Everybody who builds a battery-powered thing asks the same question, and almost nobody can answer it: how long will it last?This book answers it with arithmetic you can do with a pencil, on one real device.
The Long Watch is a sensor the size of a paperback that wakes once a minute, records how cold and how damp it was, and tells you
how many hours it was past a line you care about. Hours below freezing. Hours above 70% damp. Hours under the humidity a mushroom flush needs. A thermometer tells you it
hit −2 °C; this tells you it was below zero for five hours, which is the number that decides whether the crop lives.
It runs
375 days on three AA cells — and you work that number out in Chapter 3, long before the thing exists.
Who this is for: Arduino and Pi Pico beginners, weekend makers, growers with a greenhouse or grow tent, and anyone whose battery project died in a fortnight and never found out why.
Thirteen chapters, thirteen finished numbers:- One LED, one resistor — and why you round the resistor up, not to the nearest value
- What a battery holds — and the capacity you never reach, because your circuit gives up first
- Current × time — the whole year, worked out on one sheet of paper
- Dividing a voltage — a divider that reads the pack without quietly draining it
- Volts to numbers — what one ADC count is actually worth, and when more bits buy nothing
- Making a sensor tell the truth — two known points, and the straight line between them
- R, C and time — settling, debounce, and how long a thing takes to be ready
- Switches, pull-ups and a screen that is off — what two resistors cost you per year
- PWM and brightness — why half the duty is nothing like half the light
- The regulator and heat — the part that was in every sum and never examined
- Storing and sending — the second budget, counted in bytes, and what a radio really costs
- The same node, two other jobs — when a runtime stops being a number and becomes a range
- The capstone — build it, measure it, and explain the gap between measurement and prediction
Two inversions most beginners get backwards. The battery goes into the
waking, not the sleeping — 1.2 seconds of every minute spends 87% of the budget, so the lever is a shorter wake, not a cleverer sleep mode. And two resistors too small to feel, left permanently connected, take 375 days down to 186.
Bring your own node. The demo device is worked in full, every figure shown — and beside it, on the same sheet, runs yours: your wake interval, your cells, your sensor. By Chapter 11 you hold a complete current budget for a device of your own choosing. By Chapter 13 you have measured one and explained why it disagreed with you.
Every chapter ends with a graded practice set and an answer key that walks the reasoning, not just the number. A fill-in budget sheet at the back, and
nineteen free calculators online - no account needed.
Nothing in this book touches mains electricity, and no soldering iron is needed. Battery and USB only, from the first page to the last.
Predict the battery life. Then check it in an afternoon.This book was written in collaboration with AI tools. Every number in it — every current, every charge, every runtime, every price — is recomputed from first principles by an automated arithmetic harness, and the book does not go to print unless all 825 checks pass.
This book teaches arithmetic, not procedure. It is not a substitute for the datasheet of the part in front of you. All prices are illustrative and dated, shown beside a blank for your own.
