# Selecting Solid State Relay

I am trying to select an appropriate solid state relay for a project using an ESP8266 microcontroller (from 3.3v gpio pins). I don't know much about them, but I assume the key parameters are:

1. Maximum load parameters (current and voltage)
2. Minimum/maximum control parameters (current and voltage)

Take this http://www.mouser.com/ds/2/315/semi_eng_pd1a_aqy27-1075857.pdf as an example.

WRT to max load, it looks good: 60V/2amp continuously is more than I need.

The control parameters is a bit more confusing... On page 2 the datasheet lists a number of fields next to 'input'. Is the one I want for the control 'LED dropout voltage'? It looks like that says the maximum control voltage is 1.5V (above the 3.3V GPIO logic level coming from my microcontroller). Also, it looks like the 'recommended current' is 5-10mA (which is fine for the 12mA GPIOs). Can someone help me out with this part?

The specification is saying the LED will drop a voltage of 1.5V at a current of 50mA. However, you do not want to drive it that hard as any variance will kill the LED.

You need to deliver the rated current in order to ensure the device operates. The numbers in the spec sheet are a little weird here, but recommended led current is 5-10mA so lets pick 7.5mA. simulate this circuit – Schematic created using CircuitLab

So you need to drop the micros output voltage down to 1.5V at 7.5mA, that means you need a series resistor of $(3.3-1.5)/0.0075 = 240\Omega$

Lets double check that to be sure. If the LEDs actual voltage is 1.16V, the current through the $240\Omega$ would then be $(3.3-1.16)/360 \approx 9mA$ which is still within the recommended values.

The input is a simple IR LED - you have to limit the LED current to 5 - 10 mA with a series resistor, as you would with any other LED. When calculating the resistor value, you should allow for the LED forward voltage of 1.25 Volts