How it works
The 555 has two internal comparators and an SR flip-flop. In monostable mode, a trigger pulse below 1/3 VCC on pin 2 sets the latch, output goes high, and capacitor charges through R toward VCC. When VC reaches 2/3 VCC, the threshold comparator (pin 6) resets the latch, output goes low, and the internal discharge transistor (pin 7) rapidly discharges the capacitor. Pulse width T = 1.1 × R × C; for R = 10 kΩ and C = 10 μF, T = 0.11 s. In astable mode, capacitor charges through R1 + R2 and discharges through R2 only. Frequency f = 1.44/((R1 + 2R2) × C); duty cycle D = (R1 + R2)/(R1 + 2R2) × 100%. To get 50% duty cycle, R1 must approach zero or a diode bypass around R2 is used.
Key points to remember
Eight pin functions: GND (1), trigger (2), output (3), reset (4, active low), control voltage (5, tie to 100nF to GND if unused), threshold (6), discharge (7), VCC (8). Monostable timing: T = 1.1RC. Astable frequency: f = 1.44/((R1 + 2R2)C). Output high time: TH = 0.693(R1 + R2)C; low time: TL = 0.693 × R2 × C. Supply voltage range: 5 V to 15 V. Output can source or sink up to 200 mA, directly driving LEDs and small relays. The control voltage pin (pin 5) sets the threshold to 2/3 VCC by default; connecting an external voltage here shifts both thresholds proportionally, enabling voltage-controlled oscillator (VCO) operation.
Exam tip
The examiner always asks you to calculate frequency and duty cycle of a 555 astable circuit — memorise f = 1.44/((R1 + 2R2)C) and duty cycle = (R1 + R2)/(R1 + 2R2), and confirm which resistor affects only the discharge time.