Problem using a RELAY for home alarm

Hello … I’m making a home alarm and one of the components is a relay. In pin 5 is connected the ind2 and GND and voltage are ok. 110V with siren on normally open. Pin 5 is activated with a sensor pir …

In theory everything is OK … But when the pir activates (5v) the pin 5 , the relay does NOT work …

I did 20 thousand tests and manually I manage to run the relay … !! I Change the jumper to jdvcc vcc, put GND and voltage and giving 5v to the ind2 did not work … Then I test giving GND(not 0v) to ind2 and it works !!! …

This means that the relay is NOT WORKING PROPERLY. The only way it works is passing GND (not 0v) to ind2 and changing the jumper …

Now !!! HOW I DO WITH THE ARDUINO to give GND(not 0v) to ind2…by pin 5 or another pin !!!

well, did you have a look at the schematics?

Your Inputs IN1 & IN2 are going thru a LED, thru the LED of the optocoupler, thru R1/R4 up to VCC.
So it is right that you have to switch it on when you apply LO Level!

The Jumper has to be to VCC - JDVCC!
Try on your Arduino digitalWrite(5, LOW), it should turn on your siren.

Can we have a look to your sketch please?
You have to switch D5 to LOW when your PIR gets high!

Everything should be easy!

2 channel.pdf (49.3 KB)

I set pin 5 to LOW and nothing happened. As far as I know LOW in arduino is 0v. That is why i was asking for sending GND. I'm thinking in connecting pin 5 to a resistance and the to gnd and then to ind2. Will this damage my arduino ??

attached my sketch an a diagram .. Thanks for responding

int pinrelay = 5 ; // pin for relay

void setup()
pinMode(pinrelay, OUTPUT); // Assign PIN is OUTPUT

void loop()

digitalWrite (pinrelay, LOW);

And GND from the Arduino?

How much current does your coil relay need? Arduino pins are only good for 20-25mA. 5V relays typically need more than that, so a transistor is needed to act as current buffer. You drive the Arduino output high, the transistor turns on, the relay coil is energized.
A diode is used across the coil to dissipate coil generated current when the transistor turns off.
Here’s an example showing NPN and N-channel MOSFET as current buffers.
Relay and MOSFET connections, LEDs.jpg

(C on the relay should have been connected to Gnd as well)

Solved !! Thanks I just conect GND form the external 5V to GND in the arduino . !!!

:slight_smile: didn't you do that before? :slight_smile: