Tiny audio optocoupler is about the size of a pencil eraser and has 4 leads. These are used extensively in audio compressors, audio level controls, audio limiters, expanders/noise gates, guitar tremolo effects, guitar amplifiers and music effect boxes. Inside the black epoxy case is a 2 lead photocell (not a photodiode or photo-transistor) and a 2 lead LED. This is why the device finds extensive use in audio applications (photocell instead of photodiode). The LED requires about 10-16mA current and the resistance of the photocell varies from 150K up to megohms when the LED is off to under 400 Ohms when the LED is on. New. NOTE: Dot on case marks the cathode (-) lead of the LED. Blowout price! Actual brand may vary from picture. Sold in a package of 10.