PAGE 075 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Schmitt Trigger Basics
Hysteresis gives clean switching from noisy inputs.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Two thresholds
A Schmitt trigger input switches high at one voltage and low at a different, lower voltage. The gap between them is hysteresis.
Why it helps
- Slow or noisy signals produce clean, sharp edges.
- It prevents chatter around a single threshold.
Uses
A 74HC14 inverter with a resistor and capacitor makes a simple oscillator that can blink an LED. A button with a capacitor and the same chip gives a tidy debounce. At 5 V the thresholds are roughly 2.7 V rising and 1.8 V falling, though exact values depend on the chip.
REMEMBER
A transistor lets a tiny signal control a much larger LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
Explore LED clocks, stopwatches and electronics at cosycom.com.