Inside the 555: Comparators, Flip-Flop & Discharge Transistor
- [Trigger comparator] [sets] [flip-flop when pin 2 drops below 1/3 VCC]
- [Threshold comparator] [resets] [flip-flop when pin 6 rises above 2/3 VCC]
- [Discharge transistor] [connects] [pin 7 to GND when output is LOW to discharge timing capacitor]
| Pin | Name | Function |
|---|---|---|
| 1 | GND | Ground reference |
| 2 | Trigger | Sets output HIGH when below 1/3 VCC |
| 3 | Output | Main output: HIGH or LOW, up to 200mA |
| 4 | Reset | Active-LOW: forces output LOW (tie to VCC if unused) |
| 5 | Control | Overrides 2/3 VCC threshold (bypass to GND with 10nF) |
| 6 | Threshold | Resets output LOW when above 2/3 VCC |
| 7 | Discharge | Open-collector: discharges timing capacitor when LOW |
| 8 | VCC | Supply: 4.5V–15V (NE555), 2V–15V (TLC555) |
Astable Mode: Self-Oscillating Square Wave Generator
In astable mode, the capacitor C charges through R1+R2 (output HIGH) and discharges through R2 only (output LOW). The output continuously oscillates between HIGH and LOW.
f = 1.44 / ((R1 + 2xR2) x C)
Duty = (R1 + R2) / (R1 + 2xR2) x 100%
Use our 555 Timer Calculator to instantly compute R1, R2, C values for any target frequency and duty cycle.
Frequently Asked Questions
How does a 555 timer work internally?
Two comparators set at 1/3 and 2/3 VCC control an SR flip-flop. When pin 2 drops below 1/3 VCC, output goes HIGH and capacitor charges. When pin 6 exceeds 2/3 VCC, output goes LOW and the discharge transistor on pin 7 drains the capacitor. This charge/discharge cycle creates timing.
What is the frequency formula for a 555 timer in astable mode?
f = 1.44 / ((R1 + 2×R2) × C). Duty cycle = (R1 + R2) / (R1 + 2×R2). Example: R1=1kΩ, R2=10kΩ, C=10µF → f ≈ 6.86 Hz. Use the Volt X 555 Timer Calculator for instant results.
What is the difference between astable, monostable, and bistable modes?
Astable: continuous square wave oscillation (no stable state). Monostable: trigger produces a single timed pulse T = 1.1×R×C, then returns to stable LOW. Bistable: two stable states set/reset by trigger pins — acts as a basic SR latch.
What are the 8 pins of the 555 timer?
Pin 1: GND. Pin 2: Trigger (set). Pin 3: Output. Pin 4: Reset (active LOW). Pin 5: Control Voltage. Pin 6: Threshold (reset). Pin 7: Discharge. Pin 8: VCC (4.5–15V).
Can a 555 timer control motor speed (PWM)?
Yes — in astable mode with a diode bypass for ~50% duty cycle, connect pin 3 to a MOSFET gate (100Ω resistor). The MOSFET switches the motor at the 555 frequency. A potentiometer in place of R2 gives variable speed control without any microcontroller.
Conclusion
Half a century after its invention, the NE555 remains the most versatile and beginner-friendly IC ever made. Three modes — astable, monostable, bistable — cover an enormous range of analog timing applications without any software. Every electronics hobbyist should understand the 555 timer cold.