>So I have played around with the Linear Technology
>spice program - see attached jpeg.

You will find that the 8N137 opto is a lot faster than the 4N25, but an even
better one is the Agilent (now Avago Technologies) HCPL-2300 which has an
amplifier for the PIN diode included, and has very low LED current
requirement, and is claimed to be 8M baud. Also has UL, CSA and VDE
approvals for working with mains voltages.

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