The astable 555 from earlier this week oscillates forever. Its quieter sibling, the monostable, does something more disciplined: it emits exactly one pulse of a chosen length each time you ask. Stair lights, debounce timers, and "hold the door open for eight seconds" circuits are all this one trick, and the formula is even simpler than the astable's.

The build

At sim.devoltapp.com, on a Breadboard (400) with the Bench PSU at 5 V:

1. NE555 Timer across the centre gap, supply to pin 8, ground to pin 1.
2. Timing pair: a 100 kΩ Resistor from 5 V to pin 7, pin 7 tied to pin 6, and a 10 µF Electrolytic Capacitor from pin 6 to ground. Note pins 6 and 7 join each other here, unlike the astable where a second resistor sat between them.
3. Trigger: a Push Button from pin 2 to ground, with a 10 kΩ pull-up Resistor from pin 2 to 5 V. Pin 2 idles high; a press yanks it low. After a week of pull-up posts this wiring should feel like an old friend.
4. Output: Red LED plus 330 Ω from pin 3 to ground. Tie pin 4 (reset) to 5 V so noise cannot reset the chip, and put 10 nF from pin 5 to ground.

Run it. Nothing happens, which is correct: a monostable's resting state is off. Now click the button once. The LED lights, holds for about a second, and turns itself off. Press again, another measured pulse.

The formula, and cursors to check it

The pulse width is:

T = 1.1 × R × C

Our values: 1.1 × 100 kΩ × 10 µF = 1.1 seconds. Verify it properly with the instruments rather than counting in your head: freeze the scope after a pulse, enable Measurements and Cursors, and drag cursor A to the output's rising edge and cursor B to the falling edge. The Δt readout should sit within a few percent of 1.1 s. The 1.1 factor is no mystery either: it is ln(3), because the capacitor charges from 0 V toward 5 V and the 555 calls time when it crosses ⅔ of the supply.

Watch the capacitor node on a second channel while you trigger: a clean RC charge ramp from 0 V to 3.33 V, then a vertical drop as pin 7's internal transistor empties the capacitor for the next round. The output pulse is exactly the width of that ramp.

Things worth trying while it is on the bench

• Scale the time. 1 MΩ and 100 µF gives 110 seconds, which is a stair-light. 10 kΩ and 100 nF gives 1.1 ms, which is a debouncer. Same circuit, eight orders of magnitude of usefulness.
• Hold the button down. On a real 555 wired this simply, holding the trigger low past the pulse's end stretches the output, and the simulator reproduces that faithfully (this exact behaviour got an engine fix in June when a Learn lesson depended on it). Discuss why with your class: pin 2 is level-sensitive, not edge-sensitive.
• Retrigger early. Press again mid-pulse and nothing restarts: this topology ignores triggers while the capacitor is still climbing.

Astable and monostable, one mental model

Both circuits are the same three actors: a capacitor climbing through a resistor, two comparators watching for ⅓ and ⅔ of the supply, and a discharge transistor. The astable wires them into a loop so the story never ends; the monostable lets the story end and waits to be asked again. Once students see both traces side by side, the 555 stops being a magic bean and becomes a state machine with analog inputs, which is the honest thing it always was.

Full pin behaviour and model notes: docs.devoltapp.com/parts/timer-555. Tomorrow the micro:bit meets the breadboard properly: an external LED on P0 and the 3.3 V rules that keep it alive.