Pull-down resistors and ground

Question

Please help me something that I don't understand:

Why is there a need for pull-down resistors in the circuit below with push button if the ground has already zero voltage? This is the only thing I find confusing in them.

it's unclear what you are asking. draw a circuit and add it to your question — Jasen Слава Україні, Oct 19 '23 at 02:55
Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Oct 19 '23 at 04:58
@Voloo, Really, try to formulate your question more clearly because it is in danger of being closed. If you need it, we will help you. — Circuit fantasist, Oct 19 '23 at 12:51
In the picture i dont understand the purpose of the resistor (what it does), because there is no current so gpio 14 should read always low. — Voloo, Oct 19 '23 at 16:05
@Voloo, See the schematics (simulations) 4.2.1 and 4.2.2 in my answer below. Please, give me some feedback about the written. Explain what is still unclear. Oh, I forgot about that 50 reputation limit... how does that bother me... — Circuit fantasist, Oct 19 '23 at 16:55
@Circuitfantasist - Hi, (a) While I support an OP giving relevant feedback on answers, there is no obligation for them to do so. Also, since the site disallows chitchat & empty "thank you"-type comments, any comments should comply with that linked explanation, or not be written at all. (b) "Oh, I forgot about that 50 reputation limit" That doesn't apply in this case - an OP can always write comments to posts on their question. See that page I linked where it says: "All users may comment on their own posts and any answers to their own questions." — SamGibson, Oct 19 '23 at 17:18

coal · Answer 1 · 2023-10-19T21:51:48.140

4

You could think this analogy to understand the pull-up (or pull-down) function:

_{source: personal class notes + itecnotes.com}

When you don't connect your input to "something" (i. e. you have a floating input), the voltage remains undefined. In order to ensure that there will be some voltage level defined you have to leave the input pin connected to something.

To avoid a short circuit, you make the connection through a pull-up or pull-down resistor. So, when you press the button, you are imposing the voltage level that does not come from the resistor and without producing a short circuit. And while you release the switch and leave the input pin node unconnected to it, you ensure that the other node will impose the other voltage level through the resistor.

The value of the resistor must be chosen wisely. A high value produces a low current comsumption but the slew rate and the transient between changes will be higher than the one that would be obtanied with a lower value. For typical applications, 10 kohms is ok; consider 1kohm for a high speed changing input and 100kohms for a low speed changing input.

edited Oct 19 '23 at 21:51

answered Oct 19 '23 at 04:20

coal

81
5

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That first image is either confusing, or wrong, or talking about something different - possibly the effect of NC vs NO switches – MikeB Oct 19 '23 at 13:32
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@MikeB No, it is not wrong. It's just an analogy between pull-up resistor and a spring that ensures the input will go up when switch is not pressed. Just an analogy, like every analogy it has its flaws – coal Oct 19 '23 at 15:40
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@coal The picture implies that the input would always be read as low without a pull-up resistor. This is not true; most TTL chips will read unconnected inputs as high, and CMOS chips will read randomly (which is why you should never leave CMOS inputs floating). – Hearth Oct 19 '23 at 20:31
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While the idea to make an analogy with mechanical pull force is interesting, it breaks down for the reasons @Hearth provided – Maple Oct 19 '23 at 20:44
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Well, the picture does NOT exempt you from reading the whole answer. "When you don't connect your input to something, the voltage remains undefined". That's all, I've never said the input remains on a low level when the pull-up is absent. You use pull-down or pull-up to leave the voltage level defined, end of the story. If you don't like the analogy, you could downvote or propose a better one – coal Oct 19 '23 at 21:48
@coal Except it doesn't remain undefined either. It depends on the logic family you're using. – Hearth Oct 19 '23 at 23:45
@Hearth I wouldn't assume that the voltage will remain defined. Even if it was TTL I would use a pull-up or pull-down anyway. Assuming it will be defined is a bad practice; so for a simple explanation the word undefined is perfect – coal Oct 20 '23 at 03:02
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@coal Even if you accept that, the picture you've used directly contradicts that with the wording "switch released: reads '0'" in the top left. – Hearth Oct 20 '23 at 04:02

Circuit fantasist · Answer 2 · 2023-10-20T06:59:16.237

To see the forest...

I will take this opportune moment to summarize the idea of "pull-up" and "pull-down" resistors in a "philosophy". This will help us to make a connection between different (at first glance) specific circuit solutions and thus understand them better.

The problem

The point here is how to switch two voltages (eg 10 V and -10 V) to one input using a simple 2-terminal (SPST) switch. The usual solution is to use a 3-terminal (SPDT) switch, which first disconnects one voltage from the input and then connects the other voltage to the input.

V1 connected, V2 disconnected

schematic

^{simulate this circuit – Schematic created using CircuitLab}

V1 disconnected, V2 connected

schematic

Pull-down resistors and ground

4 Answers4

To see the forest...

The problem

V1 connected, V2 disconnected

V1 disconnected, V2 connected

Basic idea

"Pull-up" resistor

"Pull-down" resistor

Analog applications

Resistor subtractor

Diode limiter

Digital applications

"Pull-up" resistor

"Pull-down" resistor

OP's wondering

Experiments