Too little current to trigger a relay? I try to use this relay with a Arduino Mega. I connected with jumper cables


5V pin to DC+, 
GND to DC- and
Digital Pin 13 (because its a lucky number) to IN.


The power LED from the relay is lighting green. 

I run this code, which should trigger the relay every 3s for 3s: 

int pinOut = 13;

void setup() {
  pinMode(10, OUTPUT);
}

void loop() {             
  digitalWrite(pinOut, LOW);
  delay(3000);

  digitalWrite(pinOut, HIGH);
  delay(3000);            
}


When initializing the sketch, the red light (from the relay) is flashing and the relay is clicking. But afterwards nothing happens. 

With my multimeter I check the currents: 


5V to GND = ~5V
Pin13 to GND, when not connected to relay = ~4.7V
Pin13 to GND, when connected to relay = ~1.2V


The relay needs 5V and 5mA triggering current. I don't think the Arduino is not suited, since they promote the relay Arduino. 

Any ideas? Do I need some kind of resistor? Or is the USB port from the laptop not sufficient? I used two different Arduino Mega's with two different relays. By the way i don't have a big background in electric.
You are using pin 13, but you set pin 10 as an OUTPUT.

pinMode(10, OUTPUT);

Try changing that to 

pinMode(pinOut, OUTPUT);

PS by not setting pin 13 to OUTPUT, digitalWrite(pinOut, HIGH); enables the internal pull-up, instead of "connecting" it to 5V. The internal resistors are around 35kOhm. So pulling a bit of current through them will result in a lower voltage at the pin.