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.
• 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).

The design is easily explained: National's LMC6484 is a comparatively cheap rail-to-rail amplifier with guatanteed operation down to 3V. One DIL-14 package contains 4 amplifiers - just what is needed for two independent detectors.

Using R6 and R7, the input is biased at 2.5V (half supply voltage). In case the signal pin connected to the measure input is H/L, the voltage at the left side of R15 will rise/fall. Using two amplifiers in comparator mode, this potential is compared to the threshold values available through the voltage divider R11, R13 and R12. R13 adjusts the potential difference between upper L limit and lower H limit. Voltages in between are considered heigher H nor L and hence floating. (R15 is used as input protection for the amplifier as suggested by the data sheed. Hence, it should not be harmful for the device to connect the measure pins when no power is applied.)

R14 is used to limit the current through the LEDs; for 5V and low current LEDs (2mA), I use 560 Ohm resulting in about 4.5mA LED current. Adjust that for 3.3V operation and/or different LEDs.