# Maximal resistance in series with input of 74LS14 Schmitt-Trigger-Inverter

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.

• To consider the allowable source resistance you would have to consider the input current for various conditions. It would appear that your measurement of the input voltage is erroneous. It's unclear what you mean when you say you used a "potentiometer" as a potentiometer is by definition a 3 terminal variable resistor wired as an adjustable voltage divider; that would be quite distinct from using a two terminal variable resistor to connect to a fixed voltage source. Nov 12, 2020 at 19:05
• For 74LS, pay attention to the input voltage thresholds and input currents in the datasheet. For 74HC, it's much less critical.
– user16324
Nov 12, 2020 at 19:23
• @ChrisStratton I used the potentiometer as a variable resistor, to be able to set the resistance between GND and the input. This means I just used two of the terminals, the third was not connected. I will measure the input voltages again, and report back. Nov 12, 2020 at 19:25
• @ChrisStratton I did indeed make a mistake when measuring the input voltage! I was told that IC could be used as a replacement for the CD40106 but apparently that does not apply to every use case. Nov 12, 2020 at 19:44
• @BrianDrummond Thanks, that turned to out to be the issue! Nov 12, 2020 at 19:44