Why is my relay not triggering? I have a 4 channel relay for my Arduino. Here's an image:



From what I can tell, it's rated for 5V direct current because of the big 05VDC at the bottom, but please tell me if I'm wrong? If I'm wrong, then that's the problem here.

I've tested out an LED on 5V DC to work and connected it to output side of the relay.

I've connected my Arduino to the input, programmed with the following code:

const int RELAY_PIN = 3;

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(96000);

}

void loop() {
  Serial.println("Relay ON");
  digitalWrite(RELAY_PIN, LOW);
  delay(1000);

  Serial.println("Relay OFF");
  digitalWrite(RELAY_PIN, HIGH);
  delay(1000);
}


I can confirm that the corresponding LED on the relay for channel 4 does blink for 1 second and stay off for 1 second, which I'm expecting it's meant to do. I've never worked with an Arduino or a relay before.

However, the 5V DC LED on the other end never lights up. Why is my relay not switching on?

Edit: About the LED circuit. It is on and fully powered. If I bypass the relay and join the two wires, the LED lights up. If I plug the two wires into the relay, the LED never lights up.

Edit2: About the jumper: I have a secondary 5V power supply connected to the VCC for what I'm told is isolation. I've read a little on the matter, but am not fully aware of how it works.

Edit 3:

Connections

I have the following connections:


Output 3 (Arduino) to Input 4 (Relay)
5V (Arduino) to Input VCC (Relay) [The one next to the input pins]
Power supply positive/ground to GND (Relay)
Power supply negative/live to VCC (the one next to the JD-VCC)


This did not work. What did work was the following configuration:


Output 3 (Arduino) to Input 4 (Relay)
5V (Arduino) to Input VCC (Relay) [The one next to the input pins]
Jumper across VCC and JD-VCC on the Relay
From the image in the question, the jumper across pins J5 (in the
below schematic) or JD as on the silk screen on the PBC is missing.

Using the above schematic, a choice to power the Ardino and the relays off of one 5V power supply can be made.  This appears to be done by connecting the 5V to the VCC pin, Ground to the GND pin &amp; placing a jumper across J5.  However, it appears that the relays can be isolated from the Arduino if desired.  This appears to be done by connecting the Arduino's 5V to pin 2 of J5.  That should be enough to light up the LED indicator and therefor the optical isolator when the Arduino's output to IN1, 2, 3 or 4 is low.  Then, connect the 2nd power source 5V to pin 1 of J5 and the ground from the 2nd power source to pin GND.  This last step should provide the power to close the relays when the Arduino takes IN1, 2, 3 or 4 low.
Lastly, make sure the polarity of the 2nd power supply (if one is used) is correct.  The output side of the optical coupler and the transistor driving the relay only work when the current is passing in the proper direction. Not to mention, the reverse bias flyback diode (D1, 2, 3 &amp; 4) would short out the power supply if connected in the reverse direction.  However, this catastrophe is avoided here as the transistors (Q1, 2, 3 &amp; 4) would not pass current in the reversed direction.