Abstract: In accordance with the present invention there is provided a bi-stable oscillator circuit for detecting a load imparted to a surface. The bi-stable oscillator comprises an electrical amplifier, at least one resonator comprising an electrical transducer having a resonant frequency, a surface of the resonator forming the surface on which the load is to be detected and an impedance network having a resonant frequency.

Abstract: In accordance with the present invention there is provided a bi-stable oscillator circuit for detecting a load imparted to a surface. The bi-stable oscillator comprises an electrical amplifier, at least one resonator comprising an electrical transducer having a resonant frequency, a surface of the resonator forming the surface on which the load is to be detected and an impedance network having a resonant frequency.

Abstract: An ocscillator circuit having: a) a piezoelectric crystal connected to a surface; b) a variable frequency generator for generating a driving signal which is supplied to the crystal to cause the crystal to oscillate, thereby causing the surface to oscillate; and, c) an analyser for monitoring the phase shift between the voltage across the crystal and the current flowing through it and, in response generating an adjustment signal which relates to the difference between the oscillation frequency and a resonant frequency of the crystal, the variable frequency generator being responsive to the adjustment signal to vary the frequency of the driving signal to cause the crystal to oscillate at the resonant frequency.

Abstract: An apparatus for oscillating a surface is described. The apparatus comprises an oscillator circuit having: a) a piezoelectric crystal connected to the surface; b) a variable frequency generator for generating a driving signal which is supplied to the crystal to cause the crystal to oscillate, thereby causing the surface to oscillate; and, c) an analyser for monitoring the phase shift between the voltage across the crystal and the current flowing through it and, in response generating an adjustment signal which relates to the difference between the oscillation frequency and a resonant frequency of the crystal, the variable frequency generator being responsive to the adjustment signal to vary the frequency of the driving signal to cause the crystal to oscillate at the resonant frequency.