A relay is a current-controlled switching device. The working principle of the relay is that when the 5 pin relay 24v relay coil is energized, the iron core in the coil generates a strong electromagnetic force, and the armature is attracted to drive the reed, so that the contacts 2 and 5 are disconnected, and the contacts 2 and 4 are connected. When the coil is de-energized, the spring resets the reed, causing contacts 2 and 5 to close and contacts 2 and 4 to open. As long as the circuit to be controlled is connected between contacts 2 and 5 (2 and 5 are called normally closed contacts) or between contacts 2 and 4 (called normally open contacts), the relay can be used to achieve some kind of control. Purpose. 

On the side of the three feet, the middle pin 2 is the common terminal of the output contact, and the other two pins 1 and 3 are the coil, which is connected to the driving end. Pins 4 and 5 are normally open and normally closed contacts respectively.
The 5 pins are marked as 30 (5 pins), 87 (4 pins), 85, 86, 87a (2 pins), 30 is connected to the live wire (BATT constant power), 87 or 87a is connected to electrical appliances, 30 and 87a are connected, 85 and 86 are two ends of a coil, 85 is grounded, and 86 is connected to the switch.