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🔌 Logic Levels and Thresholds

In digital electronics, we don’t care about exact voltages.
We only care whether a signal is interpreted as:

  • LOW (0)
  • HIGH (1)

But real voltages are analog, not perfect 0s and 1s.
So logic levels and thresholds define where LOW ends and HIGH begins.

📏 Logic Voltage Levels​

🟢 5V Logic System (Classic Arduino, TTL/CMOS)​

  • LOW (0):
    0 V→0.8 V0\text{ V} \rightarrow 0.8\text{ V}

  • HIGH (1):
    2.0 V→5.0 V2.0\text{ V} \rightarrow 5.0\text{ V}

  • Typical output HIGH:
    4.7–5.0 V4.7\text{–}5.0\text{ V}

🔵 3.3V Logic System (ESP32, STM32, modern MCUs)​

  • LOW (0):
    0 V→0.8 V0\text{ V} \rightarrow 0.8\text{ V}

  • HIGH (1):
    2.0 V→3.3 V2.0\text{ V} \rightarrow 3.3\text{ V}

⚠️ The Gray (Undefined) Zone​

Between LOW and HIGH is a danger zone where the logic level is undefined.

Example (5V system):​

  • LOW guaranteed: 0–0.8 V0\text{–}0.8\text{ V}
  • HIGH guaranteed: 2.0–5.0 V2.0\text{–}5.0\text{ V}

👉 Gray zone:
0.8 V→2.0 V0.8\text{ V} \rightarrow 2.0\text{ V}

In this region:

  • The input may randomly read HIGH or LOW
  • Noise can flip the logic state
  • Circuits behave unpredictably

❌ Never design signals to sit in the gray zone

🎯 Threshold Voltage​

A threshold is the voltage where a logic gate decides:

  • Above threshold → HIGH
  • Below threshold → LOW

For CMOS logic (roughly):

Vthreshold≈VCC2V_{\text{threshold}} \approx \frac{V_{CC}}{2}

Example (5V CMOS):​

Vth≈2.5 VV_{th} \approx 2.5\text{ V}
  • Input = 3V → HIGH
  • Input = 2V → LOW

🛡️ Noise Margin​

Noise margin tells how much noise a signal can tolerate without failing.

HIGH Noise Margin:​

VOH(min)−VIH(min)V_{OH(min)} - V_{IH(min)}

LOW Noise Margin:​

VIL(max)−VOL(max)V_{IL(max)} - V_{OL(max)}

Example:​

  • Guaranteed HIGH output: 4.7 V4.7\text{ V}
  • Threshold: 2.4 V2.4\text{ V}

Noise margin:

4.7−2.4=2.3 V4.7 - 2.4 = 2.3\text{ V}

👉 Up to 2.3V of noise won’t break the logic.

🧱 TTL vs CMOS Logic Levels​

Logic FamilyLOWHIGHGray Zone
TTL (5V)0–0.8V2–5V0.8–2V
CMOS (5V)0–1.5V3.5–5V1.5–3.5V
CMOS (3.3V)0–0.8V2–3.3V0.8–2V

🔀 Mixing Logic Levels (Common Mistake)​

Example:​

  • Arduino output: 5V
  • Sensor output: 3.3V
  • Arduino input threshold: ~2.5V

Result:

  • 3.3V might be HIGH
  • But noise margin is small
  • Not safe for industrial or noisy environments

👉 Solution:
Use:

  • Level shifter
  • Voltage divider
  • Logic buffer

❌ Common Beginner Errors​

  • Feeding 3.3V logic into strict 5V TTL inputs
  • Letting signals float (no pull-up / pull-down)
  • Long wires without buffering → noise
  • Assuming “almost HIGH” is good enough

✅ Design Rules of Thumb​

  1. Always drive signals fully HIGH or fully LOW
  2. Avoid gray zones completely
  3. Check datasheet thresholds, not assumptions
  4. Add pull-up or pull-down resistors
  5. Use level shifters when mixing voltages
  6. Short wires for digital signals, buffer long ones

🏁 The Bottom Line​

  • Digital signals are not exact voltages
  • They have safe LOW zones, safe HIGH zones, and danger zones
  • Logic thresholds decide the boundary
  • Noise margin keeps systems reliable

Stay out of the gray zone,
respect logic thresholds,
and your digital circuits will work every time.