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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Solar cell powered supercapacitor charger with strange output voltage regulator problem

I have a strange problem in a low power circuit design. I designed a buck-boost converter as a maximum power point tracking circuit (control the output of solar cell within a small voltage range ...
2
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3answers
85 views

Murata OKR-T/10 pulldown value

On the Murata OKR-T/10, the data sheet specifies a 49kohm pulldown resistor but I've seen people using a 1k. Is there a more correct value or does anything 1k or above work just as well?
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1answer
70 views

Effects of “loading down” Voltage-Divider Circuits

Hello I am currently a beginner in the world of electrical engineering and would like some help on voltage-divider circuits. My question mainly concerns the "loading down effect". Can someone please ...
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1answer
161 views

Voltage divider circuit and the Pull-down resister

I am reading pull-down resister here. PART-1 The circuit schematic of a light sensor using a voltage divider circuit is as shown: LDR has a resistance of 500\$ \Omega \$, 0.5k\$ \Omega \$ , in ...
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1answer
100 views

How do you size pull-up/down resistors? [duplicate]

The attached photo is pulled from a Piccolo DSP reference design from TI. They use 57.6k pull-up resistors and 2.2k pull-down resistors to allow configuration of boot pins at startup. I've seen all ...
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1answer
43 views

Detect current on intercom

I'm trying to get an input from an intercom and apparently there is GND and another wire for the call. I'd like to know when there's voltage between the call wire and the ground. I'm thinking of ...
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2answers
170 views

How does the pull down resistor work in this circuit?

Here's an image from another question: I get what the pull down resistor is there to do, but I don't really understand exactly how it works. When there is no voltage applied through the base wire, ...
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1answer
66 views

Separate PICAXE Download board Serial In pull down resistor

I have a PICAXE (20M2) project that I am working on and I have two separate boards one programs the chip, the same chip then gets transferred to the other board, to which the peripherals (LEDs, ...
2
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4answers
95 views

Pull up circuit current flows?

Im trying to understand how pull up/down circuits works . Let's consider this schematic : I understand that there are two possible current flows : When the button is not pressed, the current ...
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1answer
168 views

Pull down resistor not pulling current down to 0 V? - Arduino + LED's

I was recently connecting an LED strand to my Arduino nano using the following wiring diagram: I was using 10k ohm pulldown resistors in order for the LED's to be off when the Arduino is outputting ...
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2answers
218 views

Getting rid of floating input in a logic level converter

Is it possible to make the output of this circuit 0V, when the input is floating as shown in the picture . I tried adding a pulldown resistor to the input, but it only worked at a very low value(100 ...
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1answer
119 views

When a pin needs to stay LOW or HIGH, is it okay to connect directly to VCC or GND?

For example, I'm using an Exar Exar SP3220EBCY-L RS-232 transceiver and the typical application loop back test shows that the EN pin is connected to GND and the SHDN pin is connected to VCC. Should I ...
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1answer
54 views

CMOS 4066 pull up resistor

I'm controlling three 4066 (Quad Bilateral Switch) with two chained 74HC595 (8-bit serial-in, serial or parallel-out shift register with output latches; 3-state) Do I need to use pull up resistor ...
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0answers
391 views

How to assign a pull-up/down resistor in Verilog for inputs?

As a newbie in FPGA world, I realized that it is possible to set pull-up/down resistors in Verilog but I don't know how. I have written my code that works just fine but when I connect my XC3S400 to ...
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0answers
305 views

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
188 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
89 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
2k 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
194 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
213 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
1k 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
133 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
246 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
502 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 ...
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1answer
1k 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
2k 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 ...
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2answers
339 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 ...
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2answers
696 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
761 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
575 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. ...
3
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2answers
226 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
467 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
362 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 ...
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1answer
323 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 ...
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2answers
258 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 ...
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4answers
2k 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
242 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
2k 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 ...
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3answers
431 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
811 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 ...
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votes
3answers
5k 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 ...
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votes
7answers
5k 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 ...
8
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
votes
1answer
222 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 ...
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votes
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 ...
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votes
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
168 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. ...