555

Do you mean the 555 timer IC? If so, here’s a concise refresher you can use for exams.

Pinout (DIP-8)

• 1: Ground (GND)
• 2: Trigger (TRIG)
• 3: Output (OUT)
• 4: Reset (RESET)
• 5: Control Voltage (CV)
• 6: Threshold (THRESH)
• 7: Discharge (DISCH)
• 8: Vcc (supply)

Modes

• Astable: free-running oscillator. No external trigger. Produces a continuous square wave.
• Monostable: one-shot. Output goes high for a fixed period when triggered.
• Bistable: flip-flop (latching). Output changes state with trigger.

Key equations

Astable (uses RA, RB, C)

• Frequency: f = 1.44 / [(RA + 2 RB) C]
• High time (output high): t_high = 0.693 (RA + RB) C
• Low time (output low): t_low = 0.693 RB C
• Duty cycle = t_high / (t_high + t_low) = (RA + RB) / (RA + 2 RB)

Monostable (uses R and C)

• Pulse width: t = 1.1 R C
• Typical config: RA is from Vcc to DISCH/THRESH node, C from THRESH to GND; TRIG is pulled high and momentarily driven low to trigger.

Bistable

• No simple timing formula; output toggles with each trigger/reset. Requires external resistors to set bias.

Example calculations

1. Astable example

• RA = 1 kΩ, RB = 2 kΩ, C = 100 nF
• f = 1.44 / [(1k + 2×2k) × 100nF] = 1.44 / (5k × 100nF) ≈ 1.44 / 0.0005 ≈ 2.88 kHz
• t_high = 0.693 × (1k + 2k) × 100nF ≈ 0.693 × 3k × 100nF ≈ 0.000208 s ≈ 0.208 ms
• t_low = 0.693 × RB × C = 0.693 × 2k × 100nF ≈ 0.000139 ms ≈ 0.139 ms
• Duty cycle ≈ t_high / (t_high + t_low) ≈ 0.208 / (0.208 + 0.139) ≈ 0.60 (60%)

2. Monostable example

• R = 100 kΩ, C = 10 nF
• t = 1.1 × 100k × 10nF = 1.1 ms

Practical notes

• Use a decoupling capacitor (e.g., 0.1 µF) near Vcc and a 0.01–0.1 µF on the control voltage input (pin 5) for stability.
• Tie Reset (pin 4) to Vcc if not used (through a resistor or directly, as per datasheet).
• Don’t leave the control voltage pin floating; it normally ties to a 2/3 Vcc reference.
• The 555 can operate from about 4.5 V to 16 V (varies by package/version).

Want to practice with a specific configuration (e.g., design an astable for 1 kHz at 5 V) or go through a particular exam-style problem?