Put an LED on it: observability for breadboard computers

Software people pay real money for observability. Dashboards, traces, structured logs, a vendor bill. On a breadboard, the entire discipline costs three cents: a resistor and an LED on every signal you care about.

While building the discrete 8-bit CPU, I put an LED on every register bit, every bus line, and every control signal. Forty-something lights. It looks like a carnival and it is the only reason the machine works.

What the lights actually buy you

State is visible at wire speed. No probe, no meter, no context switch. You clock the machine by hand and watch the instruction move: bus lights flicker, the instruction register catches the pattern, the control word lights shift to the next microstep. When a value is wrong, you saw where it went wrong.

Bugs announce themselves. My favorite failure: the A register lights dimmed — not off, dim. Two outputs were fighting over the bus, each winning half the time. A logic analyzer shows that as a confusing mid-rail voltage. The LED just looks sick.

They enforce honesty about the clock. With LEDs on the clock and control lines, you can single-step and verify each microinstruction does exactly what the microcode table says. I found two microcode bugs this way before they ever executed at speed.

The three-cent rules

One LED per bit you’d ever want to ask about. You will ask.
Buffer them (or use high-efficiency LEDs at low current) so the indicator doesn’t load the signal it’s watching — a 74LS output driving ten inputs and a hungry LED starts missing spec.
Group them physically like the data flows logically. The bus display goes between the things that share the bus.

The general principle

Every system should expose its state at the granularity you’ll debug it at. On the bench that’s an LED per bit; in firmware it’s a status struct you can dump over serial; on a website it’s build output you actually read. The medium changes, the move doesn’t: make the invisible thing visible before it breaks, because afterward you’ll be too busy guessing.