Top lamp is a 1. bottom is a 0.
When S1 is in a closed state, power from Vcc (positive power) will flow to S2 which defines the bit state (1 or 0). When power is put through D+ (in a USB, D+ represents an ON state (1), and D- represents an OFF state (0)), the bit state is 1 and vice versa. The remaining power goes trough a resistor (R1) that is connected to ground (negative power).
