Powering a Relay from a 3.5mm Headphone Jack

Discussion in 'The Projects Forum' started by rjhlondon, Jan 11, 2011.

  1. rjhlondon

    Thread Starter New Member

    Jan 11, 2011

    I have been asked to find out if it would be possible to power a relay via the power a 3.5mm headphone jack on a pc.

    Basically the current phone system in the office plays a tone every time there is a queue of calls on hold. It’s been decided that the noise is inappropriate and they would rather have a signal light.

    So what I had imagined was wiring up the lights and in between the lights and the mains power supply placing a relay. This relay would need to be activated by the AC power output from the 3.5mm head phone jack on the pc when the sound is played and remian closed for a designated length of time. I can modify the sound file the system plays to a continuous tone to produce as much constant power out as possible.

    I'm not sure if this is even possible, but does anyone knows of a realy like this?

    Or if anyone has any better ideas?

    Thanks in advance for any help you can give me!


    Senior Member

    Jun 29, 2010
    Use transistor or other component amplifier.
  3. rjhlondon

    Thread Starter New Member

    Jan 11, 2011
    Hi thanks for your reply,

    do you have any more details on that? examples of the parts you mentioned?

  4. Len Whistler


    Dec 10, 2010

    I think you would have to start by measuring the output with a multimeter, get some cheap headphones then cut and strip the wire. Once you know what is being outputted from the pc head phone jack you can build the circuit.

    PS ..... I have a feeling that a multimeter won't do the trick, perhaps an oscilloscope?

  5. wayneh


    Sep 9, 2010
    The short answer is NO. The low power audio (alternating current) for the headphones is not what a relay normally requires; a DC voltage with generally more power than the headphone signal.

    The longer answer is that you can still accomplish your goal - it'll just be a bit more complicated. It'd be great if you could get at the signal the phone uses to turn "on" the tone, and hijack it to turn on a light instead.

    But assuming you cannot do that, you're going to want a "tone detector". By that I mean something that can detect an AC audio signal meant for a speaker, and for that to trigger a light. Someone here probably has a great idea for detecting the signal (comparator with coupling capacitor on the input?), but once that is done the rest is easy. See the sections on LED flashers, 555 timer circuits and such.

    Can you have the tone be a square wave at a frequency of your choosing? That could simplify things.

    Hey, wait a minute. There are a ton of lights-to-music circuits meant for flashing LEDs and such to the rhythm of your music. One of those circuits would probably work great, perhaps with minor modification.
  6. bertus


    Apr 5, 2008