Logic probe - instant logic level indication

The circuit diagram shows perfectly clearly that T1 together with R3, R4, D5 and D6 constitute a current source for LEDs D3 and D4. As a result, the current to the LED will be approximately 12 mA, irrespective of the operational voltage. The LED cathodes are grounded by either N1 or N2 enabling. The LEDs are switched on and supplied by a constant current. The circuit"s other task depends on the voltage applied to the disconnected end of R 1. If, for instance, a relatively high voltage with respect to the ground potential is applied, N1 will invert the "high" level, grounding the cathode D3. D3 lights to indicate a logic "1". But D4 remains unlit, as its cathode is "high". It won"t light until a very low voltage (less than 1/3 of the supply signal) is applied to R1, in which case the "low" level will be inverted twice before reaching the cathode of D4. R1, D1 and D2 protect the circuit against an input overload. The high-impedance 10 MS2 input resistor (R2) limits the load to the c...