I'm going to buy two 16 channels relay boards from Sainsmart : http://www.sainsmart.com/16-channel-12v-relay-module-for-pic-arm-avr-dsp-arduino-msp430-ttl-logic.html

I want to control it with a SBC66EC (modtronix.com/sbc66ec.html?sef_rewrite=1&currency=EUR) which has 32 3.3V I/O.

The relay board must be powered with an 12v alim. My question is : can i control the relay board with 3.3V GPIOs because of the Optocouplers of the relay board ? There is a lot of different theories on the web ... I'm a bit lost as i'm not specialized in electronic/electricity.

Here is the schematic of the relay board : https://www.dropbox.com/s/pm2p3q04sj65x5e/16ch%20schematic.pdf?dl=0

Thanks !


Based in the schematic and pictures, that board is hardwired without the typical jumpers. Four sections. 12V in and a builtin 5V regulator. The 5V powers the optocoupler led side. It also powers the optocoupler transistor side and relay driver side. Finally the 12v powers the relay section.

Some different relay modules have a jumper that let's you disconnect the optocoupler led side from the transistor side, and then you could power that side from 3.3v instead. But the pictures and the schematic don't show that.

In this case, you can either cut a trace and inject 3.3v at the optocoupler, or use a transistor plus resistors for each input. As is, it will not work directly with a 3.3v gpio.

  • \$\begingroup\$ Just a clarification for my sake, the Optocouplers are active when a low is present, surely with the led being a 'diode' and the 5V only going to ground through the IC and when the MCU sets the I/O high the 'diode' functionality should stop problems. Or did I just miss something? \$\endgroup\$ – RSM Jul 6 '15 at 19:05
  • \$\begingroup\$ No, because when vcc is 5V, and the gpio is at 3.3v, that's still 1.7v at the optó and it triggers. And your risking your board if it's not 5V tolerant, regardless of the diode drop. \$\endgroup\$ – Passerby Jul 6 '15 at 19:09
  • \$\begingroup\$ OK, I thought I missed something with the voltage drop. :/ Its pretty stupid then with there design then, guess it would be better if you could have driven the signal with the MCU Vcc. \$\endgroup\$ – RSM Jul 6 '15 at 19:25
  • \$\begingroup\$ Thanks for the answers ! I thought the led side could be powered by 3.3v. \$\endgroup\$ – Thibault B. Jul 6 '15 at 19:28
  • \$\begingroup\$ Some designs allow you to switch. That board is really meant for a 5V arduino, but with extra parts can work around arbitrary voltages. \$\endgroup\$ – Passerby Jul 6 '15 at 19:33

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