A Pulldown Resistor brings a line down to a voltage (Typically Ground Potential), while allowing the line to be pulled up.
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ULN2003A Darlington array behavior
I'm testing a ULN2003 Darlington transistor array (Datasheet Here).
simulate this circuit – Schematic created using CircuitLab
With GND or 5V input it works fine, but if I unplug an ...
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2answers
130 views
How to use a transistor to pull a pin low?
I'm trying to make a simpler approach to flash LPC1114 MCU.
If the MCU is reset while pin1.1 is held low it enters bootloader mode. I'm using a simple arduino sketch to drive reset & 1.1 pins via ...
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1answer
79 views
pin is pulled up to something
I was reading 17xx series` data sheet, and I had trouble understanding a specific explanation given for a major number of pins:
5 V tolerant pad providing digital I/O with TTL levels and ...
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5answers
511 views
What makes a pull-up/down resistor strong or weak?
A "strong" pull (up/down) resistor would be one of a relatively low value, while a "weak" one would be of a relatively high value.
For example, a pull-down resistor would be used to keep an I/O pin ...
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2answers
147 views
Can a pull down be used to tie a line to ground?
From my recent introduction to pull-up/pull-down resistors, I understand that an input with a pull-down will stay low instead of floating. Since the pull-down resistor leads to ground, couldn't I ...
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1answer
152 views
Pull Up and Pull Down Resistor Values [duplicate]
I usually use 10K Ohm resistors for pulling up or pulling down micro-controller lines. This is only because I have tons of them.
Recently while looking up pick and place surface mount information I ...
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4answers
629 views
Why use a pull-down resistor with a LDR and microcontroller?
I am looking at this:
http://learn.adafruit.com/photocells/using-a-photocell
It's connecting VCC <-> LDR <-> resistor <-> GND. And the analog ...
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2answers
103 views
How and why is “floating input” a valid state for RF/IR encoder IC's?
RF/IF encoder/decoder IC's such as this one, will accept the address-pin in one of 3 states:
Floating
High
Low
As per this excellent answer, I believe "Floating" input can assume either High or ...
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1answer
162 views
Logic NOT gate only works with Oscilloscope probe connected
I have a very simple circuit that is misbehaving in a way that I cannot explain with my very limited experience.
The circuit that consists of a motion detector:
...
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3answers
309 views
How do I calculate the required value for a pull-down resistor in a PWM circuit?
I'm using 74HC595 with SoftPWM to control some LED's.
When the 74HC595 is off, its outputting about 0.57v, enough to turn my LED's on somewhat.
How do I work out the value of the pull-down resistor ...
4
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1answer
743 views
Using Piezo Vibration Sensor with Pull-Down Resistor - Arduino
I'm using a MEAS piezo sensor (this one: https://www.sparkfun.com/products/9196) to detect very slight vibrations, but I'm having trouble selecting a pull down resistor that allows it to be sensitive, ...
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5answers
1k views
Pull-up and Pull-down Resistor Usage on Input or Output MCU Pins
Are pull-up/down resistors (whether internal or external) only needed for MCU INPUT pins? In contrast, an MCU pin configured as an OUTPUT "knows what level it's at" because it does the driving - a ...
4
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2answers
245 views
Pull-down resistor value for FSR's voltage divider circuit connected to Arduino
I am working on a project which includes two FSRs (force sensitive resistors) connected to an Arduino Uno. I have each FSR setup as in http://www.instructables.com/id/FSR-Tutorial/?ALLSTEPS, but each ...
18
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2answers
594 views
Heuristic explanation of the usage of a pull-up (or pull-down) resistor needed
First, I acknowledge that there are several questions regarding this topic in the forum, however, the answers assume too much background knowledge of electronics to be of use to a true beginner (like ...
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1answer
391 views
Pull down resistor for Arduino input to read LOW when no voltage from switch
I'm building the Arduino spaceship interface from the project book and everything works fine. It involved connecting 3 Arduino outputs to 3 leds, and 1 Arduino input to the a switch. For a photo of ...
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1answer
304 views
Pull-down resistor with ADM485
I have ADM485 already mounted and an output of some device, buffered by CD4050BM. For the circuit to be safe for the RS485 bus, the ADM transiver initially should either be in Z-state or read-state. ...
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2answers
176 views
Increase resolution of voltage divider
I have a lead-acid battery on which I am using a voltage divider to measure the remaining charge.
Its my understanding that an example battery could discharge from 100-0% over say 10.5V - 12V.
...
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2answers
351 views
Must IO pins each have their own pull-up/down resistor?
A very basic question:
Must each IO pin of a microcontroller that is serving as an input from a switch or jumper have its own pull-up/down resistor?
In this example, each pin is pulled down by a ...
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1answer
246 views
Phototransistor as a pull-down in digital circuit
I have very little experience with phototransistors. I'm trying to create a circuit that will effectively function as collision detection. I'm using OPB608A which utilizes an NPN transistor. My goal ...
2
votes
1answer
264 views
Need of External Pull Up/Pull Down for Processor I/O pins
For a particular I/O pin, its been mentioned in the processor data manual that there is Internal pull down, after power on reset, in the device with a default drive state of the pin as tristate(Z).
I ...
5
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2answers
210 views
Chosing a pulldown resistor
Setup:
A digital output from a device is connected to an output buffer to drive an optocoupler. If device power is lost the output will go in high-impedance mode.
Problem: I want to make sure that ...
2
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4answers
1k views
Pulldown resistor on output pin, how does output still work?
On a part I'm working with, it says that you can configure the part to operate in a particular mode by "strapping" a particular pin to ground. That is, it wants you to use a pulldown resistor to pull ...
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2answers
210 views
Arduino - connecting a button to a digital pin [duplicate]
Possible Duplicate:
Pull Down Resistors
I'm new to Arduino, can you please tell me why in this scheme the gray wire and the resistor are needed? Why won't it work without them? Thank you!
...
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4answers
1k views
Pull Down Resistors
In my quest to understand electrical engineering, I have stubmled across this tutorial:
http://www.ladyada.net/learn/arduino/lesson5.html
I have understood the diagrams until I got to switches. I am ...
4
votes
3answers
357 views
Which side should I put the resistor?
I'm a beginner and trying to understand some basics of electricity.
One thing I'm having a hard time understanding is the correct side where I should put a resistor. For example, in the following ...
5
votes
3answers
640 views
Why would an input pin have both a pull-up and pull-down resistor?
In his answer detailing the various types of input pins, Russell McMahon leaves the following note [referring to input pins]:
there are special cases where a resistor to high and low at once is ...
4
votes
3answers
4k views
Logic Conversion 3.3V to 5V (Current Related) 74HC 74HCT
A beginner style question.
I'm attempting to perform some logic level conversions on 3.3V signals to 5V using the 74HC4050D powered at 3.3V and 74HC541 powered at 5V. Is it OK to directly connect the ...
9
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7answers
4k views
Which configuration is better for pulling down an NPN transistor's base?
I was discussing on pull down resistors with a colleague of mine. Here are the two configurations for transistor as a switch.
The input signal can either come from a microcontroller or an another ...
6
votes
2answers
2k views
Why does a pull-up resistor not stop current flowing to the input pin when the switch is open?
Sorry this is such a silly question, but I can't seem to understand this. In the third diagram here it shows a pull-up resistor.
I understand that when the switch S1 is closed, current is pulled ...
3
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1answer
206 views
Increasing current capability of a Pin
This is a followup from my previous question.
Here's a summary of how the circuit works:
I'm designing a circuit that can test for short circuits and open circuits in a wiring harness. The wiring ...
8
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3answers
3k views
Recommendation for default settings for unused pins on an STM32 (ARM Cortex M3) - pull up/pull down?
We are currently using various variants of the STM32 microcontroller family. I would like to know the following:
1) What are the recommended default settings for pins of the microcontrollers in ...
7
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4answers
1k views
Why are pull up resistors more common than pull down resistors?
I noticed that pull up resistors are much more common than pull downs, why?
For example, the Arduino's MCU has internal pull ups but these tend to invert the physical logic of things you are working ...
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1answer
155 views
Mismatched Pull Up Resistors
Is it OK to have pull up or pull down resistors which aren't all the same value (if you're running low on components for example)? I can't think of a reason it should affect simple logic circuits.
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