# 555 timer

#### AlexDani

Joined Apr 3, 2021
14
Hello guys, i have a question about the pin 4 of the 555 timer IC. So, i tried to make a circuit with 555 timer in astable mode and also tried to use pin 4. But i notice something i can't really explain. So i used a 1k resistor for pin 4 and everything worked fine but when i switched to a 10k resistor the circuit starts running even tho the other side of the resistor was connected to ground. The question is how can i really calculate the resistance for pin 4?

#### crutschow

Joined Mar 14, 2008
27,709
Pin 4 is the /Reset.
It should go to V+ with no resistor if you are not resetting the 555.
What are you trying to use it for?
What are you connecting it to besides a resistor?

#### AlexDani

Joined Apr 3, 2021
14
This is the schematic, im trying to make a water level indicator

#### crutschow

Joined Mar 14, 2008
27,709
When all else fails you (gasp) read the device's data sheet (snippet below).
As you can see, you need to keep the voltage below 0.4V with a maximum current to ground of 0.4mA to reset the 555.
That means the maximum resistor value to ground to reliably reset the 555 would be 0.4V / 0.4mA = 1kΩ.

#### Audioguru again

Joined Oct 21, 2019
3,511
You have a green wire connecting pin 4 to pin 8, then R4 does nothing. Maybe the wire is the water?
Pure water does not conduct. Tap water can also have a resistance so high that the 555 cannot detect it.

#### AlexDani

Joined Apr 3, 2021
14
I see. To be honest i make this calculation before i post the comment but i didn't know if it was correct or not. But now i know for sure. Thank you, sir.

#### AlexDani

Joined Apr 3, 2021
14
I have one more question, sir. Can be R4 considerate also a pull-down resistor?

#### Hymie

Joined Mar 30, 2018
991
In your circuit, R4 is doing nothing other than being a 1KΩ load across the battery.

#### crutschow

Joined Mar 14, 2008
27,709
I have one more question, sir. Can be R4 considerate also a pull-down resistor?
Yes.

#### MrChips

Joined Oct 2, 2009
24,173
Water is not a very good conductor. R4 value has to be much higher than 1kΩ, try 330kΩ.

#### MrChips

Joined Oct 2, 2009
24,173
Before jumping to conclusions, ask the TS how the water sensor is designed. Maybe it is not a continuous piece of wire from pin-4 to Vcc.
Maybe it is a float switch?

#### AlexDani

Joined Apr 3, 2021
14
Thank you

#### crutschow

Joined Mar 14, 2008
27,709
So what's your definition of a pull-down resistor if it's not to pull a signal down to ground????????????

#### MrAl

Joined Jun 17, 2014
8,474
A man of few words

One word answers dont help too much you should describe why you came such a conclusion and that would help identify the problem you are referring to.

#### MrAl

Joined Jun 17, 2014
8,474
This is the schematic, im trying to make a water level indicator
View attachment 234393
What is that wire going from pin 4 to pin 8. That just shorts pin 4 to pin 8 and will not 'detect' anything.
You need at least two contacts, and that is assuming that you intend to detect water with some mineral content such as we find in an application like a sump pump.

There are also IC's made specifically for liquid detection such as the LM1830 although i have never used one but you might look into that.

The contact area for both contacts should be as large as possible as current flow is increased with area.
It is also a good idea to use an AC current rather than DC so that the contacts last longer.