I'm using a split supply, using 2x 9V batteries for a headphone amplifier. In lieu of an on off switch I'd like to just turn on or off when I put in the 3.5mm jack. Unfortunately the female jacks I've seen all have normally closed switches. When the user inserts, the switches open. I can't figure out a low power way to use a closed circuit to kill the power! I feel like this should be very simple but I'm coming up with complicated solutions like xoring the input and using a SSR, but SSR have 1-1.4V drops...ideas?


1 Answer 1


If the diagram to be believed, this 3.5 mm jack mates with a 3 conductor stereo plug, and also has an completely independent normally open switch:

enter image description here

Granted, these are rare; out of 170 3.5 mm audio barrel connectors at Digi-Key, this is the only one I could find in this configuration.

  • \$\begingroup\$ Are you sure it's normally open? How can you tell? This question: electronics.stackexchange.com/questions/2381/… indicates a similar diagram, which is normally closed. \$\endgroup\$
    – Paul L
    Jan 12, 2015 at 7:11
  • 1
    \$\begingroup\$ Note in the diagram above, the contacts are not touching (there is a little gap), and when the plug is inserted, pressure against the little open triangle on contact 4 will cause contacts 4 and 5 to close together. In the other diagram, the opposite occurs -- the contacts are initially shown touching, and when the plug is inserted, contact 1 will pull away from contact 2. \$\endgroup\$
    – tcrosley
    Jan 12, 2015 at 7:50

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