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Converts an input square wave into a proportional output voltage.
The input frequency range for this circuit is dependent on the 100k resistor and the 100n capacitor, which is 0Hz - 118Hz. The maximum output voltage for the maximum input frequency is 3.71V which corresponds to a resolution of 31.4mV/Hz. Increasing or decreasing the timing capacitor or resistor will increase or decrease the maximum input frequency. This means that if you need to measure a single hertz change within a large frequency range, say 0 - 100kHz, you would need to be able to detect a single 37.1uV/Hz change. So if you don't need to detect single hertz changes in a large range, then make your frequencies you want to detect change in large steps, in this same example make them 1kHz steps.
Input frequencies higher than the max input will result in this circuit acting as a frequency divider where the input frequency is halved, if the output is taken at the collector of Q2 instead of the aggressive lowpass filter. The circuit has a much quicker response in reality and can be used in many applications where you need to convert the rotational speed of an object into a voltage, or for feedback circuits in which the output needs to be maintained at a setpoint.
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