Abstract: An interlock switch for the service door of a computer. The switch prevents power from being automatically resupplied to the computer if the door is opened while power has been interrupted, even if the door is subsequently closed. The interlock switch includes a magnetic reed switch, an electromagnet for closing the reed switch contacts when the power-on switch of the computer is actuated, and a permanent magnet which holds the reed switch contacts closed as long as the service door has not been opened. The reed switch and electromagnet are mounted on the frame of the cabinet of the computer. The permanent magnet is mounted on the edge of the service door adjacent the reed switch. The use of the switch in a security system is also disclosed.

Abstract: A method and circuit for detecting a fault condition in a voltage distribution bus which includes sensing a first voltage drop across a test resistor wired in series with the bus, sensing a second voltage drop across a second resistor wired in series with the bus and comparing the first and second voltage drops to detect a fault condition in the voltage supply connected to the bus.

Abstract: A relatively inexpensive electronic delay timer particularly suitable for portable use in such applications as control of a camera shutter. The timer can be used in either a leading or trailing mode and employs a small 6V battery as a power source. An integrated circuit timer device is arranged to provide an output signal at the end of a time delay period which can be variably selected by means of a built-in capacitive-resistive network and which is arranged to start in response to a movement of an external control switch. The output signal triggers a second time period of predetermined duration in a second integrated circuit timer during which time a second output signal is generated thereform which is used to activate a transistor device, allowing a capacitor in the circuit to discharge therethrough and through a solenoid which is mechanically coupled to the camera shutter. When the second output signal ceases, at the end of the second time period, the transistor is deactivated and the capacitor recharges.