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);

  digitalWrite(pinOut, HIGH);

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.

  • 1
    but what do you want with not so lucky 10?
    – Juraj
    Mar 6, 2019 at 14:40

1 Answer 1


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.

  • OMG it is working now. I was trying for hours and in the end it was just a wrong number. Thanks a lot. Mar 6, 2019 at 14:59
  • We've all been there. (-;
    – Gerben
    Mar 6, 2019 at 15:36
  • There but for the grace of god go I. The hardest bugs to fix are ones where you see what you expect to see, not what's there. Good catch. (Voted)
    – Duncan C
    Mar 6, 2019 at 15:42

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