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3.5mm Jack with NC switch for on/off

Discussion in 'General Electronics Discussion' started by Charles92027, Oct 4, 2014.

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  1. Charles92027


    Oct 4, 2014
    I have a Nightfire Electronics lm386 headphone amp that I'm embedding inside a solid-body ukulele. I'm wiring it to a 3.5mm jack for headphone play.
    I would like to have the circuit switch on and off when the headphones are plugged in or removed, but all of the 3.5mm jacks I can find have a normally-closed switch for disabling a speaker system when headphones are plugged in.
    Anyone have any ideas how I can turn the normally-closed switch into an on/off, or maybe forget the switch and use some other technique to turn it on when the headphones are plugged in?
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    There are several possibilities.

    You can get mono and stereo 3.5 mm jack sockets with fully isolated switches built in. These are a bit of a specialist item and I can't find any on the usual web sites - Digi-Key and Mouser, but you may be able to find one if you have a good search.

    You can also use a bit of extra circuitry to switch the power supply to your circuit using a socket with a switched contact, or even a socket without a switched contact!

    Do you have stereo or mono headphones?

    What is the supply voltage for the headphone amp?
  3. Charles92027


    Oct 4, 2014
    Hi KrisBlueNZ, thanks for the response.
    The supply for the amp is a nine-volt battery, and I'm going to use stereo headphones, earbuds actually. Although the output of the amp is mono.
    I have tried for a long time to find a 3.5mm jack with an isolated, normally open switch. I have a quarter-inch jack that's that way, but not a 3.5mm.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    OK, thanks for the answers. Here's my suggestion. It uses a stereo jack socket with no switch.


    It works by applying a low DC voltage (about 0.7V), at a very low current, to the left and right contacts of the plug. When no headphones are plugged in, this voltage biases transistor Q1 ON, and it pulls its collector down to 0V, turning off Q2, the MOSFET that supplies power to the amplifier.

    When headphones or ear buds are plugged into the socket, the DC resistance of the headphones (about 32Ω per side, usually) pulls the DC voltage down close to zero, and Q1 no longer has any base voltage so it turns OFF. RC pulls its collector voltage up to about +9V, which makes Q2 conduct and connect the negative side of the battery to the negative supply input of the amplifier.

    The components are all standard except CC and Q2. CC should be a non-polarised electrolytic. Q2 can be any N-channel MOSFET, but for best performance, its ON-resistance, RDS(on), should be as low as possible. Preferably less than 50 mΩ. The device I've specified is excellent in this respect, and is also quite small. It's kind of specialised though, so it won't be available from stores like Radio Shack or Maplin.

    Here are links to the components on the Digi-Key web site. If you have a different supplier preference, let me know and I'll see what options they have.

    BT1 and CN1: I assume you already have these.
    RA, RC: 1M8 resistor:
    RB: 33k resistor:
    CC: 10 µF 16V non-polar electrolytic:
    DB: 1N914 diode:
    Q1: BC548C transistor:
    Q2: NTD4906N MOSFET:

    Q2 is static-sensitive. Keep a bit of aluminium foil around the leads until it's fully soldered into the prototyping board. (Suitable stripboard for building the circuit:

    The circuit draws about 10 µA from the battery when there's nothing plugged into the socket. The battery won't last quite its full shelf life, but it will last a long time with such a low load on it.

    Any questions, ask away :)
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