VU meter Voltage level detection

Discussion in 'The Projects Forum' started by stryker23, Apr 25, 2009.

  1. stryker23

    Thread Starter Member

    Apr 15, 2009
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    I'm working on building a VU meter that will detect voltages from 0-5 volts. If the voltage exceeds 5 volts it will trigger a 12 volt relay. I found a circuit diagram using a lm3915 ic. The audio input will be from a small microphone. Can someone tell me how I can set it up so that it detects the 0-5 volts and lights the leds in 0.5volt increments? Also, how can I make it trigger the 12 volt relay when it exceeds 5 volts? Thanks
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    I just wanted you to know that your post has been read. It's late and I need some sleep. I will look at this in the morning. ;)
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The LM3915 is a logarithmic function LED driver. You can't use it as a linear LED driver.

    You want an LM3914 for evenly spaced voltage increments per division.

    However, audio is better displayed using the LM3915 or LM3916 IC's.

    Use 980 Ohms for R1 and 2.77k for R2; that should give you a Vref of 5v and an LED current of 15mA.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    After you change your chip, as Sgt. Wookie suggested, you can use this as a relay driver and latch. J1 will be connected to the output pin that goes low when the input >=5V.
     
    Last edited: Apr 26, 2009
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Did you want the relay to simply trigger, or stay latched?

    Pin 10 of the LM391x is where you would connect CDRIVE's circuit.

    This is very similar to CDRIVE's circuit, but just requires a SPST N.O. relay:
    [​IMG]

    Note that the base resistor isn't really needed, because the LM391x series IC's will limit the sink current according to the values selected for R1 & R2. The 980 Ohm and 2.77k Ohm resistors will limit the 2N2907's base current to 15mA, which (depending on your relay) may be enough current to energize it (roughly 300mA).

    Posting your relay's specs would help.

    S1 is a normally closed pushbutton. If you don't want the relay to latch, eliminate S1.
     
    Last edited: Apr 26, 2009
  6. stryker23

    Thread Starter Member

    Apr 15, 2009
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    Thanks for the replies. The relay circuit is very helpful as I didn't have much of an idea of how to set it up. I think as for the chip, I just misunderstood how it works. I want the input to be from a microphone, but I assumed the audio could still be related to voltage. Is the lm3914 purely for voltage inputs (no microphone),and the lm3915 for audio so the leds light up according to decibels?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The audio level IS related to voltage. However, we have no clue as to what your microphone's specifications are, or even what type it is.

    Audioguru is a senior member here who designed/built a line-level meter using a couple of LM3916's and some precision single-supply opamps. If I remember correctly, he had the LM3916's connected in series for a total range of 60dB in 3dB increments.

    The LM3914 and LM3915 are suitable for cascading, the LM3916 really isn't due to it's internal divisions; -20, -10, -7, -5, -3, -1, 0, +1, +2, +3. For audio, you really want to use either the LM3915 or LM3916, as people's hearing is logarithmic.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Two points here. As you can see I put the relay coil in the emitter circuit. I did that because I didn't have enough coffee in me and the wife wanted me to do yard work....ggrrrr. It's not that it won't work in the emitter but Sgt, Wookie's design is better because the transistor will saturate, making the Vce voltage near zero. This leaves nearly all of the Vcc across the coil. This is desirable because the less the Vce is the less power the transistor has to dissipate. I don't like leaving sloppy mistakes laying around so I will be changing that schematic.

    Point 2: I decided to latch the relay because stryker23 said he was going to be inputting voice audio. If the relay doesn't latch then the relay is going to chatter as the voltage goes above & below Vthreshold . ;)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If I had a nickel for every time my wife interrupted me while I was helping someone with a project on here, I'd have a lot of nickels... :rolleyes: :D

     
  10. stryker23

    Thread Starter Member

    Apr 15, 2009
    13
    0
    Ok, well I think I have a good understanding for now. I will have to wait to get the chip, but hopefully once I start setting it up, it will work the way I understand it from your explanations. I'll be back if I run into trouble. :) Thanks for helping and providing the circuit schematics.
     
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