Questions tagged [pullup]

A Pullup Resistor brings a line up to a voltage (Typically VCC), while allowing the line to be pulled low.

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How do I use a pull-up resistor to trigger a relay?

I have a 12v automotive electrical system that has a pressure switch that switches at 40psi. It is normally closed to ground (low pressure), and goes open when switched (high pressure). The switch ...
Chris S's user avatar
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How big a pull-up resistor is needed to pull up an internally pulled-down GPIO (ESP32-C6)?

I'm feeding a BCD diode matrix of switches with Vcc at 3.3 V and running the BCD outputs into GPIOs on the ESP32-C6. The plan is to set each GPIO input to internal pull-down so I can pull it up ...
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Alternative to pull-up resistor

I am building this AND gate, and I managed to get it to work properly. However, I had to increase the value of the pull-up resistor to 3 megohms which is too big for what I need. However if I don't ...
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Multiple NAND gates built with transistors linked together

I am trying to build a full adder with NAND gates. I have a transistor implementation: I am getting the wrong output voltages for Cout and S; is there a way to fix this? Is it my resistors that have ...
Jake's user avatar
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Connecting an open-collector output to a transistor base using a single pull-up resistor

I have a Hall-effect sensor with an open-collector output rated for max. 25 mA. This sensor will be used to switch an NPN transistor powering a 12 V LED strip. The load is 300 mA. Could I get away ...
Joshua Dotson's user avatar
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Esp32 INPUT-pullup

I have an enable pin that i want to drive high always except when a limit switch is closed, and at the same time I want to monitor the state of the limit switch So is that possible with input_pull-up ...
hjeij2000's user avatar
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Do I need pullup resistors for a voltage level shifter driven by a push-pull GPIO output?

I need to shift a 3.3V, push-pull SPI bus from my STM32F3 controller up to 5V to drive a MAX7219 LED controller. To do this, I aim to use an NVT2022 level shifter, datasheet here. The first paragraph ...
kiryakov's user avatar
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ECP5: Determining max current an FPGA pin can sink from datasheet

I have a circuit where I want to drive the CS# input on a SPI flash chip from an output pad on an FPGA. The flash IC's datasheet recommends putting an external ...
jemalloc's user avatar
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How can I read the value of an open drain pin connected to an LED

I'm developing a circuit with a charger inside. The charger has 2 STAT pins that indicate the functionality of the board (charging, standby, fault, etc.) As the datasheet suggests, I've connected two ...
Filo_Gold's user avatar
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Identifying the 4 pins on a 4-pin connector - Is this the correct approach?

Identifying the 4 pins on a 4-pin connector Is it possible (and safe for the electronics with the power OFF) to identify the 2 signal pins on a 4-pin connector, located on a mother board, by checking ...
AlphaBob's user avatar
4 votes
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What causes a negative voltage dip on the collector of common emitter NPN circuit at turn-off?

Circuit diagram: simulate this circuit – Schematic created using CircuitLab Observed waveforms: (Yellow: 3.3V square wave in, Magenta: Collector output) When the input square wave goes low and ...
Unimportant's user avatar
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2 votes
4 answers
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Pull-down resistors and ground

Please help me something that I don't understand: Why is there a need for pull-down resistors in the circuit below with push button if the ground has already zero voltage? This is the only thing I ...
Voloo's user avatar
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Power sequencing: enable signal hold below threshold if asserted after supply

Short version: When I power my PCB and the SYS_EN signals at the same time, all voltage rails turn on. If I first power the PCB, then pull high the SYS_EN signal, the 3.3V and 1.8V rails don't turn on....
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Custom i2c with Arduino UNO

For educational purposes, I would like to write my own i2c protocol. Is it possible to write a custom i2c library to communicate using regular ports, or should I use the SCL and SDA ports? Can I mimic ...
Anton Tsvayer's user avatar
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MAX3362 RS485 IC connection with ESP32

I am using MAX3362 RS485 serial data transfer IC with ESP32. Marked pins 1, 2 and 3 are connected to GPIOs of ESP32. Do I need to add any additional pull-up resistors or some termination resistors or ...
Zunuran Nasrullah's user avatar
2 votes
3 answers
374 views

Why AP331A comparator doesn't work without pullup?

I was designing a PCB and had to include some kind of analogue signal peak detector, so I used comparator to get +3.3V when signal is above 2V and 0 when signal is bellow 2 V. I chose AP331AWG ...
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Can a voltage divider be used to safely lower pull-up voltage?

I'm working with an LTC2950 pushbutton on/off chip that's meant to toggle the enable pins of voltage regulators based presses from a momentary pushbutton. The LTC's enable pin is open drain and is ...
E.HP.S's user avatar
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2 answers
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Thoroughly confused about open drain

I've been reading a tutorial on open drain and I'm thoroughly confused. First, the terminology makes little sense. How can a resistor pull up or pull down current when its job is to resist current? ...
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3 votes
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Datasheet says pull up/down resistors are not necessary, but application of part uses resistors?

I am using a TPS2546 in a USB charging hub, and I'm basing it on a preexisting design I found online (link to writeup for design). In the schematic for the hub design the ILIM_SEL, EN, CTL1, CTL2, and ...
Cole Morrel's user avatar
4 votes
2 answers
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MAX485 module, pull ups for RX and TX

I recently bought the below RS485 module. I see from the schematic they use pull-ups on pins 1 and 4 (TX and RX), can someone explain why are those pull-up resistors needed (R1 and R4)? Secondly, pins ...
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What are pull-up resistors for and how are their values calculated?

I have some questions related to pull-up resistors. Just trying to understand in depth here. Why do we use pull-up resistors on the input pins of a microcontroller? Can I directly connect GPIO input ...
Wajahatulla Sharif's user avatar
3 votes
1 answer
170 views

A pull-up resistor increases the low side of a signal, how can I counteract it?

I have an ebike computer that has two inputs that accept square wave signals. The range is 0-5V. One of the inputs has no pull-up/down resistors on it. When I hook up a device producing signal, the ...
Nicolay Doytchev's user avatar
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Need help with MAX31865

I am trying to program an ESP32-S3 microcontroller to write and read the registers of a MAX31865 IC over SPI, but I cannot get anything meaningful out of the device. The following image shows the ...
Marcell Juhász's user avatar
5 votes
3 answers
920 views

When you have a pullup resistor, does the system consume power?

If you have a pullup resistor setup: Image Source: Sparkfun - Pull-up Resistors If the button is open, is voltage and current constantly being drawn through the input pin or does the high impedance ...
Thomas Stokes's user avatar
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Do all u-blox ZED-F9T input pins have internal pull-up resistors?

The ZED-F9T datasheet says in section 5 Communications Interfaces: All the inputs have internal pull-up resistors in normal operation and can be left open if not used. This statement on its own ...
MattHusz's user avatar
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Do I have to use pull up/down resistor with switch with common terminal?

I have seen some examples why we need to use pull up/down resistors, but do I need to use it if I use switch with common terminal? If I have the schema like so, do I need to use the pull up/down ...
krystof18's user avatar
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3 answers
195 views

Pull-up resistor is running slowly (inverting MOSFET circuit)

There's a problem with my fan control circuit. It works well, but when I first power up the circuit, the fan runs for about 0.2 seconds. The reason seems to be the time until the pull-up resistor is ...
KimJiHoon's user avatar
4 votes
7 answers
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How can I control PNP and NPN transistors together from one pin?

I'm trying to understand how it is possible to control a PNP and a NPN transistor simultaneously from one controlling pin. My issue is that when the INPUT pin is left floating, the transistors drive ...
AKTanara's user avatar
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Battery isolation circuit with inverted logic

I asked a question some time ago in regards to preventing back charging my back-up battery question. I was just using a diode, but a circuit was suggested for more protection. It doesn't lend itself ...
Feynman137's user avatar
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5 answers
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"Pull-up" and "pull-down" or "push" and "pull"?

I thought about this question when I read an interesting comment by @Simon Fitch in his answer. In the complementary amplifier stage, in total there are three "pulling" elements - Q2 is a &...
Circuit fantasist's user avatar
1 vote
1 answer
223 views

How to choose pull-up resistor values for level shifting circuitry for serial communications

I recently joined a new project where two ESP32-WROOM-32UE modules on separate circuit boards talk to each other via the UART pins. The distance between the two boards is roughly 1 meter of cable ...
user2218339's user avatar
1 vote
1 answer
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Confused by i/o expander datasheet note re internal pull-ups

The inputs on the NXP PCAL6524 i/o expander can be configured with internal pull-up or pull-down resistors. The datasheet states, "External resistors are required for inputs... that may float. If ...
Slothrop's user avatar
2 votes
2 answers
135 views

Debugging a problematic old TTL pull-up that's not pulling up

I am trying to bring back into operation a late-70s disk controller board (all 74xx TTL). I'm stuck trying to figure out why a small pull-up network is resulting in a far out of spec voltage not doing ...
BZo's user avatar
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2 votes
1 answer
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Value of resistor clarification

I'm using this device - USB2514B Table 3-3 says that the required resistor value is: "47 - 100 kohm". I assumed that the 47 was 47 ohms and not 47 kohms. I used 10 kohm resistors in my ...
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Calculation of pull-up resistor for open-drain output - multiple loads

Link to voltage supervisor Link to AND gate I would like to drive an open-drain output to multiple (5) loads, all of the same type. I calculated the pull-up resistor range needed and wanted to double ...
Matty's user avatar
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11 votes
7 answers
4k views

Has anyone measured what a high-impedance pin looks like?

I understand that when an IC pin is in a high-impedance state, the pin is not connected to anything internal. It will be at the voltage to whatever it is being pulled from the outside. I am just ...
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1 answer
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L6924U output status pins in the datasheet description are open-collector, but schematic shows a MOSFET

The battery charger L6924U has two status pins, ST1 and ST2, to tell what is happening inside the charger: charge in progress, charging done, stand-by mode, etc. In the datasheet STM always writes ...
Lyoner's user avatar
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2 votes
1 answer
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Trigger feedback from camera hot shoe to embedded controller

I am trying to build a circuit using the signal from a Fuji XT-20 camera hot shoe to drive an interrupt input pin on a Teknic ClearCore controller. My understanding is that all the hot shoe does is ...
user11271728's user avatar
0 votes
1 answer
122 views

I2C transmission gate transistors

I have been looking at a design that uses DMN31D5UDJ-7s for gating I2C signals that are connected to 4.7 kΩ pull-ups to VIO = 1.8 V. I was wondering if I could replace these transistors with ...
Firas Abd El Gani's user avatar
4 votes
3 answers
313 views

How does a pull-resistor produce the low signal?

When using a pull-up resistor with a switch connected to an Arduino, the input pin reads either high or low when the switch is open or closed respectively. I understand how when the switch is open, ...
CustomCalc's user avatar
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1 answer
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Reading fan speed by ESP32

I have a case type 12 V 3-pin fan and I would like to connect its tach signal wire to an ESP32 input pin. Power supplies to ESP32 and the fan are separate, but the ground wire is common. Based on some ...
Serge's user avatar
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1 vote
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FPGA Loopback (two pins) - What is the way to create a pullup to set one (or the other) pin to high when it is tri-stated to high impedance?

I need to wire up the four GPIO pins (connected to the FPGA pins) in a way that keeps the value of the pin at a pulled up value of appropriate voltage. I have an option to use the pullup resistors. ...
Vahe's user avatar
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1 answer
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TCA9548A : power down while pull-up resistors are still supplied on one side

To keep RF as low as possible in a music synthesizer, I power down the SoC (XIAO SAMD21) which performs autotune once the autotuning procedure is completed. On one side of the TCA9548A there is the ...
Synthlink's user avatar
4 votes
2 answers
467 views

How to programmatically connect/re-connect pull-up resistors?

I’m using a MT6701 magnetic absolute encoder that supports both I2C readout and quadrature pulses (ABZ). Inconveniently, the pins for the quadrature outputs A and B are the same pins for I2C’s SCL and ...
Roy Li's user avatar
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1 answer
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How I can read a 5 V input state correctly with an Arduino?

I want to read the output of a PLC with an Arduino. If the PLC sends a HIGH (5 V) on a OUTPIN, the Arduino should read this HIGH, else LOW. I am generally using this code for this: ...
mehmet's user avatar
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Why does the DS18B20 temperature sensor require a stronger pull-up resistor when Node MCU is powered through the Vin pin?

I have a DS18B20 temperature sensor connected to a Node MCU (ESP-12 / ESP8266) like shown below, where μP is the MCU (any GPIO gives the same result) and Vpu = VDD = 3.3 V, coming from the 3.3 V pin ...
guiweb's user avatar
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-1 votes
3 answers
310 views

Using 5 V instead of a pull-up resistor?

I was researching pull-up resistors. One reason to use them is to give a solid high to our input pin, but I can't figure out why I should use a resistor in this setup. What if we connect 5 V without ...
mohas's user avatar
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2 votes
1 answer
163 views

MCP3204 problem with SPI comunication

I have problem with the MCP3204 ADC. I am trying to connect 8 devices by SPI. A single device works perfectly, but if I try to attach the second MCP3204 it doesn't work. On a logic analyzer I can see ...
Gregor's user avatar
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1 answer
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Solving different voltage for I2C pull-up resistors

I am designing my own PCB (based on the STM32F4). I am communicating with some devices using I2C. The thing is that this devices are connected to the I2C bus, which uses two 4.7 kΩ resistors (pull-ups)...
Developing Electronics's user avatar
1 vote
3 answers
101 views

Are pull-up resistors necessary for the TUSB320HA's EN pin if it's never set low?

The TUSB320HA IC has a pin named EN that needs to be set high to enable the chip. According to the datasheet: VIH_EN, the voltage needed to set EN high, should be between 1.05V and 3.65V IIH, the ...
ide's user avatar
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