How do I connect the pull-up resistor to pin 7 of the LM555 timer?

[KAKU]

Hi, all, newbies here!
I have a question about the lm555 timer. I try to connect the pull-up resistor to pin 7 of the LM555 timer but it was failing and I found that the pulse width became narrower after connecting.
Hope help please! Thanks!

Moderators note : replaced link with manufacturers link, using the link again will lead to a ban

 

[Bluejets]

Show your circuit.
If you want clear and concise information on 555 timer, go to talkingelectronics web site.

Look down the left hand side index for both decriptive and animated circuits.

http://www.talkingelectronics.com/te_interactive_index.html

 

[crutschow]

I try to connect the pull-up resistor to pin 7 of the LM555 timer

Pin 7 is the discharge transistor for the timing capacitor.
What are you adding a resistor, which makes little sense?

Post a schematic.

 

[Audioguru]

The circuit you showed has a potentiometer (a variable resistor) connected to pin 7. If it is adjusted to a low resistance then of course the pulse width is narrow.

 

[KAKU]

Thanks for ur quick reply. A simplified circuit is shown below.

 

[Harald Kapp]

I do not understand the original question. You mean the wire indicated in red, do you?
Without the connection to pin 7 the 555 will not work correctly.
Please describe in more detail your problem.

 

[Martaine2005]

How is R3 adjustable? Shouldn't the wiper be connected to pin 7?

Martin

 

[bertus]

Hello,

Put a resistor of 330 Ohms or higher in series of potmeter R3, to limit the discharge current.
The discharge current is max 50 mA.
The attaced PDF will tell you more.

Bertus

 

[Harald Kapp]

How is R3 adjustable? Shouldn't the wiper be connected to pin 7?

No. The connection as shown makes the potentiometer an adjustable resistor (rheostat):
- wiper on top: R3 = 100 kΩ
- wiper to bottom: R3 = 0 Ω (usually a few Ω as the potentiometer will not create an ideal short circuit. That is where the series resistor proposed by @bertus comes into play).

 

[KAKU]

You mean the wire indicated in red, do you?

Hello! Yes! So it cannot work, right?

 

[KAKU]

No. The connection as shown makes the potentiometer an adjustable resistor (rheostat):
- wiper on top: R3 = 100 kΩ
- wiper to bottom: R3 = 0 Ω (usually a few Ω as the potentiometer will not create an ideal short circuit. That is where the series resistor proposed by @bertus comes into play).

Hello,

Put a resistor of 330 Ohms or higher in series of potmeter R3, to limit the discharge current.
The discharge current is max 50 mA.
The attaced PDF will tell you more.

Bertus

Hello, Bertus! Thank U!