This is how to connect a tongling relay to a d1-mini (ESP8266)

d1mini_relay_wiring.md

Connect a 5V relay

I had issues connecting the relay to the wenmos d1-mini (ESP8266 or ESP-12F), turns out you must use the 3v3 pin not 5V.

relay	d1
VIN	3v3
GND	G
IN	D2

You can use any pin you want for IN, but check diagram below for safe GPIO pins that aren't used by something else.

Example code that turns relay on and off:

int state = 0;
int relay = 4; // D2 on d1-mini
void setup() {
  pinMode(relay, OUTPUT);
}

void loop() {
    if(state == 0){
      state = 1;
      digitalWrite(relay, HIGH);
      delay(3000);
    } else {
      state = 0;
      digitalWrite(relay, LOW);
      delay(3000);
    }
}