An electronic personnel safety sensor is provided capable of causing a machine or operation to be shut off to prevent injury to personnel or damage to equipment protected, by detecting, indicating, and acting upon a predetermined increase or decrease in effective capacitance caused by the introduction, or intrusion, or withdrawal of material to or from a field or protection zone created by a suitably designed and connected electrode structure. The sensor is operable in a fail-safe manner inasmuch as it does not contain any single component, part or electrical connection, the failure or removal of which would cause the sensor to permit the protected machine or operation to operate. The sensor embodies circuitry wherein all activated units thereof are connected in series and are furnished an enabling signal from a common source. The initial signal may be provided from a crystal controlled, amplitude regulated oscillator signal generator source and is AC coupled to a modified null electronic capacity sensing type bridge. The latter bridge has two symmetrically disposed arms, one of which contains a fixed reference capacitor and the other of which contains the combination of an antenna and an adjustable capacitance. The latter is used to bring the two arms of the bridge to the "null" set condition that corresponds with normal operation of the protected machine or implied non-penetration of the protection field. The original signal from the oscillator is modified by the bridge according to the state of balance. The portion of the signal passed on by the bridge is larger at a state of unbalance and a minimum but not zero, at optimum balance. This modulated signal is again AC coupled to the next stage, i.e., an operational amplifier via a suitable transformer but at the input of the operational amplifier the signal is processed to produce rectified AC with a polarized or DC component that in a specific manner in combination with the adjusted characteristics of the operational amplifier causes the latter and the following units including a normally open relay which has been moved to a closed contact position to deenergize, i.e., to completely and precipitously to effectively stop amplifying or passing the original signal if the unbalance of the bridge falls outside of prescribed limits.