A capacitance presence sensing apparatus (10) having a fail-safe circuit (13) and modular units (100). The fail-safe circuit (13) includes delivering a low voltage (FIG. 5) to a sensor and connecting voltage through a resistor to an oscillator (18). An opener ground in the circuit changes frequency of the oscillator (18) to indicate a problem With the fail-safe circuit (13). The apparatus also contains a number of inductance coils (28) which may be selected to tune the frequency of the system. Circuitry is provided in cards which may be combined together with receiver transmitter cards (106) and to easily facilitate the use of the system for desired applications.