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DIY Mosfet based 3-wire Solenoid control

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  • DIY Mosfet based 3-wire Solenoid control

    I know that it may seem strange that my first post is directly in the How to section,
    but i gained alot of knowledge just by reading in this forum, and wanted to share this.

    Im in the middle of my first retrofit in BMW e39 Pre.FL Headlights with Bosch/AL Bi-X Projectors,
    so i came in need to control the 3 wire Pull and Hold Solenoids.

    I don't wanted to use relays and big caps because of their size, as i wanted to keep everything inside my headlight housings.
    I searched the forum and found nothing except the wolstentech controller which as far as i know isnt available anymore.

    Because of that, i rigged up a very simple mosfet based circuit which sends a short pulse to the Pull Coil when the high beam is activated.

    The impulse length depends on the mosfet, resistor and capacitor, the given values are around 0,2 seconds.

    Its very small,and needs only a few cheap and easy to source components.

    I thought this could be useful for some of you, so i wanted to share it to give a little
    bit back for all the good info on here.

    Needed parts:

    (per solenoid)

    1x N-Mosfet IRFZ44N
    1X resistor 470K 0.25w
    1X diode 1N4148 (1N4001-1N4007 also works)
    1X capacitor 220nF/0,22uF/224 (i used ceramic)


    The hold coil is wired directly to the Highbeam 12v and permanent GND, pull coil gets switched GND via mosfet.

    i built it "dead-bug-style" and shrinktubed it inside the Headlight housing.
    the mosfet doesn't need any heatsink, as long as you dont want to flip the highbeams on and off every second for a few minutes.

    So first, twist the diode and resistor together in parallel;

    Then solder it to the mosfets gate and source pin, diode ring to gate:

    Last step, solder the cap to gate, and wire it up:

    I hope this may be useful for some of you.

    Last edited by mac_gyver; April 29th, 2020, 09:42 PM. Reason: added images and tags