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LED VU meter with transistors, dotted...

Discussion in 'LEDs and Optoelectronics' started by blixt139, Jan 16, 2013.

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  1. blixt139

    blixt139

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    Jan 16, 2013
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    You can also use the LM3915.

    That transistor-based circuit has a linear response, like the LM3914, and unlike the LM3915 and LM3916, which have a logarithmic response and are designed for audio level monitoring.

    It's possible to change that design to a dot graph but it is not well designed and it will never give the correct response. I recommend using an LM3915 or LM3916. You may be able to order them from Digikey or Mouser or Newark or even through eBay, if you can't get them locally.
     
  3. blixt139

    blixt139

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    Jan 16, 2013
    Just figured out an alternative way to achieve similar effect. I have a little mickey mouse chinese multimeter with a pointer, which can detect the small changes in voltage in the smallest 10 volts range. Full volume peaks of my source audio signal are about 0.5 V, what kind of amplifier circuit should I use in order to make it 10 V for the multimeter pointer to be able to move in full range?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You can use an op-amp-based inverting or non-inverting amplifier.

    Does the multimeter measure DC or AC? The difference is important.

    If it measures DC, you will need an envelope follower to convert your AC signal into a corresponding DC voltage. That DC voltage can then be amplified by an op-amp to give a 0~10V DC output for your multimeter. The op-amp will need a supply voltage of at least 12VDC.

    If the multimeter measures AC you will need to amplify your signal so that at maximum volume, the AC output voltage is 10V RMS. This will require power rails of +/- 16V at least. You can use an op-amp in inverting or non-inverting amplifier configuration to boost the signal voltage.

    Both of these alternatives will not give you a proper display because they will have a linear response to the signal voltage. The ear has a logarithmic response, and the LM3915 and LM3916 simulate this response to give an indication that shows the apparent volume. A circuit that responds linearly to the signal amplitude will indicate changes in volume but not in a way that follows the perceived volume.

    If you want to continue with that approach, Google the keywords I've given you here. There are suitable circuits around.
     
  5. blixt139

    blixt139

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    Jan 16, 2013
    Thanks for the answer, I think that the LM3916 approach would work best for me.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Good choice.

    Good luck!
     
  7. blixt139

    blixt139

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    Jan 16, 2013
    I``ve made the thingy using this schematic:

    http://imageshack.us/photo/my-images/189/lm3916schematics.jpg/

    However, I still get effect similar to bar mode even when I`m not using pin 9. I use 2.2 mf electrolytic cap and 1.4kOhms resistor. Each LED light of the picture has three more connected to it in series for the pointer effect.
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Right. That's because you're feeding your audio signal directly into the LM3916. The LM3916 responds constantly to the instantaneous voltage at its input.

    An audio signal consists of a complex series of "ups and downs", with the signal going above and below the 0V reference line hundreds or thousands of times per second, according to the frequencies in it. The volume of the sound is related to the height of the positive and negative excursions of that signal.

    If you feed a signal like that into the LM3916 it will respond continuously as the instantaneous voltage goes up and down, hundreds or thousands of times per second. So with the LM3916 configured as a dot graph, you will get a bar graph effect, as the dot whizzes around along with the instantaneous voltage of the audio signal. (The LM3916 will ignore the negative half of the waveform.)

    You need to convert the AC signal into an "envelope" signal that represents its amplitude or volume, using a circuit called an envelope follower. See http://en.wikipedia.org/wiki/Envelope_detector.

    For this application, an envelope follower is best made using a precision rectifier feeding a low-pass filter. This can be made with a few op-amps and some passive components. It sits between the circuit's audio input and the LM3916's input connection. I'm still looking for a ready-made design on the web. I'll update this post when I find one.
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I've had a good look around and the best example of an envelope follower for your application is actually in the LM3916 data sheet! It's in Figure 10 on page 14.

    Unfortunately that circuit needs a negative supply voltage for the negative supply pins of the op-amps. It would be possible to convert it to a single-supply design but this would require an extra voltage reference. It's probably best to generate a negative supply voltage.

    This can be done with an ICL7660 (Intersil or Maxim) but these can easily be damaged by excess supply voltage. Intersil make a version that will accept up to 12V but you need to be sure that your supply will never go above 12V. You could use a few diodes in series to drop the supply voltage down to a safer voltage for the ICL7660 (e.g. five 1N914 diodes in series will drop about 3.2V so your 12V rail would drop to 8.8V).
     
  10. blixt139

    blixt139

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    Jan 16, 2013
    Thanks for the response!
     
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