Fault indication for seven element LED display - compares element voltages of two independent displays with time shared values passed to logic gate tree

摘要

With two independent 7-element LED displays, driven by the same data input, a fault in either of them is detected by comparing the voltages on corresponding elements using 7 comparison circuits. A significant difference detected by any of the 7 cirtuis lights a 'fault' lamp. The comparison circuits themselves are checked by switching off the data and applying a test voltage to each of the 14 inputs in turn, using a shift register. Failure to detect the test voltage lights a fault lamp. The digits of a multi-digit display are time shared. The time sharing voltages are compared in a logic gate tree to detect if any two overlap. If so, a fault lamp is lit. If only one display is needed, the second may be a dummy, formed of resistors and diodes.