part · Level 1 · Light · power
What a 3×AA battery pack does
3×AA battery pack (4.5 V) — The push that gets electrons moving. 4.5 volts — safe to touch.
What it is
A battery is a little chemical pump. Inside each AA cell, a slow reaction pushes electrons out of one end (the − terminal) and pulls them in at the other (the +). That push is called voltage, measured in volts (V). One fresh AA gives about 1.5 V.
Your pack holds three AAs end to end, + of one touching − of the next. Voltages in a chain add up: 1.5 + 1.5 + 1.5 = 4.5 V. Two wires come out — red is +, black is − — and every circuit you build is a loop from red, through your parts, back to black.
4.5 V is far too small to feel. Your skin is a resistor of tens of thousands of ohms, so almost no current can get through you. That is why every early TinyOhms project is skin-safe, and why the rule is batteries only, never the wall.
How it works
Voltage on its own does nothing. Current — the flow of charge, measured in amps (A), usually milliamps (mA) here — only appears when the loop is complete, and how much flows depends on what is in the loop. Ohm's law says current = voltage ÷ resistance:
- A red LED with a 220 Ω resistor drops 2.0 V across the LED, leaving 4.5 − 2.0 = 2.5 V for the resistor. 2.5 V ÷ 220 Ω ≈ 11 mA.
- A fresh alkaline AA holds roughly 2000 mAh (milliamp-hours). At 11 mA that is about 2000 ÷ 11 ≈ 180 hours — a week of nonstop light.
Now the scary case. Touch red straight to black and the only resistance left is the battery's own insides plus the wire — well under 1 Ω. 4.5 V ÷ 0.5 Ω = 9 A in theory. The cells cannot actually deliver that much, but they will try, and the wire and the cells get hot within seconds. That is a short circuit, and it is the one mistake at this voltage that can actually hurt (a burn, or a leaking cell).
As cells run down the voltage sags: a pack reading 3.9 V still lights an LED, but every current you calculated is now smaller. If a project is dim or a buzzer is weak, measure the pack first.
The numbers
| Voltage, fresh | 4.5 V — 3 × 1.5 V alkaline cells in series |
|---|---|
| Voltage, nearly flat | ≈ 3.3 V — 3 × 1.1 V — time to swap them |
| Capacity per cell | ≈ 2000 mAh — alkaline AA at low current; rechargeable NiMH ≈ 1.2 V and 1900–2500 mAh |
| Red wire | + (positive) — black wire is − |
| Short-circuit current | several amps — hot wires, hot cells — never join red to black |
See it working
The First Light circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Red touching black through nothing but wire. The pack warms up and the batteries die in minutes. Fix: pull a wire out immediately, then find the missing resistor or the wire in the wrong breadboard row.
- One cell in backwards. The pack reads 1.5 V instead of 4.5 V and nothing lights. Fix: match the flat end of each cell to the spring.
- Mixing old and new cells. The weak one drains fastest and can leak. Fix: replace all three together.
- Trusting the label. "4.5 V" is the fresh value. Fix: keep a multimeter nearby and measure before you blame the circuit.
- Using a USB charger or wall adapter "because it is 5 V". Adapters can push amps with no chemistry to slow them down. Fix: stay on AA packs for every Level 1–6 project.
Questions people ask
- Why does a 3×AA pack give 4.5 volts?
- Each alkaline AA cell produces about 1.5 V. Cells in series add their voltages, so three in a row give 1.5 × 3 = 4.5 V.
- Can 4.5 volts from AA batteries shock you?
- No. Skin resistance is tens of thousands of ohms, so 4.5 V pushes far less than a milliamp through you — below anything you can feel. The only hazard is heat from a short circuit.
- Why is my battery pack getting warm?
- Something is drawing far too much current, almost always a short circuit: a wire from + landing in the same row as −, or an LED wired without its resistor. Disconnect one wire right away and check the loop.
- Can I use rechargeable AA batteries?
- Yes. NiMH cells are about 1.2 V each, so the pack reads about 3.6 V. Every current in your calculations drops a bit — an LED with a 220 Ω resistor goes from 11 mA to about (3.6 − 2.0) ÷ 220 ≈ 7 mA — still bright.
Projects that use this part
- 🔦 First Light — Build a real flashlight circuit and learn why every circuit is a loop. L1 · 15 min
- 📡 Morse Code Messenger — A push button, an LED, and a secret code — send real messages with light. L1 · 20 min
- 🎈 Twin Reds — Two red LEDs on one loop — same current, half the leftover push, and a lesson in how voltages stack when parts sit end to end. L1 · 15 min
- 🚦 Tabletop Traffic Light — Three LEDs on three parallel lanes, all lit by one switch — build a tabletop traffic light and discover parallel circuits. L1 · 25 min
- 🔐 The Vault — Two buttons, one loop — the LED only lights when BOTH are pressed at the same time. L1 · 15 min
- 🚪 Do Not Disturb Sign — A green "chill" LED plus a switched red "leave me alone" LED — two lanes on one battery, and a switch that only touches one of them. L1 · 20 min
- 🤝 Handhold Chain — Three switches in a line — the LED only lights when all three are closed at once. AND, one step past The Vault. L1 · 15 min
- 🚪 Two-Door Signal — Two slide switches in parallel, one shared LED — either switch alone lights the lamp. The OR gate, at Level 1. L1 · 15 min
- 🚨 Signal Light — A slide switch and a push button in parallel — the slide is the steady beacon, the button is the extra flash. Either lights the lamp. L1 · 15 min
- 🛟 Escape Hatch — The Vault with a big red emergency button wired around both locks — turn BOTH keys, or hit the button, and the light comes on. L1 · 20 min
- 🧪 The Brightness Lab — Run a real experiment: predict, measure with your eyes, and prove Ohm's law. L1 · 30 min
- 🔔 Door Buzzer — Your first sound circuit — a real doorbell for your bedroom door. L2 · 20 min
- 🎛️ Dial-a-Brightness — Twist a knob and watch the LED fade up and down — your first "how much" input. L2 · 20 min
- 📻 Dial-a-Buzz — Dial-a-Brightness for sound — one knob rides the buzzer between full alarm and library silence, with no button in the way. L2 · 15 min
- 🔧 Continuity Tester — Your first real tool — two probe wires that beep and light up whenever the tips touch anything that conducts. L2 · 20 min
- 🚨 Panic Doorbell — Two buttons wired so either one fires the same buzzer and light — your first OR gate. L2 · 20 min
- 🔈 Whisper Alarm — A doorbell with a volume knob — the pot lives in the buzzer lane, so twisting it changes the beep without touching the light. L2 · 25 min
- 🛎️ Two-Tone Doorbell — Two call buttons, one doorbell: LOUD rings the buzzer and lights the porch LED; QUIET starves the buzzer and shows why it can't just be dimmed. L2 · 25 min
- 🚨 Arm & Ring — A doorbell that ignores you unless a hidden slide switch is armed first — series-AND with sound, plus an 'armed' LED that shows the state. L2 · 20 min
- 🔔 Twin Bells — One doorbell button rings two buzzers in parallel — one in your room, one in the kitchen — with a slide switch that mutes just the bedroom bell at night. L2 · 20 min
- 🖐️ Two-Hand Bell — Two push buttons in series feeding a buzzer and an LED — both hands must stay pressed for the alarm to sound. L2 · 20 min
- 🔕 Silent Alarm — Three panic buttons wired in parallel, all inside an armed lane — any one of them fires the buzzer, and a small LED tells you the alarm is watching. L2 · 25 min
- 🎮 Quiz Buzzer — Two teams, two big buttons: yours lights your LED and rings your buzzer, theirs does the same on their side. Two independent doorbells sharing one battery. L2 · 20 min
- 🌙 Automatic Night Light — A light that switches itself on when the room gets dark — no hands, no code. L3 · 35 min
- 🌅 Sunrise Alarm — The night light — flipped. Swap two parts and the LED wakes up with the sun instead. L3 · 25 min
- 🍪 Cookie Jar Alarm — The Sunrise Alarm's brain wired to a buzzer instead of a lamp — quiet in the dark jar, screaming when the lid comes off. L3 · 25 min
- 🪤 Trip Wire Alarm — A thin wire across a doorway holds the alarm silent. Break the wire and the buzzer screams — until you connect it back. L3 · 25 min
- 🌡️ Warm Hand Alarm — Cup your hand over the sensor and the LED lights up — a thermometer that turns into a switch. L3 · 30 min
- 📖 Reading Light — The Automatic Night Light with a knob on top — turn it to dial in how dark the room has to get before the lamp fires. Your first tunable threshold. L3 · 30 min
- 🏝️ Vacation Nightlight — The Automatic Night Light with a manual override — flip the slide switch off and the lamp stays dark even in a pitch-black room. L3 · 25 min
- 📚 Study Lamp — The Automatic Night Light with a force-on override — flip the slide switch and the lamp stays lit even in daylight, ignoring the sensor. L3 · 25 min
- 🌆 Curfew Beeper — The Automatic Night Light's brain with a buzzer where the LED used to sit — beeps the moment the room goes dark, so you never miss sunset. L3 · 30 min
- 🤒 Fever Beeper — The Warm Hand Alarm's brain wired to a buzzer instead of a lamp — silent in a normal room, beeping the second a warm palm covers the sensor. L3 · 25 min
- 🌇 Dusk Chorus — The Curfew Beeper with a red LED wired in parallel with the buzzer — one dusk sensor drives a chime and a lamp at the same time. L3 · 30 min
- 🎺 Warm Hand Chorus — Fever Beeper with a red LED added in parallel with the buzzer — one warm-hand sensor drives a chime and a lamp at the same time. L3 · 30 min
- 🌀 First Spin — Your first motor. A switch, a coil, and a diode that catches the voltage kick when the current stops. L4 · 15 min
- 💨 Handheld Fan — Press the button, the fan spins. A tiny finger current commands a hundred-milliamp motor through a transistor. L4 · 20 min
- 🌞 Sunrise Fan — Sunrise Alarm with the LED swapped for a motor. The sun rises, the fan starts — an automatic wake-you-up desk breeze. L4 · 25 min
- 🆘 Panic Fan — Two buttons, either one fires the fan. Panic Doorbell wearing a motor — an OR gate that spins. L4 · 20 min
- 🎉 Party Spinner — One button, three mouths: a spinning motor, a red LED, and a buzzer all fire together. One transistor drives them all. L4 · 25 min
- 🚗 Twin Wheels — Two motors from one button — a Level 9 robot drivetrain, five levels early. L4 · 25 min
- 🎚️ Dial-a-Spin — Dial-a-Brightness with a motor instead of an LED. Twist the knob and watch the fan speed slide from full spin to a slow crawl. L4 · 20 min
- 🕸️ Trip Wire Fan — Trip Wire Alarm's brain wearing a motor. The wire is intact — the fan is silent. Snap it — the fan spins as the alarm. L4 · 25 min
verified against: SparkFun — What is a Battery? · SparkFun — Battery Technologies · SparkFun — Voltage, Current, Resistance, and Ohm's Law