🚨 Signal Light

What you're building

A tail light with two modes on one lamp. A slide switch flipped on holds the red LED steady — a beacon anyone can see from behind. A push button on top of that fires the same lamp only while you're holding it — a momentary flash to grab a driver's attention when you really need it.

Two-Door Signal proved that two slide switches in parallel make an OR gate: either one alone lights the lamp. This project builds the same OR gate with two different kinds of switch: a persistent one that stays where you left it, and a momentary one that only counts while your finger is down. Same wiring, same math — but the two behaviours you get out are wildly different.

The big idea: OR of a slide and a button

Two switches wired side by side — both connecting the same two spots — give the loop two ways to close. Electricity is lazy: it takes any available path. So the LED lights as long as at least one switch is closed.

• Slide switch closed, button up → path exists (through the slide) → LED steady on.
• Slide switch open, button pressed → path exists (through the button) → LED on while held.
• Both closed → two paths at once → LED on, same brightness.
• Both open → no path anywhere → LED off.

The mix of switch types is the fun part. The slide decides your default state: on if you flipped it on, off if you flipped it off. The button overrides that default upward — it can turn a dark lamp momentarily on, but it can never turn a lit lamp off. In logic terms it is still A OR B, but you're using the two inputs for two very different jobs.

Do the math

When at least one switch is closed, the loop is exactly First Light — one battery, one resistor, one LED. Extra closed switches in parallel don't slow it down; they just add a second wire from the plus rail to the same merge point.

• Battery push: 4.5 V
• Red LED eats: 2.0 V
• Voltage left for the resistor: 4.5 − 2.0 = 2.5 V
• Through 220 Ω: I = 2.5 ÷ 220 ≈ 0.0114 A = 11 mA

Same 11 mA whether the slide is on, the button is pressed, or both. A closed switch — slide or button — is basically a short piece of metal (a few milliohms). Whether one such piece of metal delivers the current or two do in parallel, the LED still sees the same 4.5 V push through the same 220 Ω speed bump.

That's why the "extra flash" doesn't look any brighter than the steady beacon: the resistor already sets the current, and the second closed path can't push it higher.

Build it

1. Power rails: battery red (+) to a top row, battery black (−) to a bottom row.
2. Fork the plus rail into two branches: one jumper from the + rail to the slide switch's common leg, another jumper from the + rail to one leg of the push button (across the centre groove — same trick as Morse Code Messenger).
3. Merge the outputs: the slide switch's outer leg and the push button's diagonal leg both land on the same row — the "someone closed a switch" merge row.
4. The LED lane: merge row → 220 Ω → LED long leg → LED short leg → − rail.
5. Slide the slide switch on: steady red. Slide it off, press the button: red only while you press. Slide on AND press the button: steady red, exactly as bright as before.

Deploy the beacon

Once it works on the breadboard, you can mount this on the back of a bike helmet, a backpack, or your handlebars:

• The slide switch is your on/off for a whole night ride — flip it on at sundown, off when you get home. A fresh set of AAs will keep the LED steady for ~110 hours at 11 mA (a typical 2500 mAh AA capacity ÷ 11 mA ≈ 227 h, halved for real-world losses and low-voltage cutoff — several long night rides on one set).
• The push button sits under your thumb on the handlebars. When a car pulls up too close from behind, tap it three or four times fast. A steady red is easy to ignore; a red that suddenly starts flashing is not. Human eyes are wired to notice motion, not brightness — that's why every emergency vehicle flashes.

Where OR-of-mixed-switches shows up in the real world

• Car headlights + high-beam flash: a stalk that pulls back briefly always fires the high beams, even if your main headlight switch is off. Momentary override on top of a persistent setting.
• Doorbells + a "hold to buzz" backdoor test button: the outside button rings normally; a hidden inside button lets you test the bell without going outside.
• Room lights + a wall-plate momentary "party mode" that adds the disco light: slide sets the base state, momentary adds a temporary layer.

Once you notice this pattern, you'll find it everywhere. Any time a system has a default state you can flip and an override you can hold, you're looking at OR-of-different-switches.