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

#### KAKU

Aug 10, 2021
9
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.

Last edited by a moderator:

Oct 5, 2014
5,995

#### crutschow

May 7, 2021
520
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

Sep 24, 2016
3,650
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

Aug 10, 2021
9
Thanks for ur quick reply. A simplified circuit is shown below.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,756
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.

#### Martaine2005

May 12, 2015
4,355
How is R3 adjustable? Shouldn't the wiper be connected to pin 7?

Martin

#### bertus

Moderator
Nov 8, 2019
2,730
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

#### Attachments

• 555an.pdf
355.8 KB · Views: 3

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,756
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

Aug 10, 2021
9
You mean the wire indicated in red, do you?
Hello! Yes! So it cannot work, right?

#### KAKU

Aug 10, 2021
9
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!

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