Photo-scr card reader circuits

Claim 1

Electronic apparatus for providing a binary electrical output corresponding to the presence or absence of incident light, comprising: a. a binary memory light sensing device for sensing the presence or absence of incident light, having a high impedance state when no light is incident upon it, and switchable to a low impedance state when light is incident upon it, said low impedance state being maintained in the absence of incident light when current through said device is maintained above a predetermined level; b. semiconductor switching means having its input terminal connected in series with said sensing device; c. logic gate means connected to said light sensing device and said semiconductor switching means for deriving an electrical output in accordance with the impedance state of said light sensing device; d. said semiconductor switching means shunting said sensing device so that the current flow through said sensing device is limited to a level below said predetermined level when light is incident upon said sensing device; e. power supply means providing power to said logic gate means; and, f. said semiconductor switching means further comprising a transistor, and said light sensing device comprises a photo-SCR having an anode, cathode and gate, said cathode being connected to the base of said transistor, and said gate being resistively coupled to the emitter of said transistor, said photo-SCR having a holding current which must be maintained to keep said photo-SCR in a low impedance state in the absence of incident light, said transistor cooperating with said logic gate means to provide a low impedance shunt across said photo-SCR so as to maintain the current through said photo-SCR at a level below said holding current when light is incident upon said photoSCR, and to turn off said photo-SCR when no light is incident upon it.