Another LM3915 question!

Discussion in 'General Electronics Chat' started by gface83, Jul 16, 2016.

  1. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hi Guys first time posting a question here. Hopefully someone can give me some insight on this issue.
    I have recently made a LM386 amp for guitar everything is working fine with that, but what I wanted to do is add an LM3915 VU Meter after amplification.
    The problem is I cannot find anything on the net that really answers my question. So here goes.
    I have built the LM3915 meter and it is all working great (except the potentiometer doesn't seem to do anything, but that's another issue). I have tested it out with a guitar connected and an iphone (to play music) and the meter is doing exactly as expected. The problem is the LM3915 only seems to have an input stage, meaning signal goes in, but where do I connect an output stage i.e. a speaker, so I can hear the music or guitar. Does pin 5 act like an input and an output, in which case shall I just connect an output jack to pin 5 and ground as well as having the input connected to pin 5. I hope this make sense to someone. The way I imagine it is: Sound Source - Amp - VU Meter - Output.

    Here is the link of the amp I made (with some changes).
    and here is the LM3915 schematic that I used.

    Just some extra info: I have read the datasheet for the LM3915 and I cannot get the example in the datasheet to work, I have no idea why it doesn't work. The link above is the only version I have been able to get to work.

    Hopefully someone can help me understand this.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!

    The LM3915 can't drive a speaker. Post a schematic if you want suggestions.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    @dl324:
    The TS want is LM386 → LM3915
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    This is the info I could find on the circuit you are using:
    Scheme
    http://i.imgur.com/L2M3XMn.jpg?1
    lm3914
    R1=1kohm
    POT=10kohm
    12v Dc power supply or 9v battery (max 18v)

    Did you connect the wiper of the potentiometer to one of the ends as shown in the schematic?

    Bertus
     
  5. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hi yes, I have connected the wiper (middle pin) to one of the outter pins. When i run a 1K test tone through it weird things happen. At the lowest setting the top 2 LED's 9 & 10) don't light up, but as soon as I touch the pot the 9th LED will light and then go out when I leave go. As I turn it a little bit then both 9 and 10 light up but go out when I leave go. Then from about half way all LED's light up, but the top one goes out when I leave go. So it is working to some degree but it only seems to be affecting the top 2 LED's.
     
  6. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hi
    I am aware that it won't drive a speaker, that's what I built the amp for. I wanted to have the VU meter as a signal indicator before it goes out to the speaker, I you get what I mean.

    This is the schematic that I used from youtube link above:

    upload_2016-7-16_14-44-59.jpeg
     
  7. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hi there
    I'm sorry but what is TS?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you also use a breadboard for the circuit?
    In that case it could be a bac connection, as you say it works a bit when you touch the pot and it does not work when you let it go.

    A TS is the topic starter, the person who started the thread.

    Bertus
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    As shown in every schematic in the 3915 datasheet, the only "outputs" from a 3915 are LEDs or some other visual indicator. It is *not* an audio amplifier, only an audio monitor. If you want it to indicate the strength of the input signal regardless of the volume setting, it connects to the audio signal before the volume control. If you want it to indicate the strength of the signal coming from the speaker, it can connect to the output of the volume control or the output of the 386. These are very different signal levels, so the gain of the 3915 indicator circuit will have to be adjusted to fit. All of this is explained in the datasheet.

    ak
     
  10. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Yes I have used a breadboard for the circuit, I suppose it could be a bad connection but the board is pretty new, Ill try moving to a different part of the board. Thanks for the heads up on the technical jargon :)
     
  11. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hi AK
    Yes I want it to indicate signal strength after amplification. My amp doesn't actually have a speaker connected but a TS socket instead, so I can change different outputs i.e. headphones and the option to change speakers, so my plan was to connect it before the output jack before, but if I connect the output of the amp to the VU meter then where does the out jack connect to? Do I just splice it inline with the output jack? kind of like a multimeter.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you have an other potentiometer available?
    The influence of the 10K potentiometer is much less on the LM3915 than on the LM3914, as the internal resistor stack of the LM3915 is about 23K, the LM3914 has a resistor stack of 10K.
    A potentiometer of 100K will have much more influence on the circuit.

    LM3915_internal_schematic.png

    Bertus
     
  13. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hmmm no, I haven't got one to hand. I'm going up to my local electronic store tomorrow to pick some things up, I will put an 100K pot on the list. I will report back tomorrow and let you know if that makes a difference. Thanks Bertus :)
     
  14. dl324

    Distinguished Member

    Mar 30, 2015
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    From your original post, it sounded like you were looking for an output on the LM3915 that would drive the speaker:
    To which I replied it can't.

    This isn't what I got from the original post.

    What you need to do is drive both the LM3915 and speaker from the LM386. How you do that depends on what you're doing to the amplified output before connecting to the LM3915. That's why I asked for a schematic.
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
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    What I thought he was asking for was LM386 → LM3915 → speaker.
    What he really wants is LM386 driving the LM3915 and speaker.
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Yes.
     
  17. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Ah yeah sorry It does sound like thats what I wanted. Ill try wording properly next time. Yes thats what I want to drive both the LM3915 and the speaker from the LM386. I am not going do anything with the signal after amplification, just get a reading on the VU Meter and then output the signal to a TS socket. So what I meant by the original post was where on the LM3915 do i connect my output socket? Hopefully you get what I mean lol
     
  18. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Yes that is what I want, I didn't mean to cause all this confusion. What im having trouble understanding is: if I was to buy a VUmeter I imagine I would connect my souncard to the VUmeter as an input to monitor the signal, then it would have to come out of the VUmeter and connect to my speakers, so I could visually see the signal and also hear the music. This is what Im havi g trouble getting to grips with. The LM3915 will take my input signal but theres no where to connect a speaker to actually hear the music. It seems you can't have both visual and audible signal.
    So just to clarify, I want to interupt the output signal from the LM386 (before the output jack), send it through the VUmeter, get a reading and then carry the signal on to the output jack. Hopefully that makes sense.
     
  19. gface83

    Thread Starter New Member

    Jul 16, 2016
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    Hi Bertus

    I bought a 100k pot and just tried it out, it works much better than the 10k, none of that lighting up when i touch it, however, it only affects the top 5 LED's now instead of just the top 2. When the pot is turned all the way off (anti-clockwise), the first 5 LED's remain lit. Any idea what is going on here?
     
  20. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It looks like you are having trouble with a DC offset on the input of the LM3915.
    Could you try a capacitor in series with the input of the LM3915 to block the DC?

    Bertus
     
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