Adding Qi Wireless Charging to Maker Electronics Projects: Coils, ICs, and Foreign Object Detection

Adding wireless charging to a project — an ESP32 sensor puck, a 3D-printed remote, a battery-powered enclosure you don't want to open every time it needs a charge — is a lot more approachable than it looks. The Qi standard has matured to the point where the receiver side is a single small IC and a flat coil, both cheap and widely available, and the transmitter side can be a repurposed off-the-shelf charging pad if you don't want to design your own. This guide covers how Qi charging actually works, the components involved, and the practical wiring and enclosure considerations for adding it to a maker project.

How Qi Charging Works

Qi is a resonant inductive coupling standard — the transmitter coil generates an alternating magnetic field, the receiver coil picks it up and rectifies it back to DC, and the two sides communicate over the same coupled field to negotiate power level and confirm the receiver is present (this is what stops a charging pad from just dumping power into any metal object placed on it). Standard Qi (BPP, Baseline Power Profile) delivers up to 5W, which is plenty for most small electronics projects and simpler to implement than the higher-power EPP profiles used for fast-charging phones.

Core Components

• Receiver coil — a flat, spiral-wound copper coil, typically 30-50mm in diameter for small projects; buy a matched coil/IC combo rather than sourcing them separately unless you're doing custom coil design
• Receiver IC — handles rectification, communication, and voltage regulation; common choices are the IDT/Renesas P9221 family or the simpler, more DIY-friendly modules built around it that ship as a complete breakout board
• Transmitter — for most maker projects, buying a certified Qi charging pad is far easier and safer than building a transmitter coil driver from scratch; treat the pad as a black box and design your receiver to work with it
• Output regulation — Qi receiver modules typically output a regulated 5V, which you'll still want to feed through your project's own battery charging circuit (a TP4056 or similar) rather than wiring it directly to a LiPo cell

Foreign Object Detection and Why It Matters for Your Enclosure Design

Foreign Object Detection (FOD) is the mechanism Qi transmitters use to sense when something other than a proper receiver — a coin, a keyring, a paperclip — is sitting on the pad, and shut off power to avoid inductively heating that object. This has a direct design consequence for your project: any metal between the transmitter coil and your receiver coil, including some 3D printer filaments with metal fill, battery shielding, or a metal enclosure panel, can trigger a false FOD trip or block the field entirely. Keep the charging path through non-metallic material — standard PLA, PETG, or ABS all pass the magnetic field fine, but carbon-fiber or metal-filled filament directly in the charging path will cause problems.

Coil Placement and Alignment

Qi coils need reasonably tight alignment with the transmitter coil to charge efficiently — a few millimeters of lateral offset is generally tolerated, but stacking too much air gap between the coils (thick enclosure walls, a gap for a battery in between) drops efficiency fast and can push the system below the threshold the transmitter needs to maintain a connection. Keep the wall thickness between the coils under about 5mm for a hobby-grade setup, and if you're designing a custom enclosure, add a visual or magnetic alignment feature (a small embedded magnet ring, matching many phone charging pads) so the user doesn't have to guess where the sweet spot is.

Design FactorGuidance Wall thickness between coilsKeep under ~5mm for reliable coupling Material in the charging pathPLA/PETG/ABS fine; avoid metal-fill filament, foil shielding, or metal panels directly in line Coil alignment toleranceA few mm lateral offset is usually fine; add an alignment magnet ring or visual marker for the user Battery placementKeep the LiPo cell to the side of the coil stack, not sandwiched between the coils

Wiring the Receiver into Your Project

A typical wiring chain looks like: Qi receiver module → regulated 5V output → battery charge IC (TP4056 or similar) → protected LiPo cell → your ESP32 or Arduino's usual 3.3V/5V rail. Don't skip the dedicated charge IC even though the Qi receiver already outputs a clean 5V — charging a LiPo cell safely needs the current-limiting and termination-voltage behavior a proper charge controller provides, and wiring 5V directly to a bare LiPo is a fire risk regardless of where that 5V came from.

Efficiency and Heat

Expect somewhere in the 60-75% efficiency range for a typical small-coil Qi link at hobby power levels — the difference shows up as heat in both the transmitter pad and your receiver coil, which is worth accounting for if your enclosure is otherwise sealed. For a low-duty-cycle project (charges overnight, runs off battery the rest of the time) this is a non-issue; for something charging continuously while also running active electronics, add ventilation or keep total draw modest.

When Qi Isn't the Right Answer

Wireless charging adds cost, design complexity, and charging inefficiency compared to a simple USB-C or barrel-jack connector, so it's worth being honest about whether a project actually benefits from it. It shines for enclosures that need to stay fully sealed (outdoor sensors, waterproof housings), for consumer-facing builds where a charging port would look out of place, or for anything handled frequently enough that a physical connector would wear out. For a one-off shop tool or anything you'll happily plug in with a cable, skip the added complexity.

Qi wireless charging has gone from "exotic hobbyist project" to "off-the-shelf module you wire in an afternoon" largely because the receiver ICs now handle all the protocol negotiation invisibly. The parts that still require actual design thought are mechanical — coil alignment, material selection around the charging path, and giving the system enough clearance to avoid triggering foreign object detection — and getting those right the first time saves a lot of enclosure redesign later.