555

Do you mean the 555 timer IC? If so, here’s a quick refresher you can use for exams, plus a couple practice problems. If you meant something else by “555,” tell me.

Quick notes

• Pinout (8-pin DIP): 1 GND, 2 TRIG, 3 OUT, 4 RESET, 5 CTRL, 6 THRS, 7 DISCH, 8 Vcc
• Modes: Astable (oscillates), Monostable (one-shot), Bistable (flip-flop)
• Supply voltage typically 4.5–15 V

Key formulas

1. Astable mode (free-running oscillator)

• Circuit uses two resistors RA, RB and capacitor C
• t_high = 0.693 (RA + RB) C
• t_low = 0.693 RB C
• Period T = t_high + t_low = 0.693 (RA + 2RB) C
• Frequency f = 1 / T = 1 / [0.693 (RA + 2RB) C]
• Duty cycle = t_high / T = (RA + RB) / (RA + 2RB)

2. Monostable mode (one-shot)

• Trigger causes output to go high for a time t = 1.1 R C (R from Vcc to C, C to ground; exact wiring can vary)
• After t, output returns to low and the capacitor recharges
• Trigger threshold levels are at 1/3 Vcc (trigger) and 2/3 Vcc (threshold)

3. Bistable mode

• Latches between high and low with Set/Reset inputs
• No timing network unless you add RC to create a retriggerable push

Quick worked example

• Design an astable oscillator with f ≈ 1 kHz using C = 10 nF (1e-8 F)
• T = 1/f = 0.001 s
• RA + 2RB ≈ T / (0.693 C) = 0.001 / (0.693 × 1e-8) ≈ 144 kΩ
• Pick RB = 22 kΩ, then RA ≈ 144k − 2×22k ≈ 100 kΩ
• Check: t_high ≈ 0.693(100k + 22k)×10nF ≈ 0.962 ms; t_low ≈ 0.693×22k×10nF ≈ 0.152 ms; f ≈ 1 / 1.114 ms ≈ 898 Hz (close to target; adjust values if you need exact 1 kHz)

Practice problems (solve or I can walk through)

1. Monostable: If you want a pulse width of about 2 ms using a 555, what RC values would you pick? (Hint: t ≈ 1.1 R C.)
2. Astable: Design an astable with f = 2 kHz using C = 100 nF. Choose convenient RB and RA values (give one valid set).
3. Basic check: For a given astable with RA = 470 kΩ, RB = 120 kΩ, C = 10 nF, compute f, t_high, t_low, and duty cycle.

If you want, I can tailor the problems to your course (e.g., electronics, digital design) and work through the solutions step by step.