Doofus needs help with voltmeter

Discussion in 'The Projects Forum' started by Treeman, Jun 13, 2014.

  1. Treeman

    Thread Starter Member

    May 22, 2014
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    Hi all,
    Just received a 1.5v DC meter to put in my amp project just to look cool. Opened the box and DC leapt out at me - of course my guitar puts out AC!

    Not to be outdone by my own self I found this thread about converting ammeters from DC to AC.

    http://forum.allaboutcircuits.com/showthread.php?t=33550

    Can I construct a wheatstone bridge (my first ever - whoah!) to change this to measure AC guitar output as a signal strength meter?
    Are 1N4148 diodes sufficient or is the Forward voltage at 1v too little?
    I have other diodes like 1N4001 at my disposal.

    Its just a bit of fun so accuracy is less important than 'look'.

    Thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Why do you want to install a 1.5V meter on your guitar?
     
  3. Sparky49

    Active Member

    Jul 16, 2011
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    ;)


    ..........
     
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  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the meter will probably not show a lot, the output from guitar pickups is in the few milli (1/1000) volt range.
     
  5. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Thanks Alfa.

    I did ask the supplier for suitability and having rectified there is nothing to see at all. I guess this is to do with voltage drop through diodes and scale.
    Is there a milli volt meter lurking inside hiding behind a resistor? I read that all V meters are millivolt scaled up.

    So all I want is a signal strength meter that swings high (to 11 of course) and looks cool. Accuracy is not in the least important its just for show.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    You can still use the 1.5VDC meter if you add a 1.5V battery and simple transistor amplifier.

    Show us or link us to the specs of the meter.
     
  7. Treeman

    Thread Starter Member

    May 22, 2014
    158
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  8. MrChips

    Moderator

    Oct 2, 2009
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    Are you hoping to put that meter on your guitar?
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Looking at that meter i would say its got shunt resistors inside, so maybe you can bypass them and get a lower reading?
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    volt meters do not use shunt resistors, they use a resistor in series. shunts are used in ampmeters.
     
  11. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Er... no.

    I want to put on my amp.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Then er.. Can you tell us more about the amp?
     
  13. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    http://www.ampmaker.com/images/ak00kit/ak00sc2.jpg. Should be all you need. I will be trying a 12au7 or 12at7 instead of the eccc83 as I'm not keen on distortion.

    I want to plug my guitar into my amp in the normal way and have a meter waggle around f.s.d. purposefully maybe with a back light. Ideally battery free so if I can tap off any voltage needed that would be swell. Maybe the valve heater supply may provide the juice. (6.3v I think)

    thanks.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
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    You could probably drive the meter via a rectifier bridge (1N4148 or 1N400x diodes will suffice for the bridge) from either the 8 Ohm or 16 Ohm output of the output transformer. A series resistor and smoothing capacitor might be necessary. That resistor could be a trimmer, to set the meter to give full-scale deflection at typical amp output levels.
     
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  15. Treeman

    Thread Starter Member

    May 22, 2014
    158
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    Thanks - its irrelevant (I think) whether it is showing input or output just that it works nicely.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    By all means go with the output. You've got far more signal to deal with and there is zero concern about altering the sound. (well, not ZERO).

    Is this any different than just wanting a VU meter on your amp? A needle that jumps in proportion to the volume? Instead of a needle, you might like a barograph LED display. The LM3915 IC would be handy for that.
     
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  17. Treeman

    Thread Starter Member

    May 22, 2014
    158
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    Actually Wayneh you've hit the spot I think. My amp has 4 8 and 16 ohm output transformer. I only have the 4 and 8 wired and switchable. perhaps I can use the spare 16ohm output (if usable at the same time)
    Ive got an LM3916 that I bought thinking it had an integral Led display (I think NSM3916 had...) but fell for the idea of a waggly needle. Knowing me it may end up with both. I think my stubby fingers would mess with the sound more if we are talking alteration at an oscilloscope detectable level. Any human ear would just think I am as bad a jazzer as I actually am!
    Much obliged.
     
  18. crutschow

    Expert

    Mar 14, 2008
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    If you can remove the internal series resistor, that would increase the meter sensitivity. Then, if the meter movement is still to small, you can try a single Schottky diode in series with the meter instead of the bridge, to lower the diode voltage drop.
     
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  19. MrCarlos

    Active Member

    Jan 2, 2010
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    Hi Treeman

    As far as I understand, so you mention in your original message:
    You get a meter with a range of 1.5 DC (Volts).
    you want to install on your PROJECT amp. (Today probably still as a Project).
    Just to make it look cool.

    You found out, somehow, you need to convert from DC to AC your meter, to measure the AC output of your guitar as signal strength meter.

    The link where you found that converter is this:
    http://forum.allaboutcircuits.com/sh...ad.php?t=33550

    And the meter in question is the one in this other link:
    http://www.rapidonline.com/test-meas...tmeter-85-2725


    In the first paragraph your message say:
    Just received a 1.5v DC meter to put in my amp project just to look cool. Opened the box and DC leapt out at me - of course my guitar puts out AC!

    Later you mention:
    Can I construct a Wheatstone bridge (my first ever - whoah) to measure AC to Change This guitar output as a signal strength meter?

    So: are you going to connect your meter to the output of your PROJECT amplifier or just at the output of the guitar?

    If you connect it to the amplifier output is necessary to know that AC voltage gives out, when at maximum volume.
    You need to know whether your meter could handle that voltage.

    But if you connect to the output of the guitar (guitar pickups)
    You need to make a preamp. since the guitar pickups produce a very, very low AC signal.
     
  20. Treeman

    Thread Starter Member

    May 22, 2014
    158
    15
    Amp is up and running and great. Have home made preamp too so good advice thanks.
    I thought output was up to 1v for humbuckers so that's why I bought the 1.5v.
    Took advice from supplier.

    Either connection is good for me. Initially I only thought about output of guitar but maybe output is easier.

    Will take some measurments and repost result.

    Thanks for bulleting your answer. Helps with the logic flow.
     
    Last edited: Jun 14, 2014
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