from www.mitedu.freeserve.co.uk/Circuits/Testgear/lprobe.html (no longer free access)
Their Description;
"This logic probe uses a single CMOS IC and shows the logic conditions, High, Low, and Pulsing. In addition if the probes input is neither high or low (the high impedance state of the tri-output logic ic's) then no LED's will light. Power from the logic probe is taken from the logic circuit under test; using a CMOS IC enables logic
circuits to be tested using voltages from 3 to 25 volts.
IC1a is arranged as a buffer with a difference. With no input, i.e. probe not connected to circuit the gate will oscillate due to feedback from the 2M2 resistor. Output voltage
at IC1a is aprox half the supply voltage. The Hi and Lo logic indicator LEDs are also connected to a potential divider consisting of two 1k resistors. Voltage at the junction is half of the supply voltage hence with no input, or high impedance no LEDs will light.
A high or Lo logic condition will cause IC1a to rest in a permanent state indicated by either the Hi or Lo LED illuminating. With a fast oscillator or clock signal both Hi and Lo LED's will light but dimly."
This is the reason for IC1b and IC1c. These two gates form a monostable oscillator, time constant determined by 100n capacitor and 4M7 resistor. With a clock signal this is effectively slowed as the monostable is continually triggered and re triggered. IC1d acts a sbuffer to drive the pulsing LED."
©Geoff McCulloch, 2003