A three-terminal voltage regulator (78xx) turns a noisy wall-wart into a clean, fixed supply voltage

When batteries no longer suffice, a DC wall-wart can power a hacked circuit, but its output is unregulated: a case marked “12 volts” may actually deliver anywhere from 10 to 20 volts, plus AC ripple that makes circuits hum. A three-terminal linear regulator (the 78xx family) fixes both problems: it filters out the remaining ripple and pins the output to a precise voltage set by the chip you pick (7805 = 5 V, 7809 = 9 V, 7812 = 12 V). The regulator needs its input at least ~3 V above its target output, and normally runs a bit hot, so its metal tab is bolted to a heatsink. This matters for hacking because the CMOS chips used throughout the book tolerate 3–18 V but expire dramatically above 18 V, so an unregulated supply that drifts high can destroy a circuit that a regulated 9 V would run safely — a 9 V regulator substitutes neatly for the 9 V battery.

Examples

Feed a 12 VDC wall-wart into a 7809: the output is a steady 9 V, matching the battery the circuits were designed around. Add a 0.1 uF capacitor between a chip’s + and ground pins to further reduce noise and crosstalk.

Assessment

Your wall-wart is labelled “12 V DC” but your meter reads 19 V. Why is connecting it directly to a CMOS circuit risky, and which chip would you add to make it safe? What input-vs-output voltage headroom does that chip require?