5 volts to 30 volts circuit?

Discussion in 'The Projects Forum' started by mymr2, Sep 24, 2007.

  1. mymr2

    Thread Starter New Member

    Sep 24, 2007
    1
    0
    Hi all,

    I'm new to this forum and would like some help related to a new computer i just installed in my car. The tachometer doesnt work because it requires a 30 volt square wave signal but the tacho signal from the computer only provides a 5 volt square wave signal. Apparently i can use a 2n2222a for this, I have tried these circuits but with no success:

    [​IMG]

    [​IMG]

    Any help would be greatly appreciated!

    Thanks :)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That 30 volt requirement sounds steep for a car. The tach may want a signal off the electronic ignition. The circuits you have posted will only switch between 0 and 12 volts.

    You might find a DC to DC converter that will pump your 12 volts up to 30. Just use that in place of the 12 volt source on the first circuit. Use a 3000 ohm resistor in the collector-to-source position to keep the current down.
     
  3. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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  4. arthur92710

    Active Member

    Jun 25, 2007
    307
    1
    Cant he just use a voltage tripler then a doubler?
    5x3=15 15x2=30
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    As previously mentioned, you will need a DC-DC converter to create a 30V supply from the 12V battery, Maxim is a good source for a boost converter.

    The 30V supply would then substitute for the "+12V Ignition" in your given schematics.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Looks like this is just what you need:

    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3138

    [​IMG]

    But you'll need to use an input regulator to step down the vehicle's battery voltage to 11v or less. LM317 Vreg or a 78xx series fixed Vreg, where the "xx" is 05, 06, 07, 08, 09, or 10. 78xx series regulators have various prefixes depending on the manufacturer, like AN, LM, uA.

    Since the regulator is going to be a ways away from the battery, don't forget to use decoupling capacitors on the input and output of the regulator to prevent oscillations.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, he could, but he'd first have to build an oscillator circuit, then the voltage doubler/tripler, then regulate it... much easer (and much less expensive) to buy a unit for a buck or so that already has those functions integrated.
     
  8. arthur92710

    Active Member

    Jun 25, 2007
    307
    1
    oh ok. So it seams that the simpler method (my'n) is harder. I hate when you think "oh why would you do that? this is simpler the it ends up that you lost time because of the 'simpler way' and should have done the right way(dc dc converter)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I'm not knocking your idea - sure, if this was just a hobby thing, and he had a junk box full of parts - hey, go for it; you'll learn a lot.

    But in this particular situation, our OP (original poster) just wanted to get his computer interface to his tach working quickly and reliably, with a minimum of fuss and expense. To me, that says just go for the off-the-shelf solution - particularly when it's so inexpensive. He might even ask them for a free sample - and get it. ;)
     
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