I was experimenting with a 74LS14 Hex-Schmitt-Trigger-Inverter (datasheet). I notice that with the input pins are not connected, the output pins are low.
Furthermore when I had a 10k resistor in series with an input, it wouldn't react to any voltage changes at all. To be more precise: when I connected an input to GND via the 10k resistor, the corresponding output stayed low. Then I thought maybe it has some internal pullup resistor, but when I measured the input it was actually at 0V.
Then I replaced this resistor with a potentiometer (as a variable resistor between input and GND) and noticed that the output would go high if the potentiometer was below about 3.5k and it would go low if the pot was about 13k. But during the whole time I measured 0V at the input of the device.
Why is this happening? I thought as long as the input is at 0V the output should go high. And I couldn't find anything about that in the datasheet.