What the TIL113SM does in your circuit
The TIL113SM is a single-channel optoisolator from onsemi that uses an infrared LED and a photodarlington detector to pass a signal across a 4170Vrms isolation barrier. The Darlington output with a separate base pin gives you the option to tailor the switching threshold or add external feedback — useful when you need to adjust the turn-off time or compensate for LED ageing. With a minimum current transfer ratio of 300% at 10mA forward current, the output can sink up to 150mA continuously. That CTR floor means the photodarlington will always deliver at least three times the input current to the load, so a 10mA LED drive gives you at least 30mA of collector current — plenty to drive a relay coil, a logic input, or a small solenoid directly.
Package and mounting — 6-SMD gull wing
The TIL113SM comes in a 6-pin surface-mount package with gull-wing leads (6-SMD). The base pin (pin 6) is brought out — if you do not need it, leave it floating; the Darlington still switches normally.