This circuit is a dual low/floating/high logic state detector which enables you to see the logic state of a digital signal pin. Each detector has two LEDs (typically a red and a green one) which indicate the state: red LED for high state, green LED for low and no LED for floating.
Features and attributes:5V version; for a 3.3V version you just need to change R8 and R14 for LED current limitation.
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Low/high thresholds adjustable via R13.
Frequencies up to 10kHz are properly handeled (low and high LED on), higher frequencies (especially above 100kHz) are displayed like floating state (no LED running).
Cheap: The complete circuit (two detectors) costs less than 5 Euro (equivalent $5).
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