🌙 Night Pin

What you're building

The smallest sensible nightstand lamp. One battery, one slide, one resistor, one LED. Flip it on and the red LED glows just enough to find a glass of water in a pitch-dark bedroom — but in daylight it looks completely off. In a dim room you can see it from several feet away; in a fully-dark room it's bright enough to read a watch face by if you lean in.

Same four parts as First Light, with one change: a 10 kΩ resistor instead of the usual 220 Ω. Fifty times bigger. The result is a current roughly fifty times smaller — a tenth of a tenth of First Light. That current is plenty for a visible glow, and nothing more.

The big idea: a single resistor can dim an LED to any brightness

In Step Up you learned that three identical resistors in parallel multiply brightness up. In Longer Loop you learned that three identical resistors in series divide it down. This project is the simplest case of either: one resistor, one lane, one LED. The size of that single resistor sets the brightness completely.

The pattern is Ohm's law — the same law from First Light, same equation, bigger R:

I = (V_battery − V_LED) ÷ R

• First Light (R = 220 Ω): I = 2.5 ÷ 220 ≈ 11 mA → fully lit, "lamp"
• Signal Pair per lane (R = 220 Ω): same, 11 mA → fully lit
• Step Up single slide (R = 1 kΩ): I = 2.5 ÷ 1 000 = 2.5 mA → "candle"
• Night Pin (R = 10 kΩ): I = 2.7 ÷ 10 000 ≈ 0.27 mA → "pinprick"

Each row of that table is the same circuit with a different R. The LED's brightness follows R. One resistor can be a torch, a candle, or a pinprick — it's all the same math.

Do the math

• Battery push: 4.5 V
• LED eats: ~1.77 V (red LEDs drop noticeably less than their 2.0 V book value at these low currents — the Shockley equation says Vf drops by ~60 mV every time current goes down 10×)
• Leftover for the 10 kΩ: 4.5 − 1.77 = 2.73 V
• Current: I = V ÷ R = 2.73 ÷ 10 000 ≈ 0.00027 A = 0.27 mA

A quarter of a milliamp. That's forty times below the LED's 20 mA pop limit — safe-with-huge-margin. It's also roughly 3× below the LED's "clearly visible in a lit room" threshold (~1 mA), which is exactly why Night Pin looks "off" by day and "on" by night: the glow is below daylight contrast but above full darkness. The eye adapts; the LED stays steady.

Why 10 kΩ in particular

Three reasons:

1. It's already in your bag. You own 220 Ω, 1 kΩ, and 10 kΩ at Level 1 — the three decades of resistance that Level 1 covers. 10 kΩ is the biggest. For an LED on 4.5 V, 10 kΩ is also near the practical dimming floor — much bigger and the current drops below where even dark-adapted eyes see it. (For comparison, a 100 kΩ would give 27 µA — invisible to anyone.)
2. The math is dead simple. With a 10 kΩ at 4.5 V, I ≈ V ÷ R to within 10 % without even knowing the LED's exact Vf — because the 10 kΩ eats so much more voltage than the LED does. First Light is more sensitive to exactly what Vf is; Night Pin barely cares.
3. Power consumption is approximately zero. 0.27 mA × 4.5 V = 1.2 mW total. A 3×AA pack has roughly 1 500 mAh × 4.5 V = 6.7 Wh of energy. Night Pin runs 5 500 hours — seven and a half months — on one battery pack. First Light at 11 mA runs about 140 hours. The dimmer you go, the longer the batteries last.

Where you'll see this pattern

• "Power is on" LEDs on appliances. The red LED on your TV, the green one on your charger, the blue one on your wireless speaker — nearly all run at 0.5–3 mA through resistors in the 1–10 kΩ range. The LED is visible when the room is dim; otherwise it fades into the background.
• Soft bedside night lights. A tiny LED behind a frosted plastic cover, running at a milliamp or less, lasts for years on a coin cell. The resistor sets the brightness; the plastic diffuses it.
• Dim backlights for a wall clock. The faintly-glowing hands of a battery-powered wall clock use the same trick — one LED or two, a big resistor, a long-life battery.
• Decorative "fire" or "flicker" LEDs. Some novelty candles use an LED driven at a very low current from a coin cell, with a tiny chip that modulates it; the base current is in the same Night-Pin range.

The pattern is universal: when you want a glow and not a lamp, make the resistor big.

Build it

1. Power column. Battery + at the top of column 1, − at the bottom. A jumper from column 1 bottom to the breadboard − rail.
2. Master slide (top row). Slide switch horizontally with its left lead on the + rail (column 1) and its right lead in column 2.
3. The 10 kΩ. From the slide's output in column 2, a 10 kΩ (brown-black-orange) across to column 4. Note the orange band — this is the giveaway. 220 Ω has a brown third band; 10 kΩ has orange.
4. The LED. In column 5, a jumper from the resistor's right lead (column 4) to the LED's long leg at column 5. Then the red LED vertical: long leg (anode) at the top, short leg (cathode) two rows down.
5. Bottom rail. A jumper from the LED's cathode row back to the breadboard − rail, and from there to the battery − terminal.
6. Flip the slide on. In a lit room: squint. You might see a hint of red. Cup your hand around the LED or turn off the lights — the glow becomes clear. In full darkness it looks almost comfortable.
7. Compare side by side. If you still have First Light built on another breadboard, run them next to each other. Same LED, same battery, same slide. Only the resistor is different: 220 Ω burns 25 mW and lamps at 11 mA, 10 kΩ burns 1.2 mW and glows at 0.27 mA. Visibly, First Light wins in daylight; Night Pin wins at bedtime.

Troubleshoot by trusting the dimness

Night Pin is designed to look "off" in daylight. Don't rush to replace parts just because the LED isn't bright. Confirm first:

• Switch the room lights off (or go into a closet) before deciding Night Pin is broken.
• Cup your hand around the LED to shield it from ambient light.
• Lean in — the glow is small but real.

Only if those three tests show no light at all should you suspect a fault.