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Simple circuit help...

Discussion in 'Electronic Basics' started by MacGyver, Oct 4, 2005.

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  1. MacGyver

    MacGyver Guest

    At least I think it is simple...

    I have 2 12v wingtip lights that I need to be able to control with
    one switch and one wire running through a wing in an airplane.
    This is for just one wing. That's the gist of it.

    I just bought piggyback strobes for the wingtips, but I don't want to
    have them on all the time that I have the navigation lights on.

    I was thinking of a relay that has a remote control, or has a signal
    run through the positive wire or could possibly have a low & high
    setting and if we send 12 volts to the relay it trips, but it won't
    trip at say 10 volts.

    Also was thinking about if I turn the switch on once just the light
    comes on. Flip the switch off & on again in under 2 seconds and the
    lights & strobes come on. Leave the switch off & everything turns off.

    I was thinking about relays with a capacitor to keep the line live for
    2 seconds to be able to flip another relay... but now I am confusing
    myself again.

    Any Ideas???

    Seems like it should not be too difficult.
    Thanks so much for any advise!!!

    Sean Gallagher
    Pheonix, AZ
  2. Bob Monsen

    Bob Monsen Guest

    Ah, the easiest and most reliable thing is to run another wire. You can
    use that wire to either switch a transistor, or to pass the current to the

    If you can't do that, you can build a little circuit that will 'remember'
    how long ago the power was shut off, and turn on a circuit if it
    wasn't too long ago. Thus, you can flip the switch for a second, flip
    it off, then flip it on again, and the strobe will start.

    The basic memroy circuit is an RC node, A below. It charges slowly,
    and discharges even more slowly. If Vcc is dragged to ground (through
    the position light) the diode will keep it from draining.

    The voltage on this node can be compared against another node that
    charges up and drains quickly, B below. If both are charged up shortly
    after the power comes up, the circuit latches on. If either are low,
    the circuit does not latch.

    The latching circuit is a simple "SCR" configuration, using mosfets.

    A is pulled somewhat slowly up to 12V while the power is on, but
    decays very slowly back to ground when the power is off.

    Vcc-----|>|--/\/\/\/-----o-------.------- A
    | |
    | \
    | /
    | \
    22uF--- / 1 Meg
    --- \
    | /
    | \
    | |
    | |

    B creates a quick spike, which decays back to Gnd in few ms.

    || | |
    | \
    1uF | /
    | \
    - / 10k
    ^ \
    - /
    | \
    | |
    | |

    Here is the main switch. When the power comes on, A will start to
    charge up with an RC constant of about 1/5 of a second. B will spike
    up immediately, then drain in about 1/20 second. So, the first time,
    one will be low, the other will be high. Thus, one of the N-MOSFETs
    below will be off, and no current will flow through them. Because of
    this, 2N7000 will never be turned on, and the P-MOSFET will also never
    be turned on.

    However, after a second, A will be charged up to nearly 12V. If the
    power is then turned off, it'll slowly drain down to ground, taking
    about 100 seconds to get all the way down. Thus, if within about 10
    seconds, the power is restored, both mosfets will be on briefly,
    causing the gate of the 2N7000 to be pulled up to 12V. This will turn
    it on, causing current through the 100k resistor to bring the gate
    voltage of the P-MOSFET down, turning it on. Once it's on, it'll keep
    the gate of the 2N27000 high, so the load will have power until the
    12V supply goes away.

    Vcc -----o-----------------o-----------------------------.
    | | |
    ||-| \ |
    ||<' N-MOSFET / |
    A----||-. \ 100k |
    | / |
    | \ |
    ||-| | |
    ||<' N-MOSFET | |
    B----||-. o--------------------------||-|
    | | ||>' P-MOSFET
    | | ||-. Low Rds(on)
    '-----------------)-------. |
    | | |
    | | |
    '-|| | |
    2N7000 .>|| | |
    |-||----o-------\/\/\/\-------o----- to 12V load
    | 10k
    Gnd -----------------------o----------------------------------- load gnd

    Use a big P-MOSFET with low Rds(on), because that will dissipate
    almost no power, so it won't get hot, or lower the voltage too

    You may want to put a resistor from the load output to ground, perhaps
    10k. That will ensure that it won't turn on incorrectly when the power
    comes up if there isn't a load pulling the drain of the P-MOSFET to ground.

    There may be gotchas in this circuit, and I haven't built it, so ymmv.
  3. Guest

    Am I getting this right?

    With 1 switch, you want to set 4 different scenarios:

    1 - Off
    2 - Nav Lights
    3 - Strobe Lights
    4 - Nav lIghts + Strobe Lights
    Back to Off

    Please let me know if this is correct.
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