I have 4 relay module originally created for Arduino (it looks very close to the image below) and I am little confused with the way how it works.

enter image description here

There are 6 pins ins this relay:

  • GND
  • IN1
  • IN2
  • IN3
  • IN4
  • VCC

So when i connect 5V to VCC and ground to GND - I get about 3V between each of the IN1..IN4 and GND. Why does it happen?

  • 1
    \$\begingroup\$ Wait, are these guys really isolating relay coils with opto-couplers? Someone needs to tell them that relays already isolate. This looks like a design based on religious beliefs. \$\endgroup\$ Commented Nov 17, 2016 at 11:56
  • \$\begingroup\$ How about a link to the original module? \$\endgroup\$
    – Andy aka
    Commented Nov 17, 2016 at 12:42
  • \$\begingroup\$ I only know what is written over it - "4 Relay Module". \$\endgroup\$
    – moonvader
    Commented Nov 17, 2016 at 12:59
  • \$\begingroup\$ @OlinLathrop I too am wondering why people (not just these folks) opto-coulpe relay coils. My best guess is that this is done to prevent the spike from the relay coil from propagating to the microcontroller. \$\endgroup\$ Commented Nov 17, 2016 at 19:52
  • \$\begingroup\$ @OlinLathrop That type of relay in particular actually provides rather poor coil-contact isolation. The optos work quite well unless a hapless user deliberately bypasses them to use a common power supply, in which case they may have to go on an EMI hunt. \$\endgroup\$ Commented Nov 17, 2016 at 19:58

1 Answer 1


Inputs are active low. The inputs are VCC to a resistor, the optó led and a regular led. When pulled low, the optó coupler turns on, turning the relay transistor and the relay on. When left floating you will measure a voltage of VCC - the forward voltage drop of the leds. That's normal.

  • \$\begingroup\$ Thank you. Unfortunately I could catch only that this is normal) So as it is normal - I shouldn't damage Arduino's pins or Raspberry Pi's GPIO if I connect one of those pins to IN1...IN4? \$\endgroup\$
    – moonvader
    Commented Nov 17, 2016 at 11:48
  • \$\begingroup\$ This will not damage the RPi or the Arduino.As the RPI has 3v3 and 5v for Arduino. \$\endgroup\$
    – chandan
    Commented Nov 19, 2016 at 10:54

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