Control 5 V relay board with Raspberry Pi GPIO

Question

I would like to control one of these 8-relay boards with my RPI GPIOs.

Please check the below schematic - it shows the parts involved for one of the 8 relays. JD-VCC is connected to the 5 V of the RPI, because the board uses 5 V relays, that's fine.

I'm unsure if I should use 3.3 V or 5 V for VCC (e.g. driving the optocoupler). I found several tutorials in the internet, some of them say 5 V, some 3.3 V.

Since some of the tutorials recommend using 5 V for VCC. I was asking myself if that damages the RPI's GPIO, because as far as I know, they are not 5 V tolerant. However, measuring the voltage on the IN1 using my multimeter results in 3 V. I guess this is due to the optocoupler diode forward voltage and also the forward voltage of the IN3 LED.

I'm not a professional EE, just a tinkerer, and I would just like to have the assumptions mentioned above confirmed. Is it safe to use 5 V for VCC?

Answer 1

You must use +3.3V for \$V_{CC}\$.

When using +5V for \$V_{CC}\$, it's only the green LED that's saving you. It's by sheer luck that LED current drops below some "threshold" level to switch Q3 off, since without that green LED, there would still \$5.0V-3.3V=1.7V\$ across R5 and its LED.

If you want LED current to be truly zero when GPIO output is high at +3.3V, then there must be no voltage across R5 and its LED, which means that \$V_{CC}\$ must also be +3.3V. Also, if you want an indicator LED, power it directly from the GPIO output, not in series with the opto-coupler LED:

^{simulate this circuit – Schematic created using CircuitLab}

In this way, diode IN3 is no longer interfering with the potential difference placed across R5 and its own LED.

You could avoid this ambiguity altogether, and have the GPIO source current for the LEDs:

^{simulate this circuit}

This would invert the meaning of IN1 (relay is now off when IN1 is low), but that's an easy fix in software.

Answer 2

The IN must be pulled down to ground to turn on. It must be pulled up to VCC to turn off. At 3.3V IN there is still a difference of 1.7V and that may result in current through the optocoupler so it may not turn off. And when HiGH-Z or ground the gpio would send 5V potential through it.

As long as you are sure VCC and JD-VCC are isolated, then VCC should be 3.3V for the RPI to work with it. But it could also not be enough for the opto coupler and the status led to light up. Alternatively would can use a NPN transistor (2n3904 is fine) from the RPi gpio to control the Opto. This would isolate the IN so you can power the relay board from 5V.