Flip the two bit levers — the count of lit lamps is the number.
Logic Gate
Binary → Decimal
Two levers are the bits of a binary number — one worth 1, one worth 2 — so together they count 0 to 3. Three lamps light at the thresholds ≥1, ≥2 and ≥3 (an OR gate, a straight wire, and an AND gate), so the number of lit lamps is the decimal value. It starts at binary 10, which is two lamps — decimal 2.