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. The resonator is arranged to be exposed to an environment under test, and the resonator and the impedance network are connected in parallel with the electrical amplifier such that when a load imparted to the surface exceeds a pre-determined threshold value the oscillator circuit oscillates at the resonant frequency of the impedance network and when a load imparted to the surface is less than the threshold value the oscillator circuit oscillates at the resonant frequency of the resonator, switching of the circuit oscillation frequency being indicative of the load crossing the predetermined threshold value.