My 4 Relay Module is Negative Output

Dear all,

I have just bought a 4 Relay Module having Songle SRD - 5VDC- SL - C Relays, on a blue board.
I have connected the Relay Board's Ground to my Arduino Ground pin, the Relay Board's 5V pin to my Arduino 5V pin but when providing 3.3V on a the Relay Board IN (for example the 1st IN), nothing happens and when providing Ground to the same pin, the Relay gets triggered (the NC opens and the NO closes) ! Then when 3.3V goes @ the 1st IN, the relay gets un-triggered (the NC closes and the NO opens)...

What is the issue with this Relay board? Is it a Negative Output one?
How am I to distinguish the Positive from the Negative ones when ordering Relay Boards?

All the Best !

Almost all relay boards with opto couplers have negative logic.

You are supposed to only connect +5volt and the inputs to the Arduino. NOT join grounds.
And supply the relay board from a separate 5volt (or 12volt) supply.
That second supply should connect to JD-VCC and relay board ground, with the jumper removed.

If… you do want to supply the relay coils from the Arduino (grounds joined), tell us how you plan to supply the Arduino, and which Arduino you have. If you do it wrong, you could cook the onboard 5volt regulator with the ~320mA relay coil draw.

Why is this important to you? You just drive it with inverted logic in the software. After all that is what software is for.

Can I use a common power supply for providing GND and +5V to my Arduino (UNO) and the relay board or am I obligated to use different ones?


Yes you can use the same supply for both.

Try a 5volt USB cellphone charger (with USB socket), connected to the USB input of the Uno.
That bypasses the onboard regulator.
Then you should be able to power the four relays from the 5volt pin.

The real problem with these "relay boards" is that they are nearly Rube Goldberg machines. Essentially taking a really simple problem... controlling a mechanical isolation of electrically controlled contacts from a microcontroller pin (A relay driver, see what I did there?) and turning it into a more costly and overly complex solution.