audible ohmmeter - ideal for those with perfect pitch

ideal for those with perfect pitch It is sometimes useful to have a small instrument that can give a quick indication of the approximate value of a resistor. The present circuit enables an unknown resistor to be compared with a number of known resistors and in that way indicate between which two values the unknown resistor lies. The circuit is based on the well- known 555 which is connected as an oscillator (astable multivibrator). The output of the oscillator is used to 150n 10n 13, Toko PB2720 0 drive a piezo electric buzzer. The frequency of the oscillator is in- versely proportional to the value of Rx (the unknown resistor) and is determined from 1 f Hz in ohms, and C2 is in farads. By substituting one or two of the known resistors for Rx, the note emitted by the buzzer should give a fair indication of the approximate value of Rx. Of course, if you have perfect pitch, you do not need the known resistors . . . In that case, we"ll tell you that if Rx = 0, the fre- quency is abou...