Abstract: A housing for an electrical circuit is to be releasably mounted in an explosion-proof box without modifying the structure of the box. The housing is provided with at least one spring which extends away from the housing into engagement with an inner surface of the box. This engagement between the spring and the inner surface of the box deflects the spring, exerting force on the housing in such a way as to resiliently hold the housing inside the box.

Abstract: A two-wire frequency to DC converter utilizes a multistage binary counter which is controlled by a crystal oscillator and the input frequency. This arrangement provides precisely shaped square wave pulses at the output of the counter whose frequency is functionally related to the input frequency and which can be fed into a low pass filter to obtain an average DC value for the AC input. The output terminal at each stage of the binary counter has a plurality of open terminals, each associated with a particular frequency. By connecting a single diode across a selected open terminal, the converter which is constructed without regard to any input frequency, so it can be mass-produced, can be easily customized for a particular input frequency range.