The following Logic Probe Circuit uses a single CMOS IC. No LED will glow if the input of the probe is in hi or lo state (the high impedance state of tri-output logic ICs). The logic probe below shows the three logic condition, the High, Low, and Pulsing.
Logic Probe with Pulse Indicator Circuit Schematic Diagram
Power from the logic circuit under test is used for this logic probe. CMOS IC enables logic circuit is used to be tested using voltages from 3 to 15 volts. IC1 acts as a buffer. The gate will oscillate under no input due to feedback from the 2M2 resistor, since the probe not connected to a single circuit. IC1a has an output half of the supply voltage. Hi and Lo LED’s logic indicator connected to a potential divider which consists of two 1K ohm resistors. Junction voltage is half supply voltage or high impedance no LED’s will glow, when there is no input. IC1a will rest in a permanent state due to Hi and Lo condition indicated by the LED illuminating Hi or Lo. The LEDs will light dimly with a fast oscillator of clock signal, which is the reason for IC1b and IC1c, which determine the time constant. IC1d works as a buffer to drive the pulsing LED. [Source : www.hobbyprojects.com]