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Voltmeter ideas

Discussion in 'General Electronics Discussion' started by Bluejets, Sep 25, 2018.

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  1. Bluejets


    Oct 5, 2014
    Before Arduino back in 2004, mate of mine was building a 4 cylinder engine for his scale loco and I built him a water temperature gauge with over-temp alarm.

    Unit has an isolated analog voltmeter as the readout and all worked out fine. From memory I made the meter up from a VU meter but can't remember exactly.

    Anyhow, plan changed and now there is a 3 cylinder engine as an alternative and almost ready to run ( yes, 14 years) but it seems my mate has gone and lost the meter. (*&^%$#@!)

    I modded a miniature 3 wire digital Ebay cheapie to read 10.0v with 1.0v input and blacked out the decimal point BUT then realised these are 3 wire and common ground so no good. (10.0=1v=100degrees_C)

    Cannot use the 2 wire version as they need at least 4.5v to start.

    Any way of getting around this other than re-making a VU meter as before?

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Instead of referencing the ML335's output to a fixed 2.73 V potential (to convert the Kelvin scale to a Celsius scale) use a differential amplifier to subtract the 2.73 V from the sensor's output and to scale to 1 V @ 100 °C.
    The output voltage of the subtractor is ground referenced so you can use the El Cheapo meter.
  3. AnalogKid


    Jun 10, 2015
    I think that might be less accurate. An LM324 with 5%, or even 1% resistors in a standard diff amp config will be less accurate than his pot and voltage follower when adjusted with a decent 3-1/2 digit meter. Note that the adjustment tweaks out the 324's input errors, there is no gain error or gain balancing error, and its output impedance and output current capability are more than good enough to let the meter be the subtractor.

    As for the VU meter - buy one on ebay (or did I misunderstand the issue?).

  4. Bluejets


    Oct 5, 2014
    No, you got it right.
    Had a quick look for vu meters yesterday and these days they seem a bit scarce......I'll keep looking as it seems the best alternative....thanks all
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