Pull Down Resistor Value


It's only my second day with the Arduino and I'm not an electric engineer. Have a general question on pull down resistor value.

I see on project #2 of the Arduino Uno starter kit it uses 10k ohm pull down resistor for the switch, I would be imitating this setup on a different project so to speak, but I am wondering how they arrive at this value. I read somewhere the maximum amperage the Arduino Uno can supply is 40mA. Can someone confirm this number? I'm thinking between all loads they are trying to keep the total draw below 40mA as well as reducing the little push button switch current to an acceptable value?

If I were to make a build that had 3 different toggle switches, that are used independently (no two closed circuits at the same time) I would want to ohm the switches to make sure I'm not pulling more than 40mA through the circuit and not pulling excessive amperage above the switch rating? I'm guessing ideally I would want all 3 switch circuits to be below the 40mA mark in case of operator error? For the project I'd be driving a solenoid by NTN transistor off an external 12v power source, if that matters? So I'd also factor in the amperage of the transistor..?

I apologize for what may be an obvious and basic question, but any advice would be appreciated!

Thank you!

Why not use the built in pullup resistors that can be turned on by using INPUT_PULLUP as the mode in the pinMode() function ?

That way there is no messing about needed with external resistors and wiring

Ok great thank you, I will look into that! Could you possibly confirm the 40mA mark? Is that per output or total?

The 40mA is per pin

Thank you very much for the help!

This is the maximum rating, not the operational value. Pin current should not exceed 20mA under operating conditions. And there are other limits too. You cannot draw this current from all pins at the same time.

Ok, so best to keep it as low as possible? Any minimum amperage you generally prefer to stay above?


That's always the best. Why draw more current than needed?

Ok great! Thank you all very much for the help! It's kinda funny the book example in the project would fall in the "bad" category of that diagram, but I get they're trying to start with gaining an understand of the basics.

I'll give this a try. Not sure where everyone is located that replied, but if you see flames it's probably my house and then something went very wrong! Thank you again!

Stay less than 20mA.

Let’s say your load only needs 5mA, then we design for no more than a 5mA current output.

                    resistor    2V LED
5v —— Arduino Pin —— [560 Ω] ——[A->|-K] —— GND

Gives ≈ 5mA

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