Abstract: A binary multi-code combination sensor employs a magnetically coded transmitter and corresponding receiver to add a level of security to conventional low-security alarm systems. The transmitter includes a pattern of magnetic and non-magnetic portions. The receiver includes a series arrangement of magnetically sensitive switches wherein normally open switches are positioned to be held closed by the magnetic portions of the transmitter and normally closed switches are positioned opposite the non-magnetic portions of the transmitter. Added security is provided by the normally closed switches which open in response to attempted tampering using an external magnetic field. Coded sensors increase the difficulty of successfully defeating the sensor using an external magnetic field.