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Resistance-type gauges - controlling without sensors

Discussion in 'Sensors and Actuators' started by flannelgraf, Mar 27, 2013.

  1. flannelgraf

    flannelgraf

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    Mar 27, 2013
    Hi guys. I’m designing a control panel “stage prop” for a classic sci-fi fan’s home. It will contain some analog, resistance-type gauges, which need to be wired so that their needles move in interesting ways, without requiring “user” input.

    I’ve spent enough hours in this forum and elsewhere to know that I’m going to have to hire someone to make the circuits for this. But to find the right person it would help me to know whether the most cost-effective solution is likely to be analog-only, or will require digital components, programming, or whatever. And if there’s a name for these kinds of circuits or behavior, I’d love to know that too, so I don’t sound like a complete idiot when I’m trying to explain it to a prospective builder. (He’ll figure that out soon enough, but I’d rather it not be in the first 30 seconds :).)

    Here’s what I know so far: For the gauges, I’ll be using some used, resistance-type, illuminated, auto fuel gauges, with a range of 30 (full) to 250 (empty) ohms. (I have a cheap source for these, and a way to cover up their fuel-level faceplates with custom-fabricated faceplates that match the era and style of the prop.) The panel, which also includes a power switch, a few small indicator lights, and a knob for dimming the gauge lights, will be powered (I assume) from a 120VAC to DC adapter. The gauges themselves need to exhibit the following behavior:

    Gauge 1’s reading needs to fluctuate across the entire range of the gauge. Its needle must appear to be in constant motion – that is, always rising or falling to a level anywhere within the range, in a way that is not predictable to the observer.

    Gauge 2 needs to fluctuate between, say, 50 and 100 ohms. Its needle must appear to pause at a given level for a duration of anywhere between 2 and 30 seconds, and then move to a different level. Neither the levels reached within the range, nor the duration of each “pause,” should be predictable to the observer.

    Gauge 3 simply needs to fluctuate between 100 and 120 ohms, as determined by a two-position switch. When the switch is in the up position, it’s at100 ohms; when it’s down, it’s at 120 ohms.

    Again, I know I don’t have skills to make these circuits myself, but a bit of general advice and terminology from an expert here could help me find someone who does. So if any of you have thoughts on this, I’d be much obliged to hear them. Thanks!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    1,860
    Nov 17, 2011
    Although the meters are obviously analog, the behaviour you envision is easily produced by a small microcontroller with appropriate software. puls width modulation (PWM) outputs can be used to control the gauges. Due to the inertia of these gauges, no additional filtering will be required.
     
  3. flannelgraf

    flannelgraf

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    Mar 27, 2013
    Thanks, Harald, for your helpful reply. You’ve given me exactly the kind of info I needed.

    One more question, if I may, to you or anyone else who might have an idea: Pretend that I was a talented engineer who had good success at designing, building, and programming several microcontroller-based prototype projects of similar or greater scope to the above. How much time, very roughly, would it likely take me to design, build and program the gauge-control solution for this project? Less than five hours? Less than 10 hours? Greater?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,226
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    Nov 17, 2011
    On a stripboard maybe 1-2 hours for building the circuit and putting it into a neat case.
    Maybe 1-2 hours for programming.

    For an etched PCB add 2-3 hours for making the PCB.

    All assuming the "talented engineer " you are - but would you have asked then in the first place?
     
  5. flannelgraf

    flannelgraf

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    Mar 27, 2013
    Thanks again Harald -- that's just the kind of answer I was hoping for. You've been a great help.
     
  6. CocaCola

    CocaCola

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    Apr 7, 2012
    Fee free to contact me privately since you have expressed a desire to hire someone to design/build this for you... Be aware I only check this email about once a week unless I'm expecting an email so there might be some delay in getting back to you if you don't email me in the next few days...

    koca.kola (at) zoho (period) com
     
  7. BobK

    BobK

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    1,648
    Jan 5, 2010
    Harald,

    With these estimates, I would say you are about twice as talented as I am :)

    Bob
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,226
    1,860
    Nov 17, 2011
    Maybe I estimate my being twice as talented :D

    My estimate surely is a bit on the low side and assumes a quick and dirty job by someone who can do stuff like that off the cuff (the talented engineer envisioned by the OP).
    Honestly: It all depends on how neat you are going about this. If you include proper documentation, a nice case with proper cable conduits and so on, I'd say this makes for a nice weekend project.
     
  9. woodchips

    woodchips

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    Feb 8, 2013
    Looks like a slow speed motor connected to a wafer switch with the detents removed and a selection of resistors wired around the contacts. As the switch rotates the resistance/voltage whatever changes and meter waggles. Make three and interconnect them so not quite so obvious they repeat.

    Cheap, easy, no programming!

    Bob
     
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