Hardware Hacking 113The regulator chip not only keeps the supply voltage free of spurious hum and
noise, but it lets you be very sure of the exact voltage being used tom power your
circuit. A simple measurement with a voltmeter will show that even though a
wall-wart might be marked “12 volts DC” in bright white letters, it might put out
anything from 10 to 20 volts. The CMOS chips used in most of our circuits were
chosen in part for their forgiving nature, but they have limits --- upper limits:
they can run on power supplies from 3 volts to about 18 volts, but above 18 volts
they can explode quite dramatically. The 9 volt battery we’ve been using sits
comfortably between these two extremes, and a 9 volt regulator substitutes
admirably for that battery. But if you choose to use a wall wart-based supply
that is merely filtered ((by capacitors) and not regulated, always measure the real
output voltage before connecting your circuit.
Rule #22: Never trust the writing on the wall-wart.