AC multiplying and converting

Discussion in 'General Electronics Chat' started by Rallen, Oct 22, 2013.

  1. Rallen

    Thread Starter Member

    Mar 1, 2009
    10
    1
    Hey there.

    I am working on a project, where I need an Arduino to read the voltage over a speaker in a television.

    I have realized that the voltage is both AC and very low (roughly 0.3-1.3V).

    In order to get the Arduino to read this voltage, I would like to both multiply it around 3-4 times, and convert it to DC.

    What is the best way to approach this?

    I have found this circuit, which I think could do the trick:

    [​IMG]

    Would this be the right approach? Which kind of diodes and capacitors would I use?

    Thanks already! :)
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,802
    832
    Why do you need the voltage? Do you need the instantaneous voltage or the audio envelope?

    I've done this with a Schottky diode to convert the AC signal to DC half wave, fed it into an op amp to amplify the signal to the range which could be read by the microprocessor in use, followed by an RC envelope follower.

    I only needed the gross ranges and this worked well enough for me. But without more details on what you are trying to do, I can't tell if this approach is appropriate.
     
  3. Rallen

    Thread Starter Member

    Mar 1, 2009
    10
    1
    Well. I guess some more explaining is needed! :)

    I am working on a project for university, where our product is a chain of discarded consumer eletronics, which is supposed to "speak" to each other in a sort of chain connection. For that purpose, I need to get a signal from the television, which changes when different things are being played. To get this randomish signal, we have connected two wires to the speaker output, which is supposed to feed the arduino with a signal that it can afterwards work on, and use to trigger the next thing in the line.

    Does this make sense?
     
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  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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  5. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Each diode in you circuit requires over 0.6V to conduct so it won't work at all with peak signals less than that. One way to rectifiy a low level AC signal is to use an op amp in a precision rectifier circuit. They can be configured as either half-wave or full-wave rectifiers. The circuit feedback essentially eliminates the effect from the forward voltage drop of the diode(s).
     
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  6. Rallen

    Thread Starter Member

    Mar 1, 2009
    10
    1
    Thanks for both your replies! :)

    Would something like this do the trick then?

    [​IMG]

    Pacrat -> Im not sure why you are apologizing? I dont think I have seen any posts from you that was out of line :)
     
  7. crutschow

    Expert

    Mar 14, 2008
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    3,229
    Yes, that is a full-wave precision rectifier. You will need plus and minus power supplies for that circuit. If you only have one power supply there are single supply circuits, using rail-rail type op amps such as this, that will work.
     
  8. Rallen

    Thread Starter Member

    Mar 1, 2009
    10
    1
    Great. Ty!!

    I think I'll go for the one I linked myself, since those components seems to be cheaper. One thing im concerned about, though - the Tl072 is specified to have a Supply Voltage Range: 7V to 36V. Aint that way too high for my project?

    I have found those two components laying around - 386n4 and LM358. Can I use those for anything?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You must always operate a device within it's ratings, whatever they are. So if you want to operate at a 5V supply then you need to use a different divice.

    The LM358 is a single supply op amp that operates down to a supply voltage of 3V and should work in the circuit I referenced. But change all resistors to about 10kΩ to minimize offsets.
     
  10. Rallen

    Thread Starter Member

    Mar 1, 2009
    10
    1

    Thanks again!! Just to make entirely sure, I will hook everything up like this, and connect the speaker as painted?

    [​IMG]

    I didn't quite get what you meant that the circuit wouldnt need both a minus and plus supply. I guess i need to have common ground no matter what I do, right? :)

    And 5V for the lm358 op-amp, right? Does the diodes have to be of a special type, or will any rectifier diode do the trick?

    Sorry for all my stupid questions. And thanks!!
     
    Last edited: Oct 24, 2013
  11. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Your questions are perfectly fine.

    In that circuit none of the signals at the op amp input or output terminals go below zero volts so you can use a single-supply op amp (one whose input and output goes to zero volts).

    Even though the input signal goes below zero volts the U1 op amp summing junction (- input) never does due to negative feedback. And the positive (+) input of U1 is prevented from going below zero by U2.

    Yes, you must have a common ground.

    5V is fine to power the op amp as long as the input never goes above about 3V peak.

    Any small signal diode will work for D1.
     
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