Abstract: A sensor system for ink jet printers provides an output signal selectively indicative of the passage of an electrostatically charged ink drop, and of the proper operation of the sensor. The sensor includes a plurality of spaced conductors, between which the ink jet stream passes. During normal operating mode, at least one of the conductors is connected to a reference voltage for shielding the other conductors from electrical noise, conditioning the other conductors to generate an output signal induced by capacitive coupling of a charged ink drop. During test mode, at least one conductor is connected to signal generator for capacitively inducing a test signal into the other conductors to generate an output signal indicative of proper operation of the sensor.