My first schematic of a 2-channel relay board

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Hey all this would be my first try at creating a schematic in KiCad.

The schematic is a basic 2-channel relay.

The inputs are automotive 12v and gnd. This gets converted to 5v by the K78L05-1000R3.

The Wemos D1 mini has 2 digital pin outs that are 3.3v going to the LTV-356T with a 200 Ohms resistor in between.

From there it goes to a 1k resistor and then to the PMBT3904. Between that and the G5LE-1 relay is a 1N4007F.

In that same area is another 200 kOhms resistor that’s connected to an LED.

The outputs and inputs are the 2pos and 4pos 3.5mm Pitch Terminal Block Connector 300V 8A.

Just want to make sure my values are good and the components I have selected are suitable for my application. Please let me know if I am missing anything or have a value incorrect. Thanks!

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Question also asked on:

maker.pro, electronics-lab, allaboutcircuits, Elec. Stackexchange, electro-tech-online.

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Double check D5/R5 and D6/R12. Will they ever light up? I think not …

1 Like
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Btw, you are talking about 200 kOhm resistors. They are 200 Ohm.

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Looks good for a first try. Might be tidied up a bit, but that’s secondary.
Two comments:
I think you mean 200 ohms. That would fit with schematic and function.
Also, the LED indicators won’t work. Check the connections.

Cheers.

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Ah ok. Thanks for catching that!

This looks like the popular vote for the LED.

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Let there be light … :slight_smile:
This will work now.

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Ha. Very much appreciated @straubm.

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  1. Using optocouplers here does not make any sense (their both sides get no galvanic isolation).
  2. If you have 12V on board then better would be to use 12V relays.
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@Piotr What would you put in between the 3.3v and the transistor when removing the opto completely?

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Resistor.

Take relay coil current (I don’t know your relays). Divide that current by 10. The result will be the right current for transistor base. Take 2.6V (3.3 - 0.7) and divide by that current to get the right R. Than select some standard value near it.
If you use 12V relays their coil current will be smaller (about 5/12 of 5V relay coil current).

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I think Piotr is wondering why you’re powering the relay coils from 5 V instead of directly from 12 V?
Only then do the optocouplers makse sense.
I wonder too. Automotive relays are normally 12 V.

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The type of relays I am using are the Omron G5LE-1A4 DC5. It looks like the one I have in the schematic are 18v. I’ll need to change that but that is why I am using 5v there - its the trigger voltage.

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Yes. Well, this is a KiCAD forum, not an electronic design forum. I’ll leave now.

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Even then optocouplers have no sense. They only have sense if you have two separate sources of power. Separate = galvanic isolated. Optocouplers allow to have voltage difference (even 1000V or more) between grounds at both their sides. If you expect such difference (for example during storm strikes) you have to isolate both sides. Here both sides are powered from the same source so no way of isolating them. Relay also gives isolation. Its contacts can be powered from different power source than their coil. But here you also don’t use that isolation as both sides of relay are powered from the same source. In such situation you can even replace relays with transistors, but you can have some problem to protect its output. Depends on the load.
If you want to drive some output powered from the same 12V then you can consider self protected transistors (Omni-FET, Inteli-FET). But you can stay with relays.

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Mods - can we please enforce trying to keep the forum on topic and on section.

This ought be in projects if allowed at all.

The forum is for problems with the software not how-to design - there are other forums for such questions

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Good point. and 20, sigh…

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