Overview

The 74LS14 (SN74LS14) is a monolithic Low-Power Schottky Transistor-Transistor Logic (LS-TTL) IC containing six independent inverting buffer gates with Schmitt-trigger inputs. Each gate functions as an inverter with built-in input hysteresis.

Because the input switching threshold differs for positive-going signals ($V_{T+} \approx 1.6\text{ V}$) and negative-going signals ($V_{T-} \approx 0.8\text{ V}$), the 74LS14 exhibits a typical hysteresis voltage of $V_H \approx 0.8\text{ V}$. This hysteresis transforms slowly varying, noisy, or corrupted analog inputs into sharp, jitter-free square-wave digital signals without false output transitions or oscillation. It is a staple component in retrocomputing, arcade boards, button debounce circuits, and simple RC relaxation clock generators.

Quick reference

Pinout (DIP-14 Package)

             ┌───┴───┐
          1A 1│ 1   14│ VCC
          1Y 2│       │13 6A
          2A 3│       │12 6Y
          2Y 4│ 74LS14│11 5A
          3A 5│       │10 5Y
          3Y 6│       │9  4A
         GND 7│       │8  4Y
             └───────┘

Specifications

Typical Applications

1. Push-Button Debounce Circuit

Mechanical switches bounce rapidly for $2\text{ ms} \dots 10\text{ ms}$ upon contact. The RC filter absorbs rapid bounces, while the 74LS14 Schmitt trigger provides a clean, bounce-free digital transition:

      +5V
       │
      [ R1: 10kΩ ]
       │
 Switch ───┬────────[ R2: 1kΩ ]───► [Pin 1: 1A]
 (to GND)  │                         74LS14
        [ C1: 100nF ]              [Pin 2: 1Y] ──► Clean Debounced Logic Signal
           │
          GND

2. Single-Gate RC Relaxation Oscillator

            ┌─────[ R = 10kΩ ]─────┐
            │                      │
            ├───► [Pin 1: 1A]      │
            │       74LS14         ├────► Square Wave Clock Output
            │     [Pin 2: 1Y] ─────┘
        [ C = 100nF ]
            │
           GND

Frequency: $f \approx \frac{1}{0.8 \times R \times C}$ (Keep $R \le 10\text{ k}\Omega$ to accommodate TTL input bias current).

Common mistakes

• Using too large a feedback resistor in RC oscillators: Unlike CMOS gates, a 74LS input draws up to $-0.4\text{ mA}$ when below $V_{T+}$. If $R > 10\text{ k}\Omega$, this input current will prevent the capacitor from discharging down to $V_{T-}$, stopping oscillation. Keep $R$ between $330\ \Omega$ and $4.7\text{ k}\Omega$ (or use a 74HC14 if large megohm resistors are needed).
• Driving 5V CMOS directly without a pull-up resistor: 74LS outputs only reach $V_{OH} \approx 3.4\text{ V}$. When interfacing with a standard 5V CMOS input ($V_{IH} \ge 3.5\text{ V}$), add a $2.2\text{ k}\Omega$ pull-up resistor to $+5\text{V}$.
• Exceeding the 5.25V supply rail limit: Operating above 5.5V will cause severe overheating and damage the bipolar transistors.

Notes

• Pin-Compatible Modern Equivalents: 74HC14 (CMOS Schmitt inverter), 74HCT14 (TTL-compatible CMOS Schmitt inverter), 74AHCT14.