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I want to design a circuit that turns on an LED when the frequency of the input signal is between 950-1050 Hz using only passive elements and op amps aside from the actual LED.

I have designed the band pass filter but I am stuck on the best way to make the LED turn on only when the input signal has the desired frequency since an input of 925Hz wont be attenuated significantly more than 950Hz. I should note that I want the input signal to have a DC offset of 2V and an amplitude range between 2 and 5 V for the sinusoidal portion.

My initial idea was to use an op amp with no feedback to turn the LED on however I also want the input signal to power the LED so it can then be detected by a photo transistor.

Is there a better way to do this?

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  • \$\begingroup\$ Your question is very incomplete, because you haven't specified anything regarding the accuracy of the frequency discrimination. If you need to detect 950 Hz but reject 925 Hz, you'll need a very high-order filter. A micro-power microcontroller would be a much better fit for an application like this; why are you restricted to analog components only? \$\endgroup\$
    – Dave Tweed
    Oct 10, 2013 at 11:57

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Use a voltage to frequency converter - it will produce a dc output voltage that is proportional to frequency. Try the LM(C)567: -

enter image description here

Then use a "window" comparator to "accept" voltages between the appropriate two limits that correspond to 950Hz and 1050Hz. The LM(C)567 effectively does this by using a phase-locked-loop and phase and amplitude detectors, in effect it is a synchronous detector.

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You probably need a higher order bandpass filter followed by an envelope detector (diode followed by capacitor in parallel with a resistor). Depending on the amplitude of your input you may or may not want to amplify that signal. Then, just feed it to a comparator which would drive the LED when the voltage is larger than the offset. You would have to experiment with your filter and offset to make sure it turns on and off at the desired frequencies.

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  • \$\begingroup\$ The signal v(t) = 2 + Asin(2pift) is being fed into a bandpass filter before anything else so won't the DC offset be almost completely attenuated? \$\endgroup\$
    – user27365
    Oct 10, 2013 at 2:55
  • \$\begingroup\$ For clarification. My goal is that any frequency outside my low and high frequency will be attenuated more than at the cut off and using this I don't want any of these frequencies turning the LED on. My original idea was using a single op amp in no feedback as a comparator with the reference voltage being the voltage that my boundary conditions are attenuated to but I can't figure out how to do this given it will vary as a result of the varying input amplitudes. \$\endgroup\$
    – user27365
    Oct 10, 2013 at 3:15
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Essentially, you want a "Color Organ". A simple bandpass filter will do.

enter image description here

As seen here and here You could simply replace the opamp with a transistor, or use both.

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  • \$\begingroup\$ That won't be even close to the frequency descrimination required. \$\endgroup\$
    – Olin Lathrop
    Oct 10, 2013 at 13:42

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