current because the capacitor does not have a high enough voltage to allow current to pass through the PUT (I might be wrong about this).
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well I still think that you need buzzer without generator.
    Try to reduce the value of the 100R resistor. Or even remove it from the circuit.
     
  5. PeeSeeBee

    Member

    Jun 17, 2011
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    7
    The circuit you have built is an audio oscillator, so it should be feeding a speaker, rather than a buzzer.

    The buzzer you have connected to your circuit operates from a DC voltage, but your circuit is trying to switch it on and off at audio frequencies. This is why you are not hearing anything.

    Replace the buzzer with a small loudspeaker & try it again.

    Also the voltage at the anode will be changing too fast for a meter to read. You will only be able to see it charging & discharging by using an oscilloscope.
     
    Last edited: Jul 27, 2011
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  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Where does the current for the buzzer come from? it's supposed to work from 4V to 9V, but not with resistors in series... nor will teh capacitors charge be enough to power the buzzer.

    I think you'd have to trigger with your circuit (in black) another transistor to switch the load (your buzzer).

    Also, the charging rate of the capacitor will give you about 600 Hz. The buzzer already has a generator at 400Hz, doesn't it?

    What exactly want you the buzzer to do? just emit the 400Hz sound or emit the 400Hz sound in intervals of , let's say a second or so?
     
    Last edited: Jul 27, 2011
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  7. ISeeLED

    Thread Starter New Member

    Jul 27, 2011
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    I thought that buzzer and loudspeakers were the same thing, so I think that is why I am confused as to why my circuit is not working.

    Am I right in thinking a buzzer has a predefined frequency and just needs to have the correct voltage and current applied to it to buzz, but a loudspeaker you choose the frequency by changing the period of oscillation of the current applied to it?

    Thanks for all responses.
     
  8. PeeSeeBee

    Member

    Jun 17, 2011
    43
    7
    Yep. That's about it. The Maplin buzzer simply needs a 4v to 9v supply connected to it & it will buzz at 400Hz.

    A loudspeaker needs to be connected to an oscillator circuit.

    In this particular circuit, don't expect the speaker output to fill the Albert Hall. LOL. It's output will be very quiet.

    Also, you can buy the piezo elements on their own, which would also require an oscillator circuit.
     
    Last edited: Jul 27, 2011
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