Frequency Triggered Switch

Discussion in 'The Projects Forum' started by Ober1, Sep 11, 2012.

  1. Ober1

    Thread Starter New Member

    Sep 11, 2012
    Hi all,

    I have a project that I've been working on for fun, but still can't seem to get it to work the way I want it to. What I want to make is a circuit that will allow me to close a circuit when a certain frequency is detected. Amplitude is not important for the applications I want to use it in, only the sound frequency.

    In addition, I would like to be able to set a delay for the signal so that it would have to be present for a certain amount of time before activating the switch.

    My original plan was this: Electret mic->Gain Amp-> Active High Pass Filter-> 555 Time Delay Circuit -> MOSFET switch.

    The idea was that I could amplify the signal off of the mic, filter out lower frequencies, then have a 555 circuit set for a delay. The output of the 555 would be used to trigger the MOSFET and close the circuit.

    Where I'm having trouble is the signal is not being filtered well enough, and I think this might be because it's a first order filter. I can maybe just fix this later. I think I'm also running into problems with the 555 as my delay since the input is more or less a sine wave- can 555's work this way?

    Thanks in advance for any help, I'm not well versed in building circuits like this. Also I'm only looking for ideas on how this could be made better, not specific values for components.
    Last edited: Sep 11, 2012
  2. vk6zgo

    Active Member

    Jul 21, 2012
    I think you want a Bandpass Filter,as your HPF will also let all sorts of higher frequency crud in (& there is a lot out there).

    You also have to be very careful with shielding,as this will help reduce the amount of crud you pick up.

    It would also be a good idea to feed the output of your filter through a Schmitt trigger ,or even just a diode clipper,before you use it to operate the 555.

    You can buy single package frequency operated switches,but they do need a fairly clean input signal.
    Ober1 likes this.
  3. bertus


    Apr 5, 2008

    You could take a look at the NE567, wich is a PLL locked tone decoder.
    See attached datasheet for more info.

    Ober1 likes this.
  4. Ober1

    Thread Starter New Member

    Sep 11, 2012
    Perfect! Thank you both for your help, I really appreciate it.

    Based on the data sheet and what I'm trying to do it seems like the decoder might work really well. Since I'm not very familiar with how they work- Will I be able to set it to trigger off of a range of frequencies or will the frequency need to be static for the duration of the delay? I'm only wondering because it is possible for the frequency I'm "detecting" to vary a few Hz during the time it is present.

    Thanks again, cheers!
  5. #12


    Nov 30, 2010
    From the datasheet: "Independently controllable bandwidth (up to 14%)"

    In general, PLL's have a lock-in range. The "proper" signal has to be present for an output. If the input won't hold still long enough, you can add a "pulse stretcher" built with a 555 chip.
    Ober1 likes this.
  6. Ober1

    Thread Starter New Member

    Sep 11, 2012
    Ok, perfect. Thanks again all, might be able to get this thing working after all.

    Trying to become less of a noob every day, I appreciate the help!
  7. wayneh


    Sep 9, 2010
    I don't know if it helps you, but what you're describing is essentially a guitar tuner. Many will emit a beep or light up when the right tone is detected, and you can often define what "right" means, in terms of center frequency with a tolerance range. You just want to trigger a MOSFET instead of a beep or indicator light.

    There are several free tuner apps to use on an iPod or iPhone. My favorite is PanoTuner. It's not as pretty or fancy as some of the others but it works great and is fast, simple and obvious enough to use on stage (although the numerical display of frequency is absurdly hard to read).
  8. Ober1

    Thread Starter New Member

    Sep 11, 2012
    I'm not sure why I never thought of it that way, but yes that is pretty much what I am trying to do.

    The only difference between what I want to build and a guitar tuner would be setting a specific frequency (NE567 looks like it will do this) and having the "light" trigger if the tone is steady for a certain time (say 10 secs).

    Other than that...yup, it's a guitar tuner set to detect only one tone :)