A positioning apparatus (30) for a workpiece includes a base (310) with a shoulder (312), a positioning block (330) and a circuit. The positioning block includes an upper block (332) secured on the shoulder, and a lower block (334) having a front surface (336). A lay of insulator (320) is compressed between the base and the positioning block. The circuit includes a power source (50) and a relay (60). The power source provides direct current voltage. The relay comprises a coil (601) electrically connected between the power source and the positioning block, and a normal-open switch (603) electrically connected between the power source and a manipulator which acts on the workpiece. When the workpiece contacts the front surface of the positioning block, the coil is electrified and the normal-open switch closes. The interface of the manipulator turns to high level, and the manipulator starts its next programmed motion.