Overview
This lecture covers the internal structure, operation modes, and practical circuits of the 555 timer IC, one of the most widely used integrated circuits in electronics.
Introduction & Structure of the 555 Timer
- The 555 timer was developed in 1971 and is extremely popular for its versatility.
- Inside, three 5kΩ resistors form a voltage divider between power supply rails.
- Two comparators compare input voltages to fixed references set by the divider.
- An RS flip-flop (latch) determines the output state based on comparator signals.
Pin Functions and Internal Operation
- Pin 5 (control) is typically used to filter power noise with a small capacitor.
- Pins 2 (trigger) and 6 (threshold) receive input signals to set/reset the latch.
- Pin 4 (reset) can reset the circuit externally when pulled low.
- Pin 3 is the main output; Pin 7 (discharge) helps control timing by discharging a capacitor.
Static/Bistable (Flip-Flop) Mode
- Bi-stable mode means the 555 has two stable states (set/reset), much like a basic flip-flop.
- Inputs must not be left floating; pull-up or pull-down resistors are necessary.
- Proper button wiring is essential to prevent unstable states (prefer pull-ups in digital systems).
Monostable (One Shot) Mode
- Only one stable state; triggers once, then returns to rest.
- A pulse at the trigger input sets the flip-flop; a timing capacitor starts charging.
- When capacitor voltage hits 2/3 Vcc, the flip-flop resets and output returns low.
- Timer duration: ( T = 1.1 \times R \times C ) (R in Ω, C in F).
Astable (Oscillator) Mode
- In astable mode, the 555 generates a continuous stream of pulses (oscillation).
- Timing is set by resistors R1, R2 and capacitor C1; duty cycle is not exactly 50%.
- Adding diodes can help balance charge/discharge cycles for a near 50% duty.
- The output frequency and pulse width depend on the RC values.
PWM (Pulse Width Modulation) with 555
- Adjusting resistance (via potentiometer) changes duty cycle and indirectly affects frequency.
- Pin 5 (control) should be filtered with a capacitor to stabilize threshold reference.
- Modifications with added resistors or adjusted control pin help stabilize PWM operation.
Practical Considerations & Applications
- Bypass capacitors (10 nF and 47 μF) are recommended for power supply stability.
- The discharge pin can be used to drive external loads (e.g., fan, lamp, or motor).
- CMOS variant (LMC555) offers better stability, higher frequency, and lower voltage operation.
Key Terms & Definitions
- Comparator — Compares two voltages; outputs digital high or low based on comparison.
- RS Flip-Flop (Latch) — Circuit with set and reset; retains state until next pulse.
- Bistable Mode — Operates as a flip-flop with two stable states.
- Monostable Mode — Has one stable state; generates a single timed pulse when triggered.
- Astable Mode — No stable states; continually oscillates between high and low outputs.
- Duty Cycle — Ratio of time output is high vs. the total period, expressed as a percentage.
- PWM (Pulse Width Modulation) — Varying output pulse width to control power delivered to a load.
Action Items / Next Steps
- Build and test 555 timer circuits in static, monostable, and astable modes.
- Add bypass capacitors and experiment with RC values to observe timing and duty cycle effects.
- Optional: Research the LMC555 CMOS version for advanced applications.