Build a 3v Relay Module

Would love a 3v relay module that can be drive by PIRs that output 3v signal.  Thought I finally found what I needed on Ebay, but discovered this is a bogus product that actually needs 5v to be driven.  http://www.ebay.com/itm/5PCS-3V-3-3V-Relay-High-Level-Driver-Module-optocouple-Relay-Module-for-Arduino-/331413255692?hash=item4d29c4f60c:g:6koAAOSwfcVUDuEJ Buyer Beware. Advertised relay PCB as 3v relay. While the relay on the board may trigger independently with only 3 volts, the circuitry of the board demands a higher voltage to trigger the relay. 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, there is insufficient current to drive the opto and transistor. 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. In order to drive this relay with a 3v signal device, voltage level shifting needs to take place between 3v device and relay PCB.

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gmoon1 year ago

An EL817 actually has a forward voltage of 1.2-1.4V, not unusual for an IR LED (it's internal opto source). That's well below 3V. The opto-isolator can probably be set to any reasonable voltage above the forward voltage by circuitry on the module (a current limiting resistor).

Do you have a datasheet for the relay module? Are there any 3V example schematics? I see a couple of jumpers on the module. Have you explored if they can be set for 5V and 3.3V operation?

Try contacting the website printed on the back of the module for the datasheet. I don't personally read Chinese, so the website is difficult to navigate...

DiyWaterDog (author)  gmoon1 year ago

What I have found is NOTHING! Ha ha. There is no datasheet for the module. Received no response from the manufacturer. Can't figure out function of the jumpers. If they are removed, it kills the circuit to the relay completely. Just created an ible for my work around to relay problem.

Glad you got it working.

You might also be able to "reverse engineer" that module, and add more resistance parallel to any current limiting resistor that's mounted there (more resistance in parallel would reduce the overall value) and lower the operating voltage. Or at least draw the schematic...which would tell you something...

barista1 year ago

Is it practical for you to use a 5V supply and add a voltage divider/regulator for the PIRs? Also, check if the PIRs, themselves, can handle 5V.
Worth a shot, if you can't/won't return the modules.

DiyWaterDog (author)  barista1 year ago

My work around to the defective relay module works fine by connecting both PIR and Relay to a 5v source.

Glad to hear it !
DiyWaterDog (author) 1 year ago

Work around to the problem.