The ESP can control a relay board using one of it's GPIO pins. All relay boards that work with Arduino should also work with the ESP module.
Connect the relay boardj to one of the GPIO pins on the ESP module. In case of ESP-01, we will use GPIO-2
Most of the relay boards expect a 5V ttl signal like many arduino boards provide on their output pins. But 3V3 logic should also work.
- Be careful with these relais when you consider to connect them to mains. In that case, some of them are not safe to be used when they are in a circuit with parts that can be touched. For example, if you want to connect such a relay on an ESP that also controls a sensor that can be touched. - The tracks on the pcb are sometime not suitable to carry more than some 4 Amps. See also: http://www.desmith.net/NMdS/Electronics/TraceWidth.html
We can control the relay board with two simple http url commands:
http://<ESP IP address>/control?cmd=GPIO,<pin>,0
http://<ESP IP address>/control?cmd=GPIO,<pin>,1
Syntax: WiredOut 1,On/Off