Connect microcontroller 3.3V output to relay using ULN2803A

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Connect microcontroller 3.3V output to relay using ULN2803A

Post by mingu »

I have a need to connect an ARM micro-controller with 3.3V digital output pins to a few 12V solenoid water valves. I figured that I can use a ULN2803A ( this task.

The internal circuit for each input/output is depicted below:


Couple of questions:

Is the attached schematic correct?
Do I need anything else in the circuit to protect the micro-controller?
Anything else needed to protect the ULN2803A?
And do I tie the +12V and +5V grounds together?


The load I'm driving is rated at about 400mA, 100mA shy of what this part is rated for ( each output ). The datasheet says the ULN2803A can be put in parallel to handle more current. I'm mot sure how that circuit would look.

Would I just logically connect them as if they are stacked one atop the other?

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Re: Connect microcontroller 3.3V output to relay using ULN28

Post by Benj »


I think your schematic looks good. No don't tie together the 5V and 12V supplies. The 12V could be used to generate the 5V using a 7805 or other voltage regulator.

You can also do the current step up using the circuit provided here. ... 18549343c4

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