The logic level tester is a measuring device for showing the logic level of a trace signal. With a potentiometer you can control the logic level threshold.
The logic level tester is not replacing the good measuring instruments, but can help troubleshooting since it is displaying HIGH-, LOW-, and alternating logic levels. A 7-segment display on its side is used as display.
Everything is built with a full-NOR circuit where four NOR-gates are used as edge trigger for the vertical bar (needed for the alternating pattern) since this one only is used when going from LOW to HIGH or the other way round. At HIGH level T1 is activated and segment b is on. An LOW level T2 is activated and segment e is on.
Since this circuitry is in full-NOR you dont need AND- OR- and NOT- ICs. Everything is done with (in this case) 2 chips with 4 NOR gates each.
The diodes at the input are preventing the circuit from getting damaged by higher input voltages.
Following the schematic and a few pictures: