Electronics · Semester 1 — Analog · Lesson 9 · 55 min · build 2026.09.23-1644
The big idea
A chip is a sealed circuit, not a new kind of part — inside the 555 are a couple dozen of lesson 7's transistors, arranged to watch a capacitor and flip a switch at two thresholds. The pinout is the map to what's inside. Learn to read one, and every chip ever made opens.
Lesson 8, no peeking, no board: draw the night-light from memory — sensing stage, tap, base resistor, switch stage. Then mark two things on your drawing: the node the circuit watches, and the voltage where it flips. Two minutes, trade with a neighbor, check each other's topology. Keep that word threshold warm — today you meet a part that watches two of them at once.
The black brick in your kit is an integrated circuit — IC for short, chip to everyone. Inside is a complete circuit: for the 555, about twenty-five transistors with their resistors, built onto one flake of silicon and sealed in plastic. Nothing in there is a new kind of part. What's new is that you can't touch any of it — the only handles you get are the pins, and the only way to know what each pin does is the map. That map is the pinout, and it comes from the part's manual — its datasheet. Here's ours:
| Pin | Name | Job |
|---|---|---|
| 1 | GND | the chip's ground leg |
| 2 | TRIG — trigger | watcher #1: below ⅓ of the supply, it flips the switch on |
| 3 | OUT — output | the switch's handle: swings between high and low |
| 4 | RESET | the stop button; held at + the chip runs |
| 5 | CTRL — control | retunes the two marks; parked today |
| 6 | THRESH — threshold | watcher #2: above ⅔ of the supply, it flips the switch off |
| 7 | DISCH — discharge | a transistor's collector, aimed out through the pin: the chip's own drain |
| 8 | VCC | power in: our +5 V |
Seating it: the chip rides the trench — one row of legs on each side, so every pin gets its own row. That gap down the middle of your board has been waiting for this part since lesson 2. Legs fresh from the tube splay too wide; roll each side gently against the tabletop until they stand parallel, then seat the chip with even thumb pressure and eyeball it from the side — no leg folded under.
Notch first, power after
Plugged in backwards, some chips cook themselves in seconds — and you can't tell by looking which ones will. So the ritual, every chip, all year: find the notch, say which row pin 1 lands in, then connect power. If a chip ever feels warm to a fingertip, power off first, questions second.
First flight: power the chip and work the trigger and the threshold by hand — your control-wire trick from lesson 7, doubled. Wire it from the drawing; power pins (8, 4, 1) first is good chip manners.
Power on. Predict each row before your hand moves — especially the second one:
| Do this | Predict | Saw |
|---|---|---|
| Touch the ON wire to ground, hold it there | ||
| Let go | ||
| Touch the OFF wire to the + rail, then let go |
Lesson 7's switch obeyed only while you held the whisper — the instant you let go, dark. This one holds. Between its two marks the 555 keeps its last answer; a circuit that does that is called a latch, and it's your first taste of a circuit remembering. The two marks, on our 5 V board: the trigger trips below ⅓ of 5 V ≈ 1.7 V, the threshold above ⅔ of 5 V ≈ 3.3 V. Hold those two numbers; the meter meets them again shortly.
In lesson 6 you were the timing machinery: eyes on the meter, thumb on the stopwatch, calling the moment the fill crossed the line. The chip is about to take the whole job. Keep the LED loop and power pins; retire the touch wires and their 100 kΩs. Then wire lesson 6's fill circuit — supply, resistors, capacitor — with two twists the drawing shows: node A, the capacitor's + plate, lands on the trigger and the threshold at once (pins 2 and 6 tied together), and the chip's drain, pin 7, taps in between the two resistors.
Predict
Before power: what will the LED do, and roughly how often? Put a number on it — the challenge will grade your guess:
Power on. It blinks — steadily, forever, hands nowhere near it. You built an oscillator: a circuit with no resting state, so it flips between its two states for as long as it's powered. The 555 wired this way is called astable — "no stable state" — and that word will follow this chip for the rest of the year.
Predict, then swap
Trade the 100 µF for a 1000 µF — lesson 6's big can, ten times the bucket. The blink: faster or slower, and by how much? Swap it (stripe check), watch, then — before the meter touches anything — predict the highest and lowest numbers you'll see at node A. Build A handed you both.
Now park the black probe in ground and put the red probe on node A — a component leg or a bare wire stub in the row, as always. Watch a few full cycles.
Here's the whole machine, and every gear is one you've handled. While the switch is on, the drain is shut and the supply fills the capacitor through 1 kΩ + 10 kΩ — lesson 6's slow-motion pour. The moment node A crosses ≈ 3.3 V, the threshold (pin 6) flips the switch off — and the flip opens the drain, so the capacitor now empties through the 10 kΩ into pin 7, which is nothing but an internal transistor's collector, lesson 7's C-E switch aimed at your capacitor. Down past ≈ 1.7 V, the trigger flips everything back, and the pour begins again. A capacitor that fills and drains, two thresholds, a switch: sealed in plastic, yes — but no mystery inside.
Spec, no recipe: one blink every two seconds, as close as your table can get. The schematic stays; the values are yours to choose. Your levers are the ones lesson 6 taught — more ohms is a slower pour, a bigger bucket is a longer fill — and series tricks and borrowing from neighbors are both legal. Predict which way each swap pushes before you make it, then time five blinks against the class clock — you're aiming for ten seconds even — and adjust. Closest table wins.
Predict
Back to the standard build, then one last swap: the tiny 100 nF disc in place of the capacitor — a bucket a thousand times smaller than the 100 µF. On the record: what does the blink do?
Swap and power. Look closely at the LED — then don't trust your eyes until you've asked the meter: red probe on pin 3. It reads a steady in-between number, which should bother you: pin 3 is a switch, and switches are high or low, not halfway.
The truth: the LED is blinking right now — about seven hundred times a second. Your eye gives up somewhere around fifty flashes a second and reports the average: a calm half-brightness. The meter does the same thing — too slow to follow, it averages. Everything in this room that could witness the blink has failed. Next lesson a part arrives that can keep up with seven hundred a second — and you won't see what it does with the signal. (Lesson 6's promise about that little disc is still out, by the way: its everyday job on real boards is standing guard beside chips' power pins. Today it just moonlighted as the fastest bucket you own.)
Go further — for the fast and the curious
Put the 100 µF back in first. 1. The lighthouse: an eyeblink of light, then seven seconds of dark. Two moves — make the fill enormously longer than the drain (which resistor grows? which shrinks?), and let the LED light on low: its chain can hang from the + rail down to pin 3 instead of from pin 3 down to ground. 2. The tempo knob: make the lower resistor the pot (wiper + one leg) in series with the 1 kΩ, and sweep from a lazy blink to a fast flutter — about five a second. Why can't the knob reach the speeds the little disc hit? Hold that thought for the light theremin, two lessons from now. 3. The freeze button: a wire from pin 4, touched to ground, mid-blink. Predict: does it freeze lit or dark? Then, with the 1000 µF in, hold the freeze for a slow half-minute before letting go — and time the first blink against the steady ones. Something's off. Explain it. 4. The switch's drop: with the slow 1000 µF blink running, measure pin 3 during a long "on." Compare to 5 V — the chip's switch drops far more than lesson 7's 0.2 V. How does that show in the LED?
The math, for the curious — always optional
The blink has a formula — R1 is the top resistor, R2 the lower one:
Ohms × farads is seconds — lesson 6's trick — so the standard build gives:
— and 2.1 × 0.693 ≈ 1.5 s per blink; the 1000 µF makes it ten times that. Why 0.693? The capacitor only travels the middle stretch, ⅓ to ⅔ of the supply, and that slice of the fill curve costs 0.693 of one RC. Why does R2 count twice? It works both shifts — filling with R1, draining alone. The τ = RC sheet on the math shelf draws the curve this all lives on.
On your card: 1. draw the blinker from memory — the 555 as a box, numbered pins where wires land, both resistors, the capacitor, the LED. Pin numbers matter more than where they sit on the box. 2. One sentence: what does the chip watch, and what does it do when the watched voltage crosses a mark? Hand it in at the door.
Then the teardown ritual, chips or no chips: bare 220 Ω across every capacitor until the meter calls it empty. Parts go back in the box asleep.