Audio-Frequency Meter Circuit

This meter differs from the norm in that it does not use a D`Arsonval movement or digital display to give a reading of the input frequency. Instead, the measured frequency is read from a hand-cali-brated dial. Any audio signal applied to the circuit is amplified by Ul and the resulting output is divided along two paths. In one path, the output signal is applied to the mixer; in the other path, the signal is applied to the input of U2 through SI (a normally open pushbutton switch). The portion of the amplifier signal that is fed to the mixer is applied to the base of Ql, causing it to toggle on and off at the signal frequency. In the other path, when SI is pressed, a portion of the op amp`s output is applied to U2.
Audio-Frequency Meter Circuit - schematic

If the signal is within the range of U2`s internal oscillator`s operating frequency, LED1 lights, and a signal is fed to the base of Q2. If the two signals arriving at the mixer do not match exactly, LED2 and LEDS light. That means that the circuit must be fine tuned, which is accomplished by releasing SI and fine tuning R13 until LED2 and LED3 go out. The dial setting at that point gives the frequency of the input signal to within 1 Hz (or as close as the calibrated dial will allow).

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