Make it blink: a schematic puzzle
The 555 timer in its most famous job: a steady blink with no microcontroller in sight.
The 555 watches the voltage on the timing capacitor. The capacitor charges through RA and RB until it reaches two thirds of 9 V; the chip then flips, pulls pin 7 to ground, and the capacitor drains back through RB alone until it falls to one third. Then it flips again, and the cycle repeats forever. The output on pin 3 follows that flip, so the LED is on while the capacitor charges and off while it drains. With 1 kΩ, 47 kΩ and 10 µF the period is about 0.66 s, roughly 1.5 blinks per second. It is a bathtub with a float switch: fill to the top mark, drain to the bottom mark, repeat.
RA, 1 kΩ: Part of the charging path only. Kept small so the on-time and off-time stay nearly equal, and so the discharge pin is never shorted straight to the supply.
RB, 47 kΩ: Sets the tempo. The capacitor charges through RA + RB and drains through RB alone, so RB dominates both halves of the cycle. Change it and the blink speeds up or slows down.
Timing capacitor, 10 µF: The bathtub. Its size, with RB, sets how long each half of the blink lasts. It is polarised: plus toward the chip, minus to ground, or it slowly destroys itself.
NE555 timer: The referee. Two comparators inside watch the capacitor against one third and two thirds of the supply and flip the output and the discharge pin accordingly.
Control capacitor, 10 nF: Steadies the two-thirds reference inside the chip against supply noise. Not strictly required, always recommended.
LED resistor, 470 Ω: Same job as in the first puzzle: the output swings to about 8 V, so (8 − 2) ÷ 470 ≈ 13 mA through the LED.
Red LED: Shows the output. Anode toward the resistor, cathode to ground.