🎈 Twin Reds

What you're building

First Light had one LED on the loop. Twin Reds has two, wired one right after the other. Slide the switch and both light up together — same current, dimmer than a single LED at 11 mA, no extra resistor.

You just did something that trips up a lot of beginners the first time they try it: stacking loads on the same loop. Once you know what the voltage does, you can chain any number of parts without guessing.

The big idea: parts in series each take a bite

Every LED "eats" some voltage as current squeezes through it. For a red 5 mm LED that bite is about 2.0 V — its forward voltage. First Light burned that 2 V once, leaving 2.5 V for the resistor. Twin Reds burns it twice:

• Battery push: 4.5 V
• LED #1 eats: ~2.0 V
• LED #2 eats: ~2.0 V
• Leftover for the resistor: 4.5 − 2.0 − 2.0 = 0.5 V

That's what Kirchhoff's voltage law says in plain English: whatever the battery pushes out has to be spent by the parts on the way back. Ohm's law finishes the story on the resistor:

I = V ÷ R = 0.5 V ÷ 220 Ω ≈ 0.0023 A = 2.3 mA

Two milliamps — a fifth of what First Light delivered. Both LEDs are lit, both are safe, both are dimmer. That's the "same current everywhere in a series loop" rule, live on your breadboard.

Why they share the same current

Series parts sit on one road with no branches. Every electron that leaves the battery has to pass through LED #1, then LED #2, then the resistor, in that order. There's nowhere else to go. So the number of electrons per second — the current — has to be identical in every part. Different parts drop different voltages across themselves, but the current is the single number for the whole loop.

If you added a third red LED in series, the leftover for the resistor would be 4.5 − 6.0 = −1.5 V. Negative leftover is the math saying "the battery can't push that hard" — and in real life, nothing would light at all. Two reds is right at the edge of what a 4.5 V pack can drive.

Build it

1. Battery + switch: battery + → top row, then the slide switch. Same start as First Light.
2. The 220 Ω: after the switch, drop the resistor along the top row.
3. LED #1: long leg on the top row (the + end), short leg one row down.
4. LED #2: long leg on that middle row, short leg on the bottom row. Both LEDs point the same way — arrows in a queue, not head-to-head.
5. Return: bottom row → battery −.
6. Slide the switch. Both LEDs glow, gently. Take a second and compare with the flashlight from First Light — the same red LED, four times fainter. That's the math on your retina.

Debug like a series builder

Nothing lights? Trace the loop. If either LED is flipped backwards, the whole loop stops — remember, the loop is one road. The tell is easy: swap the suspect LED. If both were fine, no change; if one was backwards, both now light. One dead LED anywhere kills the whole series chain. Parallel projects (like Do Not Disturb) survive a single flipped LED; series projects don't. Different physics, different failure modes.

The takeaway

Real electronics is full of series stacks: strings of holiday lights, LED torches with the emitters chained, laser-pointer trains of three or four LEDs. Every one of them uses the same rule: add up the forward voltages, subtract from the supply, divide by the resistor. You just did the whole calculation in your head.