555 timer

Thread Starter


Joined Apr 3, 2021
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?


Joined Mar 14, 2008
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?

Show your schematic.


Joined Mar 14, 2008
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
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.

Thread Starter


Joined Apr 3, 2021
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.


Joined Oct 2, 2009
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?


Joined Jun 17, 2014
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.