In the figure a circuit that is the basic application of NE567 for the detection of a tone applied to its input. The sensitivity of this circuit is 50 mV. The frequency adjustment is done in the trimpot. See that in this circuit we have a PNP transistor exciting a load that can be a relay, indicator lamp or even a solenoid. We observed that the NE567 needs a certain amount of signal cycles for it to be recognized. This number will depend on the component values and the signal strength. In some cases, it can reach at least 14 cycles of the frequency signal that must be recognized for it to "lock" by going to the low level. The C3 capacitor is not critical, fixing the filter's pass band. R1 can have values in the range of 2 to 20 k ohms, typically, using a 100 k trimpot to adjust the tuned frequency. C2 is the low-pass filter capacitor. This capacitor must be selected depending on the strength of the input signal. In fact, the response speed of the circuit can be optimized with the appropriate choice of C2 and C3, for which there are empirical formulas that allow the calculation of these components. These formulas are:
C2 = 130,000 / fo (nF)
C3 = 250,000 / fo (nF)
Where fo is the frequency of operation in hertz and the result is obtained in nanofarads.
For example, for 100 kHz (100 000 Hz), the ideal C2 and C3 values will be:
C2 = 130 000/100 000 = 1.3nF
C3 = 260 000/100 000 = 2.6nF
