# Sizing a resistor for driving a SSR from a digital out

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.

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.

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.