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I'm working on an electronics project that was using an Arduino to drive a 5V relay module specified here.

The relay switches a 240V AC (upto) 10A load.

I have now moved to using an ESP32 which uses 3.3V control pins and have some questions about finding a replacement that I would like to eventually incorporate into a custom PCB-

  1. Does a suitable relay exist for this load than can be actuated by 3.3V?
  2. If not what is the most appropriate way to boost the 3.3v signal to 5V?
  3. What's the purpose of the optoisolator in the above design? Should this be included in a circuit to replace the relay module?
  4. Are there any other important components on the relay module I need to include in my circuit?
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    \$\begingroup\$ Before I answer your questions, one for you : Why can't you use the current module? It works with 3.3V logic signals and has transistors to drive the 5V relay, you just need to supply it with a 5V relay supply. \$\endgroup\$
    – Justme
    Commented Aug 17, 2021 at 17:13
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    \$\begingroup\$ I was able to find "arduino 3v relay module" on Amazon. \$\endgroup\$
    – jay
    Commented Aug 17, 2021 at 17:18
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    \$\begingroup\$ @Justme Ideally I would like to replace the module with my own circuit design as eventually this will be incorporated into a custom PCB. \$\endgroup\$
    – Falconar
    Commented Aug 17, 2021 at 17:25
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    \$\begingroup\$ If you have a 5V power available as well as 3.3V then you can use the 3.3V relay. Otherwise, yes, I think they do make 3.3V relays. \$\endgroup\$
    – user20574
    Commented Aug 17, 2021 at 17:31

2 Answers 2

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  1. Yes, relays that use 3.3V coil voltage do exist. However, the lower the coil voltage is, the more current they draw, so you may not want to use a 3.3V relay, but some higher voltage you have available, such as 5V or 12V.

  2. A boost converter. But again, while making a 5V supply to relay coil from 3.3V is possible, it takes a lot of current at 3.3V, so it may not be what you want.

  3. The main job of the optoisolator is to put the almost magical term optoisolator in the marketing material to increase the wow factor of the product, and to give some false sense of safety. The optoisolator does not isolate by default, but if you remove the jumper, it allows you to use isolated supplies on MCU and relay side of the optocoupler. In theory you could have two separate supplies, one 5V supply for the MCU side and another 5V supply for the relay, and have 5000 volts difference between those two supplies. But I highly doubt the PCB can handle that, even if the isolator did handle it. No, it is not a very essential component.

  4. For a single relay, you only need one transistor, one series resistor for the base, and the flyback diode for the relay coil.

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If you bypass the 1k with 330 ohm on 3.3V , it will work.

http://wiki.sunfounder.cc/index.php?title=2_Channel_5V_Relay_Module#Schematic

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