It has 4 lights: a green one, a red one and two yellow ones. The green and the red light indicate if the circuits are correct. The two yellow ones go on for approximately 1 second at each interruption of the relevant circuit and are therefore useful to show micro-breaks.
Power supply: 12V battery included (common radio controls battery A23 or VR22 type).
Sturdy case of extremely small dimensions: 3.8 x 5.5 x 2.9cm. Its small dimensions make it pocket-size. It can be used as a key-ring.