This TTL and CMOS signals tester circuit is very simple logic tester that allows analysis of TTL and CMOS signals. Circuit diagram shows that T1, R3, R4, D5 and D6 form a current source for LEDs D3 and D4. As a result, the current through the LED will be about 12 mA, regardless of supply voltage, LED cathodes are connected to ground either N1 or N2, respectively.
If a relatively high voltage is applied against the table at this terminal, N1 will reverse this high mass and connect to the cathode of D3. D3 will light and will indicate a logic "1" while D4 remain isolated as a cathode or potential. He does not light until the end of the ICI R1 does not apply very low voltage (less than 1 / 3 of supply voltage), in which case the voltage will be reversed than twice before being applied to the cathode of D4.
High impedance input resistance (R2) limits the state you applied load circuit. Also. it decouples the inverter N1 when entering the first test input is disconnected. This prevents the circuit to oscillate, regardless of the input interference.