Fast Switching with a Tone Decoder (LM567)

Discussion in 'General Electronics Chat' started by BackyardBrains, May 6, 2011.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    I am making a circuit that will pulse an LED on/off when a tone is present/absent from an iPod/iPhone. The iOS application can switch on and off a pure 10kHz signal with sub-millisecond precision. This will be used for high school kids to stimulate the neurons of genetically-modified fruit flies, you can read more here.

    I am using the LM567 Tone Detector to switch the LED. The circuit is attached as an image. It works well, and switches on when the tone is played. Only there is a substantial delay between when the tone comes on to when the LED comes on I measured to be 15ms (or ~150 cycles). For my application, this must be much quicker: 2-3ms.

    According to the spec sheet (also attached to this post), the Fastest ON-OFF Cycling Rate is:<br />
{f_o}/{20}<br />

    In my design, {f_o}=10,000Hz therefore the switching rate should be 10,000Hz/20 = 500 Hz = 2ms. The documentation has a graph labeled "Greatest Number of Cycles Before Output". But I am struggling to understand what it means.

    Has anyone ever used this chip? I am assuming that C2 may change the number of cycles required to change states, but would like to understand what that capacitor is doing. What does Bandwidth (% of Fo) mean in this situation?

    Thanks for all your help. I love this board!
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    I haven't used it, but here's what I see. Bandwidth in the datasheet refers to the input filter. If your center frequency is 10kHz and you set the bandwidth to 10%, it will trigger from any input between 9kHz-11kHz.

    It also appears the higher the bandwidth, the lower number of maximum cycles before output. So if you want the lowest number of cycles, and you're using a pure 10kHz sine wave as the input signal, you can set the bandwidth as wide as possible. If your input is only a single frequency then you don't have to worry about it triggering on any other unwanted frequency. The datasheet has a formula for calculating bandwidth.