The difficulties inherent in the design of small portable, battery-powered, infra-red detectors employing photoconductive cells are discussed, with particular reference to the signal-to-noise ratio of suitable electronic amplifiers, and the method employed to "chop" the radiation. The main novelty is the use of an electromagnetically-maintained vibrating leaf-spring with a shutter suitably mounted at its free end as a radiation chopper. The design and applications of this device are discussed in some detail.