An audible indicator circuit including a piezoelectric crystal and transistor device, that is of extremely simple and inexpensive construction and has its on-off operation selectively controlled in response to an automatic or manually applied control signal. The piezoelectric crystal is connected between the collector and emitter electrodes of the transistor, so that with an "on" control signal applied to the base electrode and a supply voltage of magnitude that exceeds the collector to emitter breakdown voltage coupled to the collector electrode, the circuit is caused to oscillate and thereby emit an audible signal.
An excitation circuit wherein the energy of a charged coil is discharged via a sound generator. A coil and a capacitance of the sound generator are tuned to one another such that a corresponding oscillatory frequency is identical to a predetermined resonant frequency of the sound generator.
A piezoelectric transducer frequency self-calibration system interrogates the transducer by driving it with various frequencies within a specified range while monitoring the transducer output power level. Two threshold frequency points are identified at which the output power level is at a predetermined threshold level less than the expected maximum output level, and the average of these two threshold frequencies is taken to be the optimum frequency.
A microwave oven includes a piezo-electric speaker for audible announcement purposes and a microcomputer for controlling operations of the microwave oven. The mocrocomputer always develops a frequency signal for activating the piezo-electric speaker. A gate signal is developed from the microcomputer so that the gate signal and the frequency signal are commonly applied to the base electrode of a switching transistor. When the gate signal bears the logic "L", the switching transistor repeats the ON/OFF operation in response to the frequency signal applied thereto. The piezo-electric speaker is connected to the switching transistor, whereby the piezo-electric speaker generates the sound whenever the gate signal bears the logic "L".
An electronic chime is created by a piezoelectric transducer with an exponentially decaying audio signal. The electronic chime has a power supply, an oscillator, a drive circuit, and a piezoelectric transducer. The drive circuit contains reactive components that function with the piezoelectric transducer to modulate the amplitude of the decaying audio signal. The amplitude modulated audio signal better simulates a mechanical chime.
An audible test device which provides a tone when a resistance being measured is less than a selected value, the selected value being variable to a few milli-ohms. A first or second source of voltage is applied to a constant current source. The constant current generated is divided between two paths, one path including test leads holding the resistance being measured therebetween, the other path through a voltage adjusting device. The output from the voltage adjusting device is applied to one input of a voltage comparator, the voltage appearing across the resistance under test being applied to the other input of the voltage comparator. The output of the comparator is applied to a voltage controlled oscillator (VCO), the output of the VCO being applied to a piezoelectric device and to a speaker operable only when the second voltage source is utilized, thereby generating an audible tone. To set the threshold below which the audible tone will be generated, test leads are placed across a known resistance value equal to the threshold desired and the voltage adjusting device is adjusted until a tone is heard. This occurs only when the output of the voltage adjusting device exceeds the voltage across the resistance under test, the output of the comparator causing the VCO to generate an output. Thereafter, when the test leads are placed across a circuit path or resistance under test having a resistance value below the set desired value, a tone will be emitted.
