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# Is there such a thing as a circuit that outputs 1 if the input is high-impedance, and 0 otherwise?

June 6Hits:1

I am trying to design a circuit that outputs a logical 1 if the input is high-impedance, and a 0 otherwise.

Any idea how I might implement this? I would prefer to use off-the-shelf parts (no programming).

Truth table:

````I|O --- X|1 0|0 1|0 `
```

I was thinking something along the lines of a pull up resistor with an NPN transistor. Thoughts?

A simple window comparator will do what you want.

The input is tied to midvoltage by the two 1Mohm resistors. The references are set at 1/3 and 2/3rds of the supply voltage. LM339 is open-collector output, so can be OR'ed together to get the desired truth table.

Please notice that this circuit will not detect a low-impedance connection to half the supply voltage, only low impedances to high or low levels.

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