Level 1 · Light · project 7 of 24 · 15 min · ●●○○○
Bike Torch
One yellow LED, two switches: a slide for a battery-friendly low beam, a push button for a bright flash, both lanes feeding the same lamp in parallel.
new here? Start with First Light — one battery, one resistor, one LED. It teaches the loop, LED direction, and why the 220 Ω resistor matters.
What you need
- 1× 3×AA battery pack (4.5 V)
- 1× Solderless breadboard
- 6× Jumper wires
- 1× Slide switch the steady LOW beam — leave it on for a battery-friendly glow
- 1× Push button (tactile) the momentary FLASH — press for the "look at me" burst
- 1× 1 kΩ resistor brown-black-red — the low-beam lane's big speed bump
- 1× 220 Ω resistor red-red-brown — the flash lane, same as First Light
- 1× Yellow LED (5 mm) the shared lamp — long leg toward + (both lanes feed the same LED)
you'll learn: Circuits are loops · Switches · Series & parallel · Ohm's law · LED polarity
Circuit map
The circuit you'll build, running on the Workbench right now.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- LED never lights, either switch. Flip the LED. Long leg (anode) has to face + — both lanes feed it, so a backwards LED kills every mode at once. Also check the AA pack is switched on.
- Low beam works, flash does not. The push button spans two rows on the breadboard. Wire your flash lane into the SAME row the button bridges — not across a gap. Try pressing harder to feel the click.
- Flash works, low beam does not. Slide switch: middle leg plus ONE outer leg only. The other outer leg is dead. Try the other outer; if still dead, the switch is toast.
- Low beam is unexpectedly bright. You slotted a 220 Ω into the slide-switch lane instead of the 1 kΩ. Colors: 1 kΩ is brown-black-red; 220 Ω is red-red-brown. Swap them.
- Flash is unexpectedly dim. The reverse of the last one — a 1 kΩ ended up on the push-button lane. Swap the resistors so the button's lane carries the 220 Ω.
- LED very bright and warm. You accidentally left the resistor out of one lane, so a switch closes a raw LED-to-battery path. Kill power immediately and put both resistors back before anything pops.
What you're building
A bike-handlebar torch with two settings on one lamp. Slide the switch on and the yellow LED glows steadily but softly — a low beam that would run for a very long time on one pack of AAs. Press the button and the same LED punches out a bright flash for as long as your thumb holds it. Do both at once and the lamp is brighter still.
One LED. Two switches. Two different resistors. This is the first project where the choice of switch changes the brightness of the light, and that's a real-world design pattern: it's how car low-beams/high-beams share a bulb, and how a phone flashlight has both a torch mode and a camera flash.
The big idea: two lanes into one merge
Two paths run from the battery + rail down to the LED's anode:
- Low-beam lane: through the slide switch, then a 1 kΩ resistor.
- Flash lane: through the push button, then a 220 Ω resistor.
Wherever the two paths meet (electrically, right on the LED's long leg), their currents add up — the LED sees whatever the sum is. If only one lane is open, only that lane's current flows. If both are open, both lanes deliver current to the LED at the same time. The rule for these merges is Kirchhoff's current law: current in = current out. The LED accepts whatever gets delivered, and its brightness reflects the total.
Do the math
Yellow LEDs eat about 2.0 V just being on — same as red. That leaves 4.5 − 2.0 = 2.5 V to push through whichever resistor is in the way.
Low beam only (slide on, button up):
I = V ÷ R = 2.5 V ÷ 1000 Ω ≈ 0.0025 A = 2.5 mA
Roughly four times fainter than First Light — a soft warm glow, and about four times the battery life while it's the only mode running. A pack of AAs at 2.5 mA lasts on the order of weeks of continuous use.
Flash only (slide off, button pressed):
I = 2.5 V ÷ 220 Ω ≈ 0.011 A = 11 mA
First Light's current, exactly. Bright, comfortable, well inside the LED's 20 mA safety line.
Both at once (slide on, button pressed):
Two resistors in parallel act like one smaller resistor. The formula for two parallel resistors is:
R_parallel = (R1 × R2) ÷ (R1 + R2) = (220 × 1000) ÷ (220 + 1000) ≈ 180 Ω
Through 180 Ω, at 2.5 V of headroom:
I_total ≈ 2.5 V ÷ 180 Ω ≈ 0.014 A = 14 mA
Brighter than flash alone, still under the LED's 20 mA red line. Perfect "extra push" — the low-beam lane keeps trickling its 2.5 mA in while the flash lane blasts 11.3 mA on top of it. The LED just adds them together and lights.
Build it
- Power rails. Battery + → top row 0. Battery − → bottom row 3.
- Flash lane on row 0 (the top): push button → 220 Ω → merge column.
- Low-beam lane on row 1 (just below): slide switch → 1 kΩ → same merge column.
- Merge and LED. Both lanes meet at the LED's long leg (anode). The LED drops down; its short leg reaches the bottom rail.
- Complete the loop. Wire the bottom rail across to the battery − wire.
- Test all four states, in order:
- Slide on, button up → soft low beam. Look at it in a dark room — the difference from First Light is obvious.
- Slide on, button pressed → clearly brighter. The 11 mA flash "adds" onto the 2.5 mA low beam. That's KCL in your retina.
- Slide off, button pressed → equal to First Light's brightness. The low beam is out of the picture.
- Slide off, button up → dark. Both lanes broken.
Why the button gets the bright lane
Small design choice, real reason. The push button is momentary — it only runs while your thumb is on it. Momentary use of the big current (11 mA) is fine even on nearly-dead AAs. The slide switch is sustained — you might leave it on for hours — so it gets the tiny current (2.5 mA) that barely bothers the pack. Match the current draw to the duration of use. That's how real gadgets stretch battery life.
The takeaway
First Light had one path through one resistor. Twin Reds stacked two loads on one path. Bike Torch runs two different paths that meet at the load, and the load sees the sum of both currents. That merge idea shows up everywhere: two doorbell buttons on one buzzer (which you'll build next), two solar panels feeding one battery, two hosepipes emptying into one tank. Wherever paths converge, add the currents at the merge — always.
🧠 Your challenge
No single right answer. That's the point.
- Reverse the roles. Put the 220 Ω on the slide-switch lane and the 1 kΩ on the push-button lane. Now the slide is the bright steady beam, and the button is a dim flash. Predict the currents first (they just swap between the lanes). Which arrangement feels right for a bike light? A closet reading lamp? A pantry-door "someone left the door open" alert?
- Two dim, one bright. Add a second push button in parallel with the first, feeding another 1 kΩ resistor onto the same merge. Now you have two dim flashes and one steady low beam. Currents at the merge when everything is pressed: 2.5 + 2.5 + 11.3 = ~16 mA. Still safe. Design a use for it — say, one button for each rider so a duo bike ride can both signal.
- Ballpark the runtime. A typical AA cell holds about 2500 mAh. Divide by the current in each mode to estimate how many hours of that mode you can get before the pack quits. Low beam: 2500 / 2.5 ≈ 1000 h ≈ 6 weeks. Flash: 2500 / 11 ≈ 9 days of constant pressing. Both together: 7 days. (In reality, pack voltage droops faster than the "capacity" number suggests — but the ratios are correct, and that's the engineering point.)
- Add a red "brake" LED. Wire a red LED + 220 Ω between the push button's output and the bottom rail. Now pressing the button gives you a bright yellow flash and a red rear light in the same moment — one button, two lamps. Which of the circuits you know already uses this pattern? (Two-Tone Doorbell and Two-Hand Bell both split a switch's output into two output lanes — same idea, different mouths.)
For grown-ups: safety notes
- Battery-safe throughout — 4.5 V, and the worst mode (both lanes active) tops out around 14 mA. Fresh AAs run this for many days of continuous full-power use.
- Both switches close their own lanes. A stuck-shut push button turns your project into an accidental "always in flash mode," which will drain the pack much faster than you expect. If the button feels mushy or won't spring back, swap it out.
- Same LED-direction rule as First Light: long leg toward + (both lanes feed the anode), short leg to −. A backwards LED doesn't get hurt at 4.5 V, but nothing will glow no matter which switch you close.
- Watch which resistor goes where. Putting the 220 Ω on BOTH lanes turns the low-beam mode into an 11 mA drain — the battery-life story falls apart. Putting the 1 kΩ on BOTH lanes makes the flash mode almost invisible. Bands: 1 kΩ is brown-black-red; 220 Ω is red-red-brown. Read them twice.
- Don't short the button. If you jumper across the push button by mistake (a stray wire between its two rows), the flash lane is always on and your "momentary" plan doesn't work anymore.
- Small parts stay away from pets and toddlers — LEDs and switch legs are the same swallow-hazard shape as any other breadboard piece.
Checked against
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