74LS19 Hex Schmitt-Trigger Inverter PDIP-14
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Each circuit functions as a NAND gate or inverter, but because of the Schmitt action, it has different input threshold levels for positive-going (V_T+) and for negative-going (V_T-) signals. The hysteresis or backlash, which is the difference between the two threshold levels (V_T+ - V_T-), is typically 800 millivolts.

These circuits are temperature-compensated and can be triggered from the slowest of input ramps and still give clean, jitter-free output signals.

Datasheet: https://www.ti.com/lit/ds/symlink/sn74ls19a.pdf