Abstract: For sealing a pipeline at selective locations in order to perform leak tests, a pipeline packer assembly capable of internal travel within the pipeline can be moved to and stopped at selected locations and the packer inflated against the interior pipe walls to prevent further movement of the unit, and to provide a seal so that the pipeline may be subjected to internal high pressures to detect leaks. According to the invention, a control device for such unit comprises a gamma ray detector and signal processor receiving the electrical analogue signal from the gamma ray detector. An energizing signal from the signal processor actuates a stepping switch which provides "inflate" and "deflate" energization of the packer, and is kept in "off" condition while the packer is inoperative as in transit within the pipeline. A radiation source on the surface or selectively inserted into a bore hole near the pipeline provides a stimulus for the gamma ray detector which controls the operation of the stepping switch.