10K resistor not pulling down input

Other Hardware General Electronics
1

I am having trouble with getting the input to go low with a 10K resistor that seems to be the recommended value. It seems to float at about 4.2 - 5v whether I have a resistor pulling it down to earth or not.

I have made sure that the internal pullup resistor is disabled using digitalWrite(input, LOW);

If I reverse the logic and enable the internal resistor using digitalWrite(input, HIGH); and pull the input to earth then it works as expected.

I have tried using lower value resistors as pull down which does work but it shouldnt be necessary.

Any suggestions where I might be going wrong?

Peter

2

Without seeing your complete sketch it's hard to see if you are doing the proper digitalMode command or not.

Lefty

3

I can't seem to find your schematic.

4

I am about to go out so can't send the specific sketch but it's basically the example button sketch in the compiler.

I will reply in more detail later

5

Make it exactly the button sketch and build in the example and get that working. It is simple enough and doesn't take long. When that works make the changes you need and you might be able to answer your own questions. This stuff makes more sense the more you work with it. Personally, I miss wired that the first time and it took me a while to see my mistake. :blush:

6

drpeej:
I am about to go out so can't send the specific sketch but it's basically the example button sketch in the compiler.

I will reply in more detail later

It is almost certain that there is a wire not making contact like you think it is.

7

It is almost certain that there is a wire not making contact like you think it is.

Right... Check the voltage an the resistor itself to make sure it's the same as the voltage on the input pin.

If there is 5V on the resistor, there has to be SOMETHING pulling it up!!!! The internal 20k pull-up with a 10k pull-down would only pull the voltage to about 1/3rd of 5V, so that's not it...

Are you sure that's a 10K resistor?

Make sure that you have not configured that I/O pin as output. (It should be input by default.) And, make sure the pin or resistor are not shorted to anything else.

If everything is wired correctly, the pin is configured as an input, and the input can't be pulled-down by a 10K resistor, maybe your Arduino is damaged or defective?

8

OK - thanks for all the comments - all things I had thought of. I'm actually an experienced constructor and programmer but new to Arduino.

So I stripped the whole lot down and rebuilt it and guess what? It now works as it should. There must have been a rogue wire somewhere.

9

I've discovered the cause. I was using digital pin 0. When I change to pin 2 it works OK. Pin 0 apparently is used by the USB port (although it still didnt work when I unplugged from my pc and ran it from a PSU)

10

drpeej:
I've discovered the cause. I was using digital pin 0. When I change to pin 2 it works OK. Pin 0 apparently is used by the USB port (although it still didnt work when I unplugged from my pc and ran it from a PSU)

See. posting your sketch would have shown us you were using pin 0 and we could have explained the 1K ohm pull-up effect the USB serial converter chip applies to pin 0, even if unplugged from a PC.

Lefty

11

And we are left to guess at your level of overall experience in these matters. There is a broad range of people asking for help starting with people that have little to no experience other than they read that the Arduino is cool to work with....

12

OK guys - I will consider my wrist well and truly slapped. :slight_smile: