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I have two square waves of different frequencies (f1 and f2) and two circuits (ckt1 and ckt2). I want to design a circuit such that on the input when f1 square wave comes, ckt1 gets activated/turned ON and when f2 square comes, ckt2 gets activated/turned ON. I am looking for a frequency-based switching between circuits. What ideas would not involve FFT or a microcontroller (because I am looking for a circuit-level implementation with low-processing capacity)?

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    \$\begingroup\$ What are the frequencies? What are the peak values? Do they both have 0.5 duty-cycle? Is it possible to make them have non-square (i.e. duty-cycle higher or lower than 0.5). \$\endgroup\$ Nov 3, 2021 at 14:32
  • \$\begingroup\$ f1 is 1kHz and f2 is 5kHz. It is possible to make them as clock pulses ('1' => 3.3V and '0' => gnd (0 V)). The duty cycle of both the signals is around 5%. It is a requirement for the other part of the design that the duty cycle and the frequency stay the same. \$\endgroup\$ Nov 3, 2021 at 14:40
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    \$\begingroup\$ Please edit your question with new information rather than putting it in comments. \$\endgroup\$
    – Null
    Nov 4, 2021 at 3:45
  • \$\begingroup\$ If f1 and f2 are fixed and the duty is fixed then this is a relatively easy task - depending on how demanding you are. eg At 1 kHz and 5% t_hi is 50 uS, and at 5 kHz t_hi is 10 uS. By examining each high pulse you can make a decision in not more than 1 ms , and as little as about 200 us. \$\endgroup\$
    – Russell McMahon
    Nov 5, 2021 at 10:34

2 Answers 2

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Use a high-pass or low-pass filter with cutoff at 3 kHz, followed by an envelope detector. Add a comparator at the output of the envelope detector to convert the analog output to a digital signal.

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If f1 and f2 are fixed and the duty is fixed at 5% then this is a relatively easy task - depending on how demanding you are.

At 1 kHz cycle time is 1 ms and 5% duty cycle makes t_hi = 50 uS. At 5 kHz cycle time is 200 us and 5% duty cycle makes t_hi = 10 uS.

By examining each high pulse you can make a decision in not more than 1 ms and in as little as about 200 us.

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