I'm trying to research how to use a 5V relay with a 3.3V GPIO trigger (ie. Raspberry Pi, ESP32, etc).

Here is the board:


Does the trigger voltage (IN) get passed to the relay or is it triggering the opto-isolator to pass the DC+ voltage?

If the trigger voltage (IN) is only used to trigger the opto-isolator, then would it be safe to use 3.3V on the IN and 5V on the DC+?

(I've seen lots of similar questions asked, but also lots of different answers and none of them really explain how this works or why it is/isn't safe)

  • \$\begingroup\$ Do you have the specs, datasheet or schematics for this product from where you bought it? \$\endgroup\$
    – Justme
    Commented May 11, 2023 at 17:31
  • \$\begingroup\$ No, but the only active (?) components on the PCB are the SRD-5VDC-SL-C relay and an I think the opto-coupler with the following markings: L K20, 354T, XA. \$\endgroup\$
    – SofaKng
    Commented May 11, 2023 at 17:36
  • \$\begingroup\$ Not true, you are missing at least the transistor. What does the jumper do? Please reverse engineer the schematic with component values, or find out if someone has already done that for you. It might be easier to buy a board that comes with specs and manual. I mean, is it even active high or low, is it turned on by sinking or sourcing current? The opto-isolation is usually useless on many of these boards as it is not really used for isolation. \$\endgroup\$
    – Justme
    Commented May 11, 2023 at 17:43
  • \$\begingroup\$ The jumper selects high or low trigger. This unit is made to work at 5 V for both the coil (DC +/-) and trigger (IN). 3.3 V MIGHT work as-is, or may need a resistor changed to a lower value. We need a schematic to help further (you can trace it out yourself then edit your question to use the built-in schematic tool). \$\endgroup\$ Commented May 11, 2023 at 18:54

1 Answer 1


You will probably have to short out the status LED (in series with the input control voltage) to get it to work reliably from 3.3V.

The schematic is typically something like this (image from here): (trace it out and compare to be sure)

enter image description here

You would want to remove the jumper and apply 0/3.3V between the Vcc and In1 terminals. JDVcc and GND would need 5V (it's isolated so you don't need to connect the grounds, and should not).


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