🔆 Reserve Beacon

What you're building

A red LED with two brightness levels: a faint always-on pilot you can see in a dark room, and a full-beam First-Light-bright that fires the moment you close a bypass switch. No microcontroller, no PWM, no fancy chip — just two resistors, one slide switch wired to short one of them out, and the LED.

This is the pattern inside every "power-on indicator that goes bright during use": car ignition pilot lights, printer status LEDs that pulse when printing, laptop charger lights that glow during idle and brighten during fast charge. The circuit holds a slow trickle through the LED all the time, and when you want to be loud about it, a switch (or a transistor, at higher levels) shorts out the throttle resistor and the LED jumps to full.

The big idea: shorting a resistor removes it from the loop

Up to now, every switch you've placed has been in series with the loop — flip it, break the loop, LED dark. This project puts a slide switch somewhere new: in parallel with a resistor. That changes what "closed" and "open" mean.

• Slide OPEN: the slide is a gap. The 10 kΩ sits in the loop as its full 10 000 Ω. Loop total = 10 000 + 220 = 10 220 Ω.
• Slide CLOSED: the slide is a wire with essentially 0 Ω. That wire sits in parallel with the 10 kΩ. The parallel combination is R1 || R2 = (R1 × R2) ÷ (R1 + R2) = (10 000 × 0) ÷ (10 000) ≈ 0 Ω. The 10 kΩ is still physically there — current just doesn't want to go through it when there's a free wire next door. We call this shorting out the resistor.

The slide picks between two Rs: 10 220 Ω (slide open) and 220 Ω (slide closed).

Same LED, same battery, same wires. The slide picks the resistance of the loop.

Do the math — two states

State 1 — Pilot (slide open):

• Battery push: 4.5 V
• LED eats at low current: ~1.8 V (the LED's Vf drops at low current — the junction squeeze is softer when fewer electrons are crossing)
• Leftover for the 10 kΩ + 220 Ω chain: 4.5 − 1.8 = 2.7 V
• Total series R: 10 220 Ω
• Current: I = V ÷ R = 2.7 ÷ 10 220 ≈ 0.264 mA

Red LED at ~0.26 mA is the "dim pinprick" — invisible in daylight, clearly on in a dark room with your hand cupped around it. Same current as Night Pin. The 10 kΩ dominates: the 220 Ω is almost ignored.

State 2 — Full beam (slide closed):

• Battery push: 4.5 V
• LED eats at ~10 mA: ~2.0 V (datasheet Vf)
• Leftover for the 220 Ω alone: 4.5 − 2.0 = 2.5 V
• Total series R: 220 Ω (the 10 kΩ has been shorted out)
• Current: I = 2.5 ÷ 220 ≈ 11.4 mA

That's First Light. The LED is ~43× brighter than the pilot state (brightness scales roughly with current for a red LED).

Why not just use one state?

• Why keep the 220 Ω at all when the slide is closed? Because without it, the slide → 0 Ω path leaves nothing between + and the LED. Current would run away: (4.5 − 2.0) ÷ 0 = very large, and the LED pops within seconds. The 220 Ω is the LED's seatbelt — it must be in the loop in both states.
• Why not just use a plain switch? Because the switched-off state would be fully dark. The whole point of a beacon is that it's always on, just quietly — the dim pilot proves power is reaching the LED, so if the bright state ever fails, you know the problem isn't the battery.
• Why 10 kΩ, not 1 kΩ? 1 kΩ in the pilot slot gives ~2.6 mA — still visibly on, which is what Dim Candle does. 10 kΩ gives ~0.26 mA — barely visible, which is what makes the bright state feel like a jump. The two patterns teach the same trick with different "ratios of jump".

Compare the siblings:

Build	Pilot current	Full current	Ratio
Reserve Beacon (10 kΩ bypass)	0.26 mA	11.4 mA	~44×
Dim Candle (1 kΩ + 10 kΩ series, 10 kΩ bypass)	0.25 mA	2.6 mA	~10×
Dim Candle's siblings (1 kΩ bypass)	2.0 mA	11 mA	~5×

The resistance you bypass controls the brightness ratio between the two states. The resistance that stays in the loop (always 220 Ω here) controls the maximum brightness.

Where you'll see this pattern

• Power-on indicator LEDs. Dishwashers, microwaves, laptop chargers — the "I'm plugged in" LED is almost always a pilot-mode design: dim trickle through a big resistor, bright during active use when a transistor shorts the resistor out. The transistor is just a smart slide switch; the circuit is this project.
• Headlight parking lamp vs. headlight. Older cars use parking lamps as a dim pilot (filament glowing through a limiter) that becomes bright when a relay (= slide switch) shorts the limiter and lets the lamp burn at full. Modern LED headlights use the same pattern, with a transistor instead of a relay.
• Dim-bright flashlight modes. Many two-mode flashlights have exactly this layout: a push-button toggles the bypass. Press once for dim (10 kΩ in loop), press again for bright (10 kΩ shorted).
• Troubleshooting LED circuits. When an LED should be on but isn't, add a 10 kΩ "pilot resistor" across the suspected break — if the pilot glows dim, the LED and loop are fine and the fault is upstream (bad switch, missing driver). If the pilot glows nothing, the fault is in the LED or its cathode return.

Build it

1. Power column. Battery + at the top of column 1, − at the bottom. Jumper from + to the top rail.
2. The two resistors in series. On row 1 (the top working row), lay down two speed bumps end-to-end: a 10 kΩ (brown-black-orange) first, then a 220 Ω (red-red-brown) immediately after it. The electrical path is + rail → 10 kΩ's left lead → junction → 220 Ω's left lead → junction → LED anode.
3. LED (col 5). Red 5 mm LED standing vertical: long leg (anode) in row 1 at the 220 Ω's output, short leg (cathode) in row 2.
4. Bypass detour (row 0, across the 10 kΩ). The clever bit. A jumper runs from the 10 kΩ's input (row 1) UP to row 0 and sits waiting. A slide switch sits horizontally in row 0, with one side on that jumper and the other side on a second jumper that runs DOWN to the 10 kΩ's output (where it meets the 220 Ω's input). This routes a parallel wire around the 10 kΩ. The slide decides whether the detour is open (10 kΩ in loop) or closed (10 kΩ shorted out).
5. Bottom rail (row 3). Jumper from the LED's cathode row down to row 3 and across to the battery − terminal.
6. Leave the slide OPEN first. In a dark room, cup your hand over the LED. You should see a faint red pinprick. That's the 0.26 mA pilot — proof the whole chain works at low current.
7. Flip the slide closed. The LED jumps to First-Light brightness — ~11 mA, fully visible in daylight. The 10 kΩ is now bypassed; only the 220 Ω throttles the loop.
8. Flip the slide back open. The LED drops back to the pinprick, in about 1 ms (not visibly instantaneous — just instant). The resistor has rejoined the loop.

What to notice as you flip the bypass

The slide isn't turning the LED on. The LED is always on. The slide is changing the throttle. If you have a multimeter, you can measure the voltage across the 10 kΩ in both states:

• Slide open: V_10kΩ = I × R = 0.264 mA × 10 000 = ~2.64 V (basically all of the leftover)
• Slide closed: V_10kΩ = 0 V (both ends are tied together through the slide — can't have voltage across 0 Ω)

The 10 kΩ is physically there in both states. What changes is whether current wants to use it.