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TONAL_VOLTMETER

2014-11-24 08:19  
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TONAL_VOLTMETER
The circuit is based on a CD4046B phase-locked loop (PLL) IC with a built-in voltage-controlled oscillator (VCO). The frequency of the VCO is determined by the voltage at pin 9, capacitor C1 between pins 6 and 7, and potentiometer R2 at pin 12, which sets the tonal voltmeter's minimum frequency. The VCO output, which appears at pin 4, is normally fed back into the comparator in-put at pin 3. However, in this circuit, the VCO output drives a speaker directly. The VCO operates while the inhibit line (INH) at pin 5 is held logic low, and it turns off when INH is logic high. A touch switch consisting of two Schmitt-trigger inverters (IC2-a and IC2-b) turns the circuit on and off to conserve power when the TVM is not being used. The touch switch can be replaced with a standard SPST switch, if so desired. This is recommended if you don't have a Schmitt-trigger inverter and don't want to purchase one. The tonal voltmeter emits a low tone when reading a logic low. As the voltage input increases, the tone pitch increases until the input voltage reaches a logic high. As the voltage input decreases, the pitch decreases.