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Working principle is that an input square wave gives a constant HIGH output voltage when it is within a certain frequency bound.
This ciruit is designed for a 250Hz square wave, with the lower frequency bound being 244.5Hz, and the high frequency bound being 257.73Hz. Frequencies outside this range are ignored.
The idea is to convert a sqaure wave into a constant DC voltage that is dependent on frequency, by converting the sqaure wave into a sine wave and then rectifing it, it produces a DC voltage that increases or decreases based on the input frequency. Finally comparing this rectified voltage to a window comparator's conditional voltages allows for voltages between 4.4V and 4V to produce the constant HIGH output from the AND gate.
Could be useful for RF receivers where multiple devices need to be controlled based on a single Tx/Rx system, whereby changing the transmitters modulation frequency allows for the activation/deactivation of many devices.
This circuit does not like to have many frequencies as inputs, it cannot differentiate them, however this could be fixed by someone much smarter than me.
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