555

Do you mean the 555 timer IC? If you’re prepping for exams, I can give a quick refresher. Tell me if you want astable, monostable, or general theory. In the meantime, here’s a compact cheat sheet you can memorize:

Key pins (NE/LM555)

• 1: GND
• 2: TRIG
• 3: OUT
• 4: RESET (active low)
• 5: CTRL (control voltage, often tied to a 0.01 µF cap to ground)
• 6: THR
• 7: DIS (discharge)
• 8: VCC

Astable mode (free-running)

• Threshold/Trigger levels: 2/3 VCC (high), 1/3 VCC (low)
• Typical circuit: R1 between VCC and DIS, R2 between DIS and THR/TRIG (pins 6/2), C from THR/TRIG to GND
• High time: Th = 0.693 (R1 + R2) C
• Low time: Tl = 0.693 R2 C
• Period and frequency: T = Th + Tl = 0.693 (R1 + 2R2) C -> f = 1.44 / [(R1 + 2R2) C]
• Duty cycle: D = Th / (Th + Tl) = (R1 + R2) / (R1 + 2R2)

Monostable mode (one-shot)

• Triggered by a negative pulse on TRIG (pin 2)
• Circuit typically: R between VCC and C (to THR/TRIG), C from THR/TRIG to GND
• High time: Th ≈ 1.10 R C (more precisely Th = 0.693 R C for some configurations; the standard is 1.10 RC when charging toward VCC to reach 2/3 VCC)
• After trigger, OUT stays high for Th, then returns low as C charges to 2/3 VCC and threshold is met
• Reset pin (4) is active low and forces OUT low if asserted
• Control pin (5) allows fine-tuning of threshold/divider via an external voltage (often decoupled to ground with a 0.01 µF cap)

Specifications to remember

• Supply range: ~4.5 V to 15 V (typical NE555/LM555)
• Output current: can source/sink a few tens of mA (check exact part)
• Pin 5 (CTRL) is optional; add a capacitor to ground (0.01 µF) for noise suppression
• Variants: NE555/LM555 (bipolar),, and CMOS versions (e.g., LMC555, TLC555) with different characteristics

If you’d like, tell me:

• Are you solving a specific problem (give circuit values)?
• Do you want a worked example for astable or monostable?
• Do you need quick derivations or a practice problem set?