I'm a newbie trying to control something through a relay. I've tried two relays with two different setups.

Setup 1

External 12V dc input to the relay (SRD-12VDC-SL-C) and signal pin from arduino to the IN pin, GND of ext. power supply to the GND of the Arduino, NO and COM to the load object (i.e. an LED).

Setup 2

Similar to setup 1, but using SRD-05VDC-SL-C, 5V from Arduino (Vin from Arduino also tested).

Test program

Modified version of the 'blink' example.


  1. The load object has been verified to be working before the testing.
  2. Both setup can be verify the input is working, the power led and switch led of the two relay module are lighted as expected, and I can hear the click sound while switching.
  3. No output can be obtained, both from the load object and using a multimeter.


relay setup
Sorry, I'm supposed to upload a Fritzing schematic, but I can't find a proper relay as I'm using.

  1. The DC+ (red) of the relay is connected to the external ATX 12V.
  2. The DC- (black) of the relay is connected to the external ATX ground.
  3. The IN (blue) of the relay is connected to the Arduino pin 8.
  4. The external ATX ground (white) is connected to the Arduino ground.
  5. The NO (red) of the relay is connected to the load object Vcc.
  6. The COM (black) of the relay is connected to the load object ground.


A simple modified version of Blink example is used for testing.

const int relaySignalPin = 8;

void setup() {
  // put your setup code here, to run once:
  pinMode(relaySignalPin, OUTPUT);

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(relaySignalPin, HIGH);
  digitalWrite(relaySignalPin, LOW);


I have no idea on why there's no voltage can be obtain between NO and COM while the switch is on, what's wrong with my setup?

  • 1
    Can you draw a schematic please? Even if you have to use pencil and paper and take a photo of it. Or a clear photo of your actual wiring.
    – Nick Gammon
    Commented Sep 13, 2015 at 4:13

2 Answers 2


It sounds, from your textual description (please post a schematic or at least a photo of your setup) that you aren't providing power to the relay.

A relay doesn't have a power output. It never provides power - it is merely a magnetically operated switch. It is used to switch an external source of power on and off.

Treat the NO and COM pins as either side of a normal button or switch and wire it accordingly - that is (for example) connect COM to the positive of your power supply, NO to one side of your load, and the other side of your load to the negative of your power supply.

You can share the same power supply as you are feeding to the logic / drive side of the relays.


So basically there are two wires connected to the same positive 12v source, one connected to VCC on the input side of the relay board and the other connected to the output side common terminal. And there are three connections from the same negative 12v source, one connected to the ground of the relay board, one connected to the ground of the arduino, and the other connected to the ground of the load. Then, of course, the connection to the relay to one of the digital pins of the arduino.

Well I'm pleased to report it works!

Now to discover what the jumpered COM/GND connection on my relay board does or does not do. I will report back.

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