Turn Signal and Parking lamp XOR

Discussion in 'The Projects Forum' started by brandini, Dec 21, 2008.

  1. brandini

    Thread Starter Member

    Feb 25, 2008
    12
    0
    I want to make the light for my parking lamp the output of turn signal and parking lamp inputs run through an XOR function.

    I have a Quad NAND chip to make the xor ( http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1345-5-ND ) and a MOSFET to handle up to the 5watts of the stock bulb (a 194) http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRL3714ZLPBF-ND

    I breadboarded them like pictured in the PDF attachment. But it doesn't work. I had been given help from a Mazda forum member but he's currently unavailable.

    Ideas why it's not working much appreciated!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It's not working because the FET gate is tied to ground through 12 ohms. To turn on the FET, the NAND gate would have to push an amp of current through it.

    There's no need to use the resistor to ground - and replace the 16K resistor in series with the NAND output with something more like 100 ohms.
     
  3. brandini

    Thread Starter Member

    Feb 25, 2008
    12
    0
    Thanks!

    Summarize:
    Scrap resistor to ground, change remaining resistor to 100ohm

    For source and drain, would it be better to have it on the + or - side of the bulb it's driving (I also plan to use an LED replacement for the output)? What would I have to change to make it correct?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Always use the FET between the load and ground. The gate has to be 10 volts or better above the source, and so placing the load in the source circuit creates problems, as the FET will start shutting itself off as soon as it conducts (making the source voltage rise toward the gate).

    Unless the LED bulb replacement has current limiting built in, you will need something like a resistor to keep LED current below the maximum rating.
     
  5. brandini

    Thread Starter Member

    Feb 25, 2008
    12
    0
    The led replacement is a 5 or 6 led array but it's a fully sealed replacement. If it's an array wouldn't it have to have a resistor somewhere?

    Also one last thing:
    Drain would go to light
    Source would go to ground?

    Thanks for the help and the lightning replies!
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That should be correct. The substitute bulb just has to have the current limiting built in, and that is the way to connect the FET.
     
  7. brandini

    Thread Starter Member

    Feb 25, 2008
    12
    0
    I got it working! Thanks for the tips it works beautifully even without the resistor my problem was that the inputs, when not high, were floating (so my friend told me) and I'd wave my hand over the circuit and it'd make the output flicker. Creepy.

    For testing I added a really high resistor from the input to ground and it works like a charm.

    Now I'm looking to find a way to power it off of the inputs, I'm thinking diodes would make this possible but need some advice on picking the components... Any ideas anyone?
     
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