A lock for the ignition circuit for an automobile including a Hall effect control sensor which is activated by a control magnet on the cylinder member of the lock so that when the cylinder is rotated by the key to its start position, the control sensor is activated. The activation of the control sensor provides one of several starting criteria to an ignition for starting an automobile. An anti-tampering sensor spaced from and adjacent to the control sensor has a trip level which is lower than the trip level of the control sensor so that magnetic forces induced from the exterior of the lock cannot trip the control sensor. A blocking plate is disposed between the control sensor and the anti-tampering sensor to prevent the anti-tampering sensor from being tripped by the control magnet when the cylinder member is rotated to its start position and to enable the anti-tampering sensor to be tripped first to produce a deactivation signal in response to a magnetic force induced from the exterior of the lock.