[fragmede]

your car is 12 volts, and USB is 5 volts; 12 or up to 20 these days for laptop charging. My computer's CPU is probably 1.8 volts but I can't remember the last time I had my multimeter on that, but that's still more than millivolts.

[raxxorraxor]

Probably meant milliampere, specifically 1 milliampere. But yes, usually lightweight engineers are familiar with TTL and limit themselves to 5V. 12V+ is another arcane realm you don't want to touch.

Some old serial ports had 12V and a high max current. The DIY things you attached here were prone to kill your mainboard.

Voltage/current is either 0 or 1. Anything higher kills software developers instantly.

[matt-p]

In telecoms 48V dc is very common and not always even connectorised! It's "safe-ish" but DC makes me more nervous than 240v, big thick 400A cables into a rack are quite intimidating to see but the main issue is DC is sticky and doesn't have the safety protections of RCDs etc. Indeed you are lucky to get a working isolator.

[littlestymaar]

That's technically correct, but irrelevant: you cannot kill yourself with 12 or 20V any more than with 10mV. 120V or 230V is another story.

That being said, it's still very easy not to kill yourself with 120/230V: just shut down the power before touching anything.