Battery-only operation is the primary safety rule for hardware-hacking live circuits

The governing safety rule of hardware hacking is: never open or modify anything that plugs directly into AC mains. Wall voltage (120 V in North America, 240 V in Europe) can trigger ventricular fibrillation at currents as low as a few milliamps — within reach of accidentally bridging two mains points with a wet finger. Battery voltages (typically 1.5–9 V) cannot drive lethal current through human skin under normal conditions, so all projects run on batteries; this also eliminates ground-loop shock risk when touching circuit boards during performance. Wall-wart adapters supply lower-voltage DC but are not trustworthy — their output can be variable and higher than the marked value, and mains voltage still lives inside the housing; a voltage regulator chip stabilizes and limits the output. The rule has no ‘just checking’ exception: momentary mains contact is enough to injure. When a hacked circuit must feed AC-powered gear, an audio isolation transformer or DI box provides ground-fault protection; a buddy who can pull you off a live circuit is standard practice.

Examples

All projects run on 9 V batteries or AA cells; the power-supply chapter is placed last because it carries the highest risk. A battery radio can be opened and touched with damp fingers safely; an alarm-clock radio still needs AC to receive and must not be opened. CMOS chips tolerate ~3–18 V but are destroyed above 18 V, so a wall-wart marked 12 V that actually outputs higher can both shock and fry the circuit.

Assessment

Explain why 9 V battery power is safe to touch with wet fingers while 120 V AC is potentially lethal at the same contact resistance. A student wants to power a CMOS oscillator from a wall-wart marked 12 V DC — list two specific dangers and a mitigation for each.