The Oscillator.
{kids ? "A bucket that fills, and a switch you can trigger." : "An RC delay, and a transistor switch."}
{kids ? <>You've met both pieces already. A capacitor is a bucket that fills up over time (Chapter 4). A transistor is a switch a tiny signal can flip (Chapter 6). Put them together and something new happens — the circuit starts flipping by itself. : <>You have both ingredients. From Chapter 4: a capacitor charges through a resistor on a τ = R·C clock. From Chapter 6: a transistor is a switch flipped by a small base current. Wire two of each so that each side's charging cap eventually switches the other side — and the circuit never settles. That's an oscillator.}
{kids ? <>A bucket alone can't blink — it just fills once and stops. You need a switch that flips when the bucket is full. Two of them, taking turns, blink forever. : <>R and C alone cannot oscillate — they only charge once and rest. Oscillation needs an active switching element. The transistor is that element; the cap sets the tempo.}
{kids ? "Two lights that take turns." : "Two transistors, taking turns."}
{kids ? <>Here are two transistors, each with its own light and its own bucket. When one side's bucket fills, it flips the other side off — so the two lights blink back and forth, forever. Try bigger buckets or tighter pipes and watch the blink slow down. : <>This is the classic astable multivibrator. Each side charges a cap that, once full enough, switches the opposite transistor — so the two collectors (and their LEDs) alternate. The blink rate is set entirely by R and C: T ≈ 1.38·R·C.}
{kids ? <>It works! But notice: that's eight parts just to blink two lights. Engineers got tired of wiring this every time… : <>It works — but it's eight-plus parts, the duty cycle is awkward to make even, and the timing drifts with temperature and supply. Engineers wanted this in one tidy, predictable package.}
{kids ? "Inside the 555: a referee and a bucket." : "Inside the 555."}
{kids ? <>The 555 chip hides a tiny referee that watches one bucket. When the bucket gets to ⅔ full, the referee shouts "stop!" and empties it. When it drops to ⅓, it shouts "go!" and fills it again. Up, down, up, down — that's your blink, and the light follows along. : <>It's not magic — it's exactly what you just built, made precise. Three 5 kΩ resistors (that's the "555") set two reference levels: ⅓ and ⅔ of Vcc. Two comparators watch the cap against those levels and drive a flip-flop. The flip-flop runs the output and a discharge transistor. The cap ramps between ⅓ and ⅔ forever.}
{kids ? "Wire your own 555 blinker." : "Wire the 555 yourself."}
{kids ? <>You've seen inside the chip — now build the outside. The 555's power pins are already hooked up. Add the two pipes, the bucket, the jumper, and the light, then switch it on and watch the wave. : <>You've seen the guts; now wire the parts that make it tick. The power pins (1, 4, 8) are pre-connected — you place Rₐ, Rᵇ, the timing cap, the THR↔TRG jumper, and the LED output, then power up and scope the real square wave.}
{kids ? "How fast will it blink?" : "Predict the frequency, then tune it."}
{kids ? <>Before you play with the sliders above, work out the blink rate for one set of parts. Engineers always do the math first — then check it on the bench. : <>The Beacon's blink engine is a 555 in astable mode. Compute its frequency from the parts, then go back up and tune the sliders to feel how RA, RB, and C trade off.}
{kids ? <>Scroll back up to the chip and drag the sliders. Make the biggest bucket — see how slow you can make it blink. That same chip is the heart of your Beacon! : <>Scroll up to the live 555 and tune RA, RB, and C. Notice RA only stretches the on-time, RB stretches both, and C scales everything. This exact circuit drives the Beacon's flash.}