Simple Logic Probe

This simple logic probe has both LEDs on with no signal at the input but due to the nor gates connected to the probe, indicates correctly when a high or low signal is present. It also works correctly for pulse trains. Normally both LEDs are forward biased and therefore on, powered by the 12V supply. When a logic high  is present at the probe, IC
Simple Logic Probe - schematic

1a`s output goes low sending IC1b`s output high. This turns off LED1 but forward-biases (and turns on) LED2. Conversely, a logic low  at the probe will send IC1b low, turning LED1 on and LED2 off.

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