This sound frequency meter circuit is simple to build and can be constructed in a portable format. In can measure frequencies with a minimum level of 10 mVrms and a maximum frequency of up to 100kHz. It can be powered by penlight batteries since its standby current is 4 mA. The input is protected from high voltage levels up to 250 Vac.
IC1 4518 is wired to give a hysteresis of only 18mV so that its sensitivity is enough for the whole frequency range. The output of IC1 is directly connected to the counter IC1a which divides the signal by 2 and the following three counters divide by 10. In any of the measurement range with this frequency meter, 50 Hz is the point where the meter will have full deflection. The signal from S2 is used to trigger the monostable multivibrator. The monotime of this circuit is set to one-half of the frequency range – 10ms.
The tester is very sensitive that the power line frequency will be displayed when the input is touched by a finger. The sound frequency meter circuit can be calibrated using the power line frequency. While touching the input with a finger, adjust P1 slowly until the correct frequency is displayed.