audio frequency meter

7-88 elektor july/august 1980 n 9 • cra Cl 330n 14 •0 JD" D1= 10 V 400 rnW M R2 N2 R3 R8 P3 R9 P4 111 100k 10k M7= > 10 k1-2/V 0 ... 2 V 9 audio frequency meter 10 V 11 N3 O • 0•O 80547 If your interest is primarily with audio, a commercial frequency meter, although very nice, is not strictly necessary since most of its range will be redundant. The simple circuit described here is used to con- vert an ordinary 10 kE2/volt moving coil volt meter into an audio fre- quency meter. The input signal is first amplified by transistor T1 (with a gain of about 40) and then passed through a Schmitt trigger formed by N4. This converts the signal to a square wave and the negative edge of this is used to trigger a monostable multivibrator (N1 and N2). Its output is then inverted by N3 and fed to the multimeter which should be switched to the 2 volts (fsd) range. The three ranges of the frequency meter are selected by S1. They are 200 Hz, 2 kHz and 20 kHz and are calibrated (with the aid...