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Topic: low power pull-up or pull-down resistor? (Read 1 time) previous topic - next topic

Seedler

Hi,

Hi I have a wireless sensor connected to a flow switch.  Most of the time the water will flowing so the switch will be in the closed position.

This is a battery powered wireless sensor so am trying to use as little power as possible.

As I understand it, current will only flow when the switch is closed, but as my switch will be closed most of the time will a larger resistor reduce the current flow?  I was thinking 47K or 100K.

Is either pull up or pull down better than the other?

Or is there a better way of doing this?

Cheers.

Damien.

DedeHai

Pullup or pulldown does not really matter. Either the switch pulls the voltage down or up, the current flowing is the same but the signal is inverted.
What is the power consumption of your wireless sensor overall? How fast do you need to be able to detect the switching? how noisy (in terms of electromagnetics) is the environment? how long is the cable to the switch?
all these things matter when choosing a pullup/pulldown resistor. Usually, 100k is a good choice but if speed is not an issue and noise is low you can use a 1M resistor or even higher.

Regards,

Damian

Seedler

Hi,

While sleeping 0.1ma, then around 40ma for a few seconds every 20 minutes.

The switching does not need to be fast as the sensor only transmitter every 20 minutes.

The cable to the switch is one meter long.

The sensor is in a rural mountain location, but there is a mobile phone mast about 400 meters away.

I read somewhere that anything over 100K wouldn't work reliably on an Arduino.

Thanks for your reply,

Damien.


Wawa

#3
Oct 07, 2016, 09:44 pm Last Edit: Oct 07, 2016, 09:54 pm by Wawa
Hi I have a wireless sensor connected to a flow switch.  Most of the time the water will flowing so the switch will be in the closed position.

This is a battery powered wireless sensor so am trying to use as little power as possible.
So a switch, not a flow sensor.
If you connect the switch between input pin and ground,
and connect the (10-100k) pull up resistor between input pin and an output pin,
And switch that output pin to HIGH just before sampling and LOW after that,
then you don't use any current at all.
Leo..

DedeHai

yes, that is probably the best method.
The other option is to use a 100nF cap on the input pin (to GND) and use a 1M pullup resistor. The extra current then is only 0.005mA.
By the way, 0.1mA seems quite a lot for a sensor node in standby... The ones I made only use 0.003mA

Seedler

So a switch, not a flow sensor.
If you connect the switch between input pin and ground,
and connect the (10-100k) pull up resistor between input pin and an output pin,
And switch that output pin to HIGH just before sampling and LOW after that,
then you don't use any current at all.
Leo..
That sounds like an excellent way.  Thanks.

Damien

Seedler

yes, that is probably the best method.
The other option is to use a 100nF cap on the input pin (to GND) and use a 1M pullup resistor. The extra current then is only 0.005mA.
By the way, 0.1mA seems quite a lot for a sensor node in standby... The ones I made only use 0.003mA
Thank for your help.  Yeah I'm still working on reducing power.

Cheers,

Damien.

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