This type of module is used in applications where you want a simple way to detect that a piece of mains powered equipment is active. It can be used to detect if a load that has been turned on has actually activated. That allows automated fault detection and indication.
It could also be used to trigger functionality when a buildings lights are turned on or a pump runs.
For 120V use it may be useful to swap in a 1W-2W 33K-56K resistor to increase the optoisolator current.
As shown in the video, this building block is often used in multiples on elevator control PCBs for sensing phase presence and direction, to detect high voltage safety circuits and monitor loads to ensure they are powered when expected. Combining it on the main PCB saves on the cost of external phase monitoring modules.
Definitely use multiple high power resistors when doing this though. Even if it does take up a bit more valuable PCB space.
You can find these if you search for 220V optoisolator. They also do three and eight way versions.
They don't seem to do specific 120V versions, but these ones may work as is, or after the resistor mod. Cost for a single channel module seems to be around 4-5 space credits.
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