Design Project: Sensitive Audio Detector

AC Electric Circuits

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  • Question 1

    The choice of step-down transformers to use in a project such as this is not arbitrary. We must consider several factors when choosing a particular transformer:

    Turns ratio
    Winding voltage rating(s)
    Isolation (safety) voltage rating
    Expense (transformers can be expensive!)

    A turns ratio between 20:1 and 10:1 seems to work good for a project such as this. I recommend a transformer with a highest high-voltage winding rating and winding-to-winding isolation possible, to provide maximum resistance between the circuit under test (primary) and your headphones (secondary). This is a safety feature - to ensure that you will not receive an electric shock if the detector is accidentally connected to a source of lethal voltage. I also recommend using a recycled transformer (i.e. salvaged from some junk equipment) rather than purchasing a new one.

    Identify some commonly available transformers that fit these criteria.

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  • Question 2

    The series resistances and parallel-connected diodes in this circuit act as a voltage clipping network. Why do you suppose it would be important to limit, or “clip,” the signal voltage before it reaches the rest of the circuitry? The detector will function adequately without the diodes or resistors in place, so why are they there?

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  • Question 3

    Can this detector be used to indicate the presence of direct current (DC), or only AC within the audio frequency range? How about AC signals beyond the audio frequency range?

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  • Question 4

    With the headphones connected to the input of your audio detector circuit, what is the lowest current level of signal you can hear, as measured with a sensitive milliammeter connected in series between your detector and a signal source?

    Compare this against the lowest amount of audible signal current for the headphones directly. Which component of the detector circuit is responsible for this increased sensitivity?

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  • Question 5

    Suppose I wish to listen to the “hum” of ripple voltage from an AC-to-DC power supply using this detector. “Ripple voltage,” in case you don’t know, is a small AC voltage superimposed on a large DC voltage. If I simply connect my detector directly to the power supply’s terminals, I hear a LOUD “click.” If I turn the volume control down until the “click” is tolerable, the hum is too faint to hear. If I turn the volume control up far enough to hear the hum, then the “click” is far too loud for comfort.

    How can I set up the detector so that it only detects the AC portion (the “ripple” voltage) of the power supply’s output, and not the DC portion?

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