Newbie needing help driving relay with only 2 pins?

Hello all! My first post here, so hopefully I'll be doing this right.

I have a setup I'm trying to build and I'm having trouble with a particular bit with a power relay. I have a Bobuino 2 board that I've hooked up to a 5V relay. Basically it's this one:

Only, the actual relay on the board is a 5V model (SLA-05VDC-SL-C).

(Here's an Amazon link, for more reference: Yeeco 30A High Current Relay Output Relay Control Module Power Switch Normally Open Normally Closed 5V DC Electric Current Amp Transfer for Cooler/Heater Cooler Heater Refit Water Heater Control: Car Electronics )

The purpose of all this is to be able to control a 120V GFCI outlet and turn the power to it on or off via the relay. I'm (loosely) following the instructions found here:

Only instead of a humidity sensor, I just have a simple program (for now) that turns power on and off every 5 seconds.

The problem is that this website, and every other example I've found, gives instructions for hooking up a relay board to the arduino that has three control pins: VCC (i.e. +5V), GND and IN. This relay board I bought only has two pins: VCC and GND.

Before I realized the difference, I tried hooking up VCC to D10 on the Bobuino board (and GND to GND). The board's LED lights up as expected to show the signal is coming through, but the relay never actually triggers (I've verified that the NC pole is always live, and the NO pole is never live). So I know I at least am commanding the right pin through my Sketch. From what I've seen, the D10 pin should be putting out enough voltage to trigger the relay, but since this relay only has two control pins instead of three, I'm guessing I need something more. The problem is: what? What am I missing? Do I just need to toss this relay and purchase a different one?

Apologies if this has been addressed on the forums already, but I tried searching both this forums and elsewhere all day and could not find a specific answer to this.

Thanks in advance for any help!

I suspect that relay is not designed to be directly connected to an Arduino I/O pin. I suspect it will need a lot more current than the 20mA that an I/O pin can provide.

I suggest you find a suitable 5v supply (NOT the Arduino) and measure the current that flows through the relay coil. If it's more than 20mA then you will need a transistor between the Arduino and the relay. The Arduino switches the transistor and the transistor switches the power for the relay coil.

The relay boards that have Vcc GND and IN have the transistors already on the board.


I suspect it will need a lot more current than the 20mA that an I/O pin can provide.

You're right there. Datasheet for the relay says 185mA !

That board is a relay and a diode, not a relay driver.

LOL so I just had flat-out the wrong part (or missing a part, depending on how you look at it). That'd explain a lot! Ok thanks for the help everybody!