Abstract: An intrinsically safe power supply employing a binary current interrupter connected between the power source and the electrical load. Normally the load is situated in a potentially dangerous environment, like a coal mine, and the intrinsically safe power supply at a safe remote location, e.g., on the earth's surface. The interrupter has a pass and switching transistor, current sensor, gating circuitry, a flip-flop switch, and means to delay the turning on of the transistor--but not its turning off. If an overcurrent or overvoltage condition is sensed between the input and output of the intrinsically safe power supply, load current will cease to flow. In normal operation, only the current interrupter pass transistor will open. A reset signal from an oscillator internal of the interrupter may be used to reset the flip-flop after actuation or upon its initial setting, thereby causing the flow of power into the load.

Abstract: In an educational television system, a constructed answer system is based on the use of code pulses that are repeated cyclically during their concealed transmission within a television video waveform. Indicia, e.g., letters of a word or formula, are preloaded into a store in the form of binary code as an output from a punched tape reader or an encoder for a keyboard. The encoder is reset following each transfer of encoded indicia to the store which is indexed by a shift register to render responsive successive columns of flip-flops in a matrixed arrangement. The store includes a column of AND gates to store a permanent "recycle code". The reading of the store is clocked by modified sync pulses applied by a shift register to a matrix arrangement of interconnected AND gates that are also connected to the output of the flip-flops. The readout of the store is repeated cyclically by a logic register that compares the number of encoded indicia in the store with the number of encoded indicia read from the store.