555

Do you mean the 555 timer IC? If so, here’s a quick refresher:

Key pins

• 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control Voltage, 6 Threshold, 7 Discharge, 8 Vcc

Operation modes

• Monostable: one-shot pulse when Trigger goes low. Pulse width t ≈ 1.1 × R × C.
• Astable: free-running oscillator (no external trigger). Frequency f ≈ 1 / [0.693 × (R1 + 2R2) × C]. Duty cycle D ≈ (R1 + R2) / (R1 + 2R2).
• Bistable: two stable states controlled by Trigger and Reset.

Common formulas

• Monostable: t ≈ 1.1 × R × C (R in ohms, C in farads, t in seconds)
• Astable: f ≈ 1 / [0.693 × (R1 + 2R2) × C], D ≈ (R1 + R2) / (R1 + 2R2)
• Control voltage pin (5) can set thresholds; bypass with ~0.01 µF to reduce noise

Tips

• Reset (pin 4) must be HIGH to enable operation.
• Ensure supply voltage matches the chip’s rated range (commonly 4.5–15 V).
• Typical wiring: for astable, connect threshold and trigger to the timing capacitor node, and use R1, R2 between Vcc and that node with Discharge (pin 7) driving the capacitor.

If you want, tell me the problem you’re working on (desired frequency, duty cycle, or a specific circuit) and I’ll tailor the calculations or walk through steps.