Using the LTC1042 chip, manufactured by Linear Technology, can be designed a TTL signals monitor circuit. LTC1042 is a window comparator that can work with some very small currents.
Comparison is set with reference diode D1. Reference voltage of 2.5 V supplied by the diode is connected directly to pin 2 (center window). And window width is set with the help of the reference voltage. Since the assembly is to monitor a power circuit TTL (5 V).
Transistors T1 and T2 control LED indicators. When D2 light, means that installation and power supply is working properly. Glowing D3, means that the supply voltage is too high. If there is on any of the two diode voltage drops too low, or even completely absent.