This small audio optocoupler is about the size of a pencil eraser and have a huge dynamic range, with a resistance ratio over 100,000:1. 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 photo-diode 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 550 ohms when the LED is on. NOTE: Dot on case marks the cathode (-) lead of the LED.
These are the same vactrols that we use in our 4093 Chaos NAND, MST Low Pass Filter, Vac Pak, etc.
The DLY/Roboto version vactrols are tested to work specifically with those modules. Those modules require a faster reaction speed than normal; most circuits don't need a fast reaction speed. There is no need to get them unless you are planning to use them in those modules. If you purchased a PCB and panel of the DLY or Roboto, the tested vactrols are included in your purchase.