I want to use a rotary knob to pulse resistance

Discussion in 'The Projects Forum' started by turgin, Aug 30, 2015.

  1. turgin

    Thread Starter New Member

    Aug 30, 2015
    I'd like to use the 3.5mm wired remote jack on the back of a car stereo that is typically used for factory steering wheel audio control integration to attach my own custom wired remote. I found a schematic online that shows the resistor values needed to control various functions on the head unit. I am specifically interested in the volume up (16k) and volume down (24k). The schematic is for momentary push buttons to activate each function but I would like to use a rotary knob for the volume.

    My though was a continuously rotating knob that would pulse 16k ohms when turned clockwise and 24k ohms when turned counterclockwise. I've linked the page below that shows the schematic and resistor values needed.

    Can someone help me with how to build a circuit for just the knob part?


    Thank you.
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Can you obtain some sort of quadrature rotary encoder? Decoding the A and B outputs and switching in the two resistance values should be easy once you identify the rotary encoder you would like to use...
    Last edited: Aug 30, 2015
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    According to the drawing there is no stable DC power source, so your external circuit will need its own power. Also, I'm not sure I'd trust that the sleeve connection is at GND potential. So a conservative approach would be to have a control circuit of some kind driving some small reed relays, with the relay contacts doing the actual switching of signal lines.

    For the control circuit: a small rotary encoder (Alps, Bourns, etc.), some logic gates (CD4093), and two drive transistors (2N7002) for the up and down reed relays. If the sleeve really is at GND, then the relays go away and the 7002's switch the signal lines directly. Is this something you can handle?

  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    First pass at an interface with a Bourns PEC-12 rotary encoder. R3/C4 make a 7 ms pulse with each encoder click. If this is too short, increase C4.

    Note: 1% resistors are not necessary; they are what's in my design library. 5% is ok.