Optocoupler/LED/nixie tube/photoelectric device

FODM217 Series Single-Channel, DC Sense Input, Optocouplers Consisting of a Gallium Arsenide (GaAs) Infrared Emitting Diode Optically Coupled to a Phototransistor in a Compact, Half-Pitch, 4-Pin Encapsulation Input-Output Isolation Voltage, VISO, rated at 3750 VACRMS.

F3 red hair red short legs Iv=5-8mcd

1210 yellow (four-legged yang)