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Discussion in 'General Electronics Discussion' started by Jasper, Mar 26, 2010.

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  1. Jasper


    Mar 26, 2010
    Hi. Please excuse my ignorance but I am a complete beginner in electronics.

    I'm interested in building a gear indicator for my motorcycle using a seven segment led.

    At the bike end I can arrange a mechanical system that will provide me with a switch that
    will activate one of six poles (six gears) or wires. These wires can either be +12V or individually go to ground.

    My question is what is the best/simplest way to drive the digital led? Is there a simple device (maybe a pre-configured PIC Etc) that would simply detect input on a particular input pin and display the appropriate number from 1 to 6?

    Thanks for the help. Hope this is not to basic.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The easiest way depends a lot on what experience you have.

    One thing that immediately springs to mind is to convert the 6 inputs to binary using a number of diodes (a little tricky since the logic appears to be inverted) and then to use a bcd to 7 seg decoder IC to directly drive a 7 segment display.
  3. Jasper


    Mar 26, 2010
    Thanks Steve,

    As you replied I was reading an article by HotWaterWizard with a drawing by John deRosa.
    Amazingly I can understand the drawing with the diodes and can understand the binary logic. So with breadboard and a LOT of diodes I could probably get something to work although I'd need help with the voltages.

    In the second part of you response you mention a bcd to 7 seg decoder IC. Can you give me a actual lead to one of these as I think that is what I need to make a project compact and robust.

    Many thanks.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    look toward the bottom of this page.

    The diode trick will be more difficult because your switch logic is inverted, i.e. when the gear is selected the lead is grounded, all others are at 12V (I think this is what you said).

    You will also need to build a voltage regulator that is robust enough to withstand the voltage spikes you may get on the 12V rail.

    Also, if this is going to be mounted on a bike, a 7 seg display, if bright enough to read during the day will be blinding at night. Conversely, if OK for night it will be invisible in direct sunlight.
  5. Jasper


    Mar 26, 2010
    Thanks very much. It took me a while to realise that this page was a link so I was searching around at the bottom of the current page (very tricky :)). The info looks very useful. If successful I imagine that the brightness is probably voltage related so maybe a
    variable resistor (maybe called a pot?). Yes I did say that the switch is grounded but think I can just reverse it.

    Where the hell is half way between China and Antartica? An oil rig? At a stretch Christmas Island. :)

    Once again thanks for the help.
  6. Resqueline


    Jul 31, 2009
    IIRC the HWW & JdR method doesn't require IC's, only diodes driving the 7-seg directly. It can also be done with "normal" instead of inverse logic.
    Here is a thread in this forum referring to it.

    Actually, I'd say steve is located more like half-way between Japan and the Antarctic Peninsula. And think bigger.. ;)
    Last edited: Mar 26, 2010
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