Pull Up Resistor Vs Pulldown

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Gail Elfert

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Aug 3, 2024, 3:49:12 PM8/3/24

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So I have three pushbuttons each with a +5v input, output to pin via resistor and a pulldown resistor to ground. They work great but I'm wondering if I wouldn't be able to just use 1 pulldown resistor to stop them from floating or if this would cause them to not function properly if they were pressed concurrently. If they were not meant to be pressed concurrently, is this a safe thing to do?

However, why not use the internal pull-up resistors instead? Then you don't need any external components. The only trade off is that the logic is inverted (unpushed buttons are HIGH and pushed buttons are LOW.)

Ah, I see what you're getting at. By using the internal pullup resistor I can sidestep the external resistors. Would this have any implications for battery life, though, since I would now have to supply a constant 5v except for when the button connection is broken (open) and electrons diverted?

SilentDirge:
Would this have any implications for battery life, though, since I would now have to supply a constant 5v except for when the button connection is broken (open) and electrons diverted?

In order for power to be burned you must have both Voltage AND current. Holding a pin up with 5V (nearly) eliminates current from flowing just the same as when tied to ground. So while you are applying a constant 5V, so little current is flowing that the Power is practically Zero. (In the P = I * E you don't use 5V. You use 5V-induced voltage which is practically zero.)

Incidentally, when using a pull-down the same reasoning applies. Keep in mind that if current starts to flow there is a voltage, even a small one. So you burn the same amount of power using a pull-up as a pull-down.

If you are worried about power consumption it is a very good practice to enable the Pull-Up resistors on ALL unused I/O pins. Otherwise, floating pins can draw over 1mA of current. Which, on a device that only consumes 20mA is pretty significant.

One last question for you then; right now I'm using those buttons to trigger an interrupt on the arduino on any difference in the interrupt pin they are all connected to (H->L or L->H). Other than changing the pin to be an input (to avoid a short) would I even need to change anything else to maintain the interrupt functionality?

I know this is an old thread but can somebody please elaborate on this. Does this mean all unused pins should be grounded to lower power consumption? If so, would this also mean one has to be extremely careful not to set a pin as output inadvertently, otherwise you run the risk of an overcurrent on that pin?

Is there any other technique of doing this.If we are doing this way the resistor might degrade on variation in temperature. If we using analog read pin to read data .The value may not same at start and in actual field.
let me know other technique

And time to also love not needing to use any external resistors at all to read switch contacts as the AVR chip gives us optional internal pull-ups to use so there is no reason to not just utilize negative logic and use the internal resistors.

if any of 3 switch has been triggered particular digital pin goes high
Assume i have connected to analog pin. Due to variation of resistor it start reading analog values.depend on analog read signal i can able to judge which mode i need to go..

Reviving this old thread again! Sorry!
In ref to the above diagram, people have said that with a shared resistor, one button push would make all pins read +. if there were 3 resistors instead, wouldnt all 3 pins still be connected thru ground or shorted and still read +?
AKA How does the diagram in the earlier posts differ from this one:

In addition, the matter is moot. Good design is that given we are connecting switches and not some other device, and have the option of how we can wire them, then they should all be connected to ground and making use of the internal pull-ups of the AVR chips rather than external pull-ups, let alone pull-downs.

Lol. Whoops Those all should have been 5V. I was doing it on my phone, and forgot to change the other two power supplies to 5V.
The reason I'm asking about this is b/c i'm trying to recreate this: -a-Fire-Breathing-Animetronic-Pony-from-FurRea/?ALLSTEPS.

The pull down resistors aren't exactly a part of the arduino circuit. So I was wondering why they put in all those pulldowns (Step 15), instead having all of them connect through one pulldown resistor.

EDIT: I should add that i've read other posts indicating that you need pulldown for each pin. But I guess I don't understand why that's the case. Put another way, if there is a pull down for each pin, everytime you you flip the switch it seems like the current will still feed into the other pins with or without resistors for each pin. Or is it that a resistor with each pin creates a "path of least/most resistance" situation, so the current would go into ground instead of feeding through the additional resistors into the other pins??

EDIT 2: Okay, I think the path of least resistance thing is why you need multiple resistors and not a single one right before ground. I just watched this video that -- while not specifically talking about single/multiple pulldowns -- suggested why multiple pulldowns were needed due to the path of least resistance concept.
The video I watched: Electronics 201: Pull-Up and Pull-Down Resistors - YouTube

No. Current will take any path as if the other path wasn't even there. The thing you are missing is that there must be POTENTIAL difference in order for current to flow. Another word for potential is "voltage".

i hooked up a pulldown resistor similar the the first picture in this thread with all 3 running to the same resistor, but.... i have a diode on each leg before it reaches the resistor, now, when i say turn on left signal, all 3 still get a signal. i then went through 3 different 10k resistors onto the same ground wire, again, all three are getting a signal.

I have connected a microcontroller GPIO pin, which is intended to be an output, to a DC-DC converter's active high Enable input pin. Since this pin is active high, and because I do not want this converter to be ON at power up or before it is required, I have used a pull-down resistor on this line to keep it disabled.

If you put the pull-down resistor close to your MCU, and have a long long trace from your MCU to the converter. If your converter's enable pin will source some current, the current will flow through the long trace and your pull-down resistor to ground. If the trace impedance is high, your converter may see a high level at the enable pin! Whatever, it will reduce your noise margin.

If all you've got right now is the Raspberry Pi, you'll want a kit with all the basics to get you up and running with simple projects - assuming it's in your budget. There are a lot of options. I've had good luck with kits from Vilros and CanaKit before, but I don't see much from them on Amazon anymore.

If you need the Raspberry Pi unit itself, the Pi is more expensive than it used to be on Amazon, apparently due to component shortages. It might be worth checking out rpilocator for a better price with other resellers, although you may have to wait a little longer to get your Pi then.

NOTE: One reader left a comment about my use of a 220Ω resistor, with a link to an authoritative site that has an article on the electrical specifications for the Pi's GPIO pins. In it, the author states that one should "never source or sink more than 0.5 mA into an input pin", although they don't explain why. According to an electrical calculator, you'd need at least a 7kΩ resistor to drop below the recommended 0.5 mA. A 10kΩ resistor should work fine too, which is what I've seen used in other examples. (Thanks Randall Stevens.)

That won't be very useful though, without a script to read the state of the circuit and take some action, even if it's just displaying a message. So here's a small Python script that does that for us. It detects when the circuit is opened or closed, and displays a message with a timestamp.

Here we specify the board numbering system, and then setup a pin to read input. Next we attach a my_callback function to the pin, so that some code runs whenever the circuit is closed or opened (the button is pressed or released). The code just displays a simple message with the current date and time.

Each GPIO pin has two states. You can call them on or off, high or low, 1 or 0, etc. A pin is set "high" when it's outputting 3.3v or reading in 3.3v, and "low" when it's off. The GPIO library calls these two states GPIO.HIGH and GPIO.LOW, and the library also helps you determine which state a pin is in.

You can specify a time during which repeat events will be ignored. For example, you can specify a bouncetime of 500 ms. That means that if you press a button multiple times in under half a second (or maybe the button's a bit loose and registers a click multiple times), subsequent clicks after the first one will be ignored for a brief time.

If a pin is set to input, then the circuit must be closed for it read that input. In my case, that means pressing the button down in order to read HIGH or 1 (since I'm connected to 3.3v... if my pin were connected to ground, it'd read LOW or 0 when closed).

But what about when I'm not pressing the button in the above circuit? It should be LOW or 0, right? That's where the problem lies. Since the circuit is open, the GPIO pin could be reading all kinds of things from the environment, and it's fairly sensitive. We need a way to force the pin to LOW (also known as "pull down") when the circuit is open (or to HIGH if the original circuit was connected to ground, also known as a "pull up").