Boards are powered by two 9volt alkaline batteries. Circuits are a compilation of circuits found in the Radio Shack Engineering Handbook for making pulse modulated LED transmitters and receivers. The laser diode `D1` is substituted with an inexpensive laser pointer pen available at most hardware stores, K-Mart, Wall Mart, etc The transmitter board
replaces the batteries used in the pen. Range 200-600 feet at night. Sound quality is excellent.