This signals logic tester indicate on a common cathode seven-segment display, if entry is at logic level "1" (one H on the display) or logic level "0" (one L on the display). If an undefined level is detected will display an "n". When the input is "0", T1 locks and T2 and T3 drive. A level of output X IC1a and "0" on that of IC1b therefore segments d, e and f will be lit.
When entry is logical "1" is saturated T1 and T2 and T3 are blocked. IC1a's output becomes "0" and that of IC1b becomes "1". Besides segments e and f, already lit will light b, c and g, which results in displaying an "H".
When entry is in an undefined state, or is not connected, all transistors are conduction (due to R1, R2 and R3) and the icon displayed is a "n" (undefined).
Switching thresholds of the tester are 1 V and 3 V (3 V threshold may be reduced slightly by increasing the resistance R4).
Input impedance tester is 5 k, so it does not affect the circuit being tested.