Control circuit for the shutter holding magnet of a camera

Claim 1

In a camera circuit of the type having a pair of terminals adapted to be connected to a power source, an electromagnet energizing coil for energizing an electromagnet to hold a shutter screen in one position, a comparison circuit for providing an ON output and an OFF output, respectively, to control the energization and deenergization of said coil during normal operation of said camera circuit, the improvement comprising, a voltage check circuit connected to said power source for providing an ON output when the voltage across said pair of terminals is at or above a predetermined voltage for operating said camera circuit and an OFF output when the voltage across said pair of terminals is below said predetermined voltage, and AND function circuit means having two inputs connected respectively to the outputs of said voltage checking circuit and said comparison circuit, said AND function circuit means having an output connected to said coil for energizing said coil when both said voltage check circuit and said comparison provide ON output voltages to said respective two inputs of said AND function circuit means and for otherwise deenergizing said coil, said AND function circuit comprises, first through fourth transistors, said fourth transistor having its collector-emitter terminals connected in series with said coil, said first transistor having its collector-emitter terminals connected in series with a first resistance across said pair of terminals, said second and third transistors having their collector-emitter terminals connected in series across said pair of terminals, the output terminal of said first transistor being connected to the input terminal of said third transistor, and the output terminal of said third transistor being connected to the input terminal of said fourth transistor, the input terminals of said first and second transistors being connected to the outputs of said comparison circuit and voltage check circuit, respectively, for turning on said first and second transistors only in response to respective ON outputs therefrom, whereby ON outputs from both circuits result in said third transistor being cut off and said fourth transistor being conducting, and a visual alarm circuit connected to the output of said AND function circuit means for providing a visual indication of energization state of said coil.