An object proximity sensing system comprises an oscillator 1 which includes a sensor 4, the capacitance of which changes in response to the proximity of an object to be sensed, such a change in capacitance causing a corresponding change in the oscillator output. The output P1 of the oscillator is applied to a discriminating circuit 2 the output level of which depends on the rate of change of the oscillator output and hence on the speed at which an object is approaching or receding from the sensor. The output P5 of the discriminating circuit is then applied to an output circuit 3 in which it is compared with a threshold value to set the detection level of the sensing system such that slow variations in the oscillator output due for example to environmental changes do not cause erroneous operation of the sensing system. <IMAGE>
