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### Topic: Indicator LED doubling as pull-up circuit (Read 983 times)previous topic - next topic

#### GilchristT

##### May 26, 2017, 06:13 pmLast Edit: May 26, 2017, 06:19 pm by GilchristT
Folks

Just a quick question on best practices. I'm designing a simple hall effect sensor based circuit as shown below.

The output requires a pull-up resistor and I'm also adding a LED to indicate when the sensor is triggered.

The circuit got me thinking, is the 10K pull-up resistor redundant as the LED circuit is carrying out the same function?

It's more a just-wondering-keen-to-learn thing, obviously removing a single resistor from the BOM isn't a big issue but I'd be interested to know if there are any good reasons not to use the indicator circuit as the pull-up as well?

thanks

Tommy

#### jremington

#1
##### May 26, 2017, 06:25 pmLast Edit: May 26, 2017, 06:25 pm by jremington
Yes, in that circuit the pullup resistor is redundant.

It is required if the output goes instead to a digital input with no pullup.

#### GilchristT

#2
##### May 26, 2017, 07:22 pm
It is required if the output goes instead to a digital input with no pullup.
I'm not sure I understand that last statement?

Surely a pull-up resistor is redundant in any situation where the digital input has a pull-up resistor enabled (setting aside arguments about the relative strenghts of integrated pull-up resistors)?

#### aarg

#3
##### May 26, 2017, 07:43 pm
The pull up resistor is a requirement, whether it is part of the circuit that you show, or part of the circuit that it is connected to.
... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

#### larryd

#4
##### May 26, 2017, 07:49 pm
If the output is On/Off the 10K is optional.

If the output is analog, I recommend using the 10K as the LED is essentially open circuit until conduction occurs, but this could be argued otherwise.

Adding a parallel 10K resistor costs little and consumes little current.

.
No technical PMs.