Logic probe

The probe indicates a high or low at 70% and 30% of V+ (5 to 12 V). One section of the voltage comparator (LM393) senses V in over 70% of supply and the second section senses V in under 30%. These two sections direct-drive the appropriate LEDs. The pulse detector is a CMOS oneshot (MC14538) triggered on the rising edge of the LM393 outputs through 1N4148 diodes
Logic probe - schematic

With the RC values shown, it triggered reliably at greater than 30 kHz on both sine and square waves.

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