Sizing a resistor for driving a SSR from a digital out

Projects General Guidance
1

I want to drive this SSR from a digital out: Digikey 425-2403-5-ND
The datasheet is here [https://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/S102,202S02.pdf]

I don't understand how to properly size the needed resistor between the output and the SSR.

The arduino "DigitalPins" wiki (https://www.arduino.cc/en/Tutorial/DigitalPins) gives a baseline of "470 to 1K" ohms on a digital out pin.

On the SSR datasheet, page 4, it lists a 20mA draw at 1.2V. I calculate R = (5-1.2)/20 = 190 ohms.
On the SSR datasheet, page 6, fig 4, I interpolate a 110mA draw at 1.2V. I calculate R = (5-1.2)/110 = 35 ohms.

Not only are these two values different, they are quite low compared to the baseline.

I'd appreciate any help figuring this out. Thanks in advance.

2

You don't tell which Arduino You use but as a general rule max output för a digital output is 20 mA so 190 Ohm or more, 220, would be useful.

3

On page 8 under Recommended Operating Conditions the spec for Input signal current at ON state is 16 to 24 mA. I would calculate the resistor value based on 20mA so for a 5V output: (Vcc - Vf) / If or (5V - 1.2V) / .02A = 190 Ohms. Use the nearest standard value of 180 or 220 Ohms.