Has Relay been falsley rated?

Thread Starter


Joined Apr 2, 2016
Hello All,

Let me start by saying I am a complete newbie electronic hobbyist. My experiences do not go much beyond cutting, stripping, and connecting wires and power sources.

With that said I thought it would be a simple enough project to create a motion sensor "something" simply by connecting together a PIR, Relay, power, and load. Not so simple based on the components I chose to work with.

Here are the two main components that bring the circuit together.

Here are the components I am using.
PIR sensor


3v Relay


While my project is not yet complete it has been discovered that I now have to create level shifting circuits to drive this 3v relay.

My question is this.... Is this relay module incorrectly rated since it truly needs more than 3 volts to drive it?

It was noted by member ericgibbs that...

"Your relay PCB has a EL817 opto-coupler for the input signal, it requires a 5V drive voltage on the Input in order to drive the onboard transistor which controls the relay coil current.
With only a 3v drive from the PIR output, there is insufficient current to drive the opto and transistor."

See.. when connecting the PIR to the Relay PCB, the voltage signal at the relay terminals is only 2.7-2.8 volts.

I realize the relay on the PCB is 3v, but the included circuitry on the PCB actually calls for 5v to drive it.

So... was this falsely spec'd as a 3v relay?

If you care to read how more on the project while I am waiting for transistors to arrive.

All thoughts welcome.



Joined Nov 23, 2012
The relay (blue cube on the board) is not improperly rated.

The module that the relay is mounted to is falsely advertised as 3.0V because the components on the board need more than 3 volts.


Joined Feb 5, 2010
The optocoupler is not the relay. The relay is not incorrectly spec'd.

Sounds like this is designed to take a 0 to 5 volt logic input, and interface with a newer device using the 3 or 3.3 volt logic level circuitry.

Thread Starter


Joined Apr 2, 2016
Thanks for the responses. As noted these were purchased on ebay from China... so there is that risk factor that you get what you actually expect.

Which then leads me to another question. Can anybody refer me to a source to purchase a similar PCB in which I will not need to boost the voltage? Would really like to just wire a PIR to a 3 volt PCB relay without the level shifting circuit between the two.