Something doesn't work

Common problems and what to do about them. Look for what you see on the screen.

Nothing changes, the switches do nothing

The power is off — the Power button, first on the left of the top bar. While it's off, the circuit isn't computed.

What the wire colours mean

1one0zeroZconnected to nothing — "floats"Xtwo outputs fight — a short
Wire colours in the simulator. Amber dashed and red signal a mistake: something is not connected or two outputs are joined.

The wire is an amber dashed line

That's Z: the pin is connected to nothing, the signal "floats". Usually:

  • a wire or a label is missing;
  • a typo in a label name. Case matters: A and a are different labels, so are CLK and Clk. This is the most common mistake;
  • a chip input isn't driven — check every pin against the build table.

The wire is red

That's X: two outputs are connected and fight — one pulls to 1, the other to 0. Usually:

  • the same label is on two outputs. Every signal must have exactly one source;
  • two labels with different names got connected by accident — by a wire or by sitting on the same pin. Then it is one signal under two names: everything connected to both labels has merged. Look for a wire between labels or a pin that has two of them.
Labels A and B accidentally joined by a wire
  1. 1 the stray wire: it joined labels A and B — now it is one signal, and the switches pull it to 1 and to 0
A typical short: everything is red (X), and the AND output "floats" (amber dashes). Delete the stray wire and the labels become separate signals again.

The lamp or the screen doesn't react

  • the transistor's top and bottom are swapped;
  • the body says P instead of N (or the other way round) — click the body to change the type;
  • a label is on the wrong pin — check the build table: the pin name on the left, the label on the right.

The chip is built but the pins are wrong or missing

A chip's pins are ports: [ — input, ] — output. Every port needs a name (double-click). No port — no pin; an input placed instead of an output — the pin is on the wrong side. To fix: double-click the chip → Edit.

A big circuit runs very slowly

Switch the engine on the top bar from Real to Fast. Real computes every transistor — exact but slow; Fast works with gates and is many times faster. Both are fine for checking a circuit.

My work is gone

The simulator saves your work by itself — in this browser. In another browser, a private window, on another computer or after clearing browser data it won't be there. So after every lesson — Export (a file on your disk); to bring the work back — Import.

I can't see where the mistake is

  • select a chip and press T — the truth table; compare it with the lesson;
  • W — the oscilloscope: shows how signals change over time;
  • open a new empty project, paste the lesson's ready circuit (Cmd+I, the text is at the end of the lesson) and compare it with yours: wherever they differ, that's the mistake.
The truth table of the AND chip
The truth table: select a chip and press T. Every input combination with its output — compare it with the table in the lesson. Works for circuits without memory.
The oscilloscope with signals A and B
  1. 1 Freeze — stop recording to look closely
  2. 2 add — a label name and Enter: the signal gets its own row
  3. 3 signals over time: high is 1, low is 0; you can see who switched when
  4. 4 knobs — time position and scale
The oscilloscope (key W) records signals clock by clock. For circuits with memory and a clock it is the main way to find a mistake: at what moment a signal went wrong.

Completely stuck

Write to me on Telegram: @daniilkhanin — send the lesson number and what you see (a screenshot helps). English and Spanish are fine.