Maker Pro
Maker Pro

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

KAKU

Aug 10, 2021
9
Joined
Aug 10, 2021
Messages
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.
Hope help please! Thanks!

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

crutschow

May 7, 2021
839
Joined
May 7, 2021
Messages
839
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
3,656
Joined
Sep 24, 2016
Messages
3,656
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
Joined
Aug 10, 2021
Messages
9
Thanks for ur quick reply. A simplified circuit is shown below.
Wp1HAfyVPwbKE6m.png
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
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.
upload_2021-8-11_10-36-6.png
 

Martaine2005

May 12, 2015
4,932
Joined
May 12, 2015
Messages
4,932
How is R3 adjustable? Shouldn't the wiper be connected to pin 7?

Martin
 

bertus

Moderator
Nov 8, 2019
3,303
Joined
Nov 8, 2019
Messages
3,303
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
13,700
Joined
Nov 17, 2011
Messages
13,700
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
Joined
Aug 10, 2021
Messages
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!
 
Top