1. Select the current suitable for the load, that is, the current that the contacts can pass. This current is already marked on the relay. Note that this current is for AC resistive loads to control DC, inductive or capacitive loads, which should be greatly reduced.
2. Determine the pickup current of the relay. Because DC pull-in current = 12v/coil resistance.

3. Release the current, regardless, as long as the control circuit can be completely turned off.
4. Select the appropriate triode or optocoupler.
5. Add back EMF elimination diodes.
The relay is divided into two sides, one side is the contact terminal, which is connected to a strong electric load. The other side is the coil end, which is connected to the weak current drive control. It is a switch that controls a high power load with a small current. The relay consists of coils, contacts, armatures, pins, etc. The contacts are divided into static contacts and moving contacts, the moving contacts are fixed above the armature, and the static contacts are connected to the pins. When the coil is not energized, the moving contact and the static contact are disconnected; when the coil is energized when it is working, the coil generates electromagnetic attraction, attracts the armature to move, and closes the moving contact and the static contact.