I want to build a simple and ultra low cost circuit that can detect a timed (let's say 200ms) burst of a single sine frequency - say,15KHz. Amplitude will be around AC ~1Vp-p. Detection will give a logic '1' on simple 3.3V TTL output while the single frequency is detected, and '0' when not.

what would be the simplest circuit? (My guess is a narrow bandpass filter and a logic level comparator/converter, but I'm looking for a simple implementation)

btw, I am running from a single 3.3V supply

  • \$\begingroup\$ Sounds like you want to detect a burst of 15 kHz, not a pulse. A pulse is apparently what you want coming out the other end. \$\endgroup\$ Commented Feb 16, 2012 at 16:26
  • \$\begingroup\$ What bandwidth? What response time? \$\endgroup\$ Commented Feb 16, 2012 at 16:26
  • \$\begingroup\$ re bandwidth - what range of frequencies should be able to trigger the filter? | How low cost does it need to be? Specify an acceptable upper cost | Are there other audio components present at the same time. If so, specifiy/ explain them. | This is an extremely easy task in some situations and a moderately hard one on others. More information helps us help you. WHAT is the context? \$\endgroup\$
    – Russell McMahon
    Commented Feb 16, 2012 at 16:30
  • \$\begingroup\$ I stand corrected on the 'burst' issue rather than pulse. .. Bandwidth for detection: 14.7-15.3KHz. .. lowcost: in the range of tenths of cents no more. The context is an (external) audio unit outputting this burst to initiate a 'wakeup' for sleeping microcontroller unit. other audio onboard is a normal audio amp pushing a 8ohm speaker. I will tap it's input for this wakeup burst detection.. response time: non-relevant (can be even 1 second) \$\endgroup\$
    – mindbomb
    Commented Feb 16, 2012 at 16:36
  • \$\begingroup\$ The LM567 Tone Decoder would do the job if you have a 5V supply. Not sure about the "tenths of a cent" restriction either! \$\endgroup\$
    – MikeJ-UK
    Commented Feb 16, 2012 at 17:15

2 Answers 2


Resistors in volume in China cost about 0.1 cent US.
Electrolytic capacitors cost around 1 cent up to around 33 uF.
Some selected ICs cost as little as 2 cents 0 eg TL431, a dual opamp such as LM358 may be 5 cents.

An NE567 would do your job but is a bit rare and may cost 10 - 20 cents.
NJM567 datasheet
I had altogether too much to do with these in another lifetime. They work quite well.

  • The NJM567 tone and frequency decoder is a highly stable phase locked loop with synchronous AM lock detection and power output circuitry. Its primary function is to drive a load whenever a sustained frequency within its detection band is present at the self-biased input. The bandwidth cebter frequency, and output delay are independently determined by means of four external components.

enter image description here

A CD4046 digital PLL would do your job - pricing here and datasheet here - TI and here -Fairchild

Price from Digikey is $US12.8 at 50,000 quantity so Asian price may be 5 cents.

Circuit diagram in real world use is not much more than shown below. Phase comparator two is just an XOR gate but works like magic. The circuit shown below would probably cost under 10 cents in 10,000 + volume in China but PCB cost and manufacturing costs will about double that.

This circuit will work superbly with minimal noise and quite well in modestly noisy circumstances. In very noisy environments a 5th order active filter can be added at a cost of 2 transistors and RC glue - but good quality R's and 's needed for a stable filter - say an additional $0.10.

enter image description here

  • \$\begingroup\$ That's a nice solution for a lot of cases, but in this case exceeds the cost spec by a order of magnitude or two. What the OP wants is simply not possible from what is available today. \$\endgroup\$ Commented Feb 16, 2012 at 18:36
  • \$\begingroup\$ @Olin - Yes. I thought I'd give him best my 2nd :-) shot and see what he said. Quite possibly the "tenths of cents"is a typo and "tens of cents" was intended - seems more likely. The 567 is a 5V min part but that's a hurdle for later. Also, if this is a wakeup system then power consumption may be too high. | "Best| solutio is a single pin on a microcontroller plus code. May even get near the 1 centrange if exceptionally lucky :-). \$\endgroup\$
    – Russell McMahon
    Commented Feb 17, 2012 at 1:41

What you are asking for is not possible. Forget it. Go home.

You want to detect 15 kHz ±300 Hz, or within ±2% of the center frequency. That is doable and will require either a resonant analog circuit or a processor with A/D converter like a low end DSP. However, either way "tenths of cents" is way out of line and totally unrealistic, even in high volumes.

  • 2
    \$\begingroup\$ Telling people to "forget it"/"go home" is not very friendly even if you think the question is silly. \$\endgroup\$ Commented Mar 6, 2012 at 15:20
  • 2
    \$\begingroup\$ @trig: It's a good way to make a point when the OP asks for something really absurd. \$\endgroup\$ Commented Sep 23, 2013 at 11:50
  • \$\begingroup\$ No, it's just rude. \$\endgroup\$ Commented Sep 23, 2013 at 17:55

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