The design that works, and the basis of the current prototype. The AD8317 covers 1 MHz to 10 GHz, which takes in every cellular band plus 2.4 GHz wifi. Four connections: power, ground, antenna in, voltage out. The detector's output has a negative slope. This is roughly −22 mV per dB, so more arriving power means lower voltage, which the firmware inverts before mapping it to pitch.

Measured on the bench: 1.5 V with the room quiet, falling to 1.1 V with a transmitting phone alongside the antenna.

600 MHz system (AD8307): Teensy 4.1 , AD8307 logarithmic detector, 9V battery.

Simpler, cleaner, and wrong. The AD8307 board covers 1–600 MHz. Cellular transmission lives at 700, 850, 900, 1800, 2100 and 2600 MHz, the entire target is above the ceiling. A useful reminder that the constraint that kills a design is often printed on the part in millimetre type. Still valid for FM and VHF work; useless for phones.

Teensy 3.1, LM2596 buck converter, LNA, logarithmic detector , 9V mains adapter.

The first attempt, and the most over-built. A low-noise amplifier ahead of the detector, and a switching regulator dropping mains-adapter voltage to run it. Both turned out to be liabilities: the LNA saturates the moment a phone gets close, and a switching supply injects exactly the kind of noise that a slow DC output cannot tolerate, the detector's reading wobbles, and that wobble becomes audible. Abandoned in favour of battery power and no pre-amplification.

A stereo electromagnetic microphone. Two coils at right angles, a preamplifier, a 9 V battery and a headphone jack and deliberately no magnet, which is the one part that separates it from a guitar pickup. It hears the immediate fields of objects: mains transformers at 50 Hz, the whine of switching supplies, the rhythmic texture of a laptop's processor, ticket gates, cash machines, LED drivers.

Its signal needs no demodulation. The coil's output is already in the audible range, so the whole chain is amplification. Its range is short; near-field coupling falls away with the cube of distance, and that is the point. You have to approach, circle, and rotate it.

Inspired in Christina Kubisch electric walks.