digital-logicinputoperational-amplifiervoltage
I have 3 inputs. Input #1 is low bias. I can hook up a 12V positive to input 1 and it will change it's state to 1 (or input is ON).
Inputs #2 and #3 are high bias. Connecting a 12VDC to either of these inputs causes no change in state.
The description for these two inputs describes +12v via 100kΩ res, with a 2V threshold. I can't change the "high" state on these two inputs.
What kind of input is expected on inputs 2 and 3?
For 74LS logic, 1.4 volts is an "undefined" value. An input must be below 0.8 volts to guarantee it will be accepted as a Low, and above 2.0 volts to be a guaranteed High.
The inputs will source 0.8 mA when taken low, so your op-amps must be able to sink that current while keeping their output voltage below 0.8 volts.
The simplest way to eliminate your problem is to reduce the effective impedance of the source, and a voltage follower will do nicely
simulate this circuit – Schematic created using CircuitLab
The choice of buffer op amp is left to you, depending on the frequency response you need.
Best Answer
Those two inputs are internally pulled up to the voltage rail using 100k resistors. In this case, you want to connect them to ground to activate the input. I'm unsure if you need to 1) pull them down 2V (to 10V), or 2) have to pull them down to 2V or lower.
If you wish to do this with a microcontroller that doesn't have a suitable output that runs from 0V to 12V, you can use an npn transistor or an n-channel mosfet.
This type of input is also referred to as "active-low".