Isolation Transformers, transistor optocoupler, light waves electrical isolation

 PS2501 Series Single Channel 80 Vce 5000 Vrms SMT Photocoupler - DIP-4

https://www.futureelectronics.com/p/semiconductors--optoelectronics--isolation-components-optocouplers/ps2501l-1-f3-a-renesas-1397785