I may be misreading this, so I really need to check, to avoid damage to the board. According to this documentation, the input resistance is massive, as in about 100 Megohms. Does this mean that I could literally take a 3.3v power supply, and just link it directly to the input, with no issues? The link is: https://docs.arduino.cc/learn/microcontrollers/digital-pins/ It reads: "Arduino (Atmega) pins default to inputs, so they don't need to be explicitly declared as inputs with pinMode() when you're using them as inputs. Pins configured this way are said to be in a high-impedance state. Input pins make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 megohm in front of the pin. This means that it takes very little current to move the input pin from one state to another, and can make the pins useful for such tasks as implementing a capacitive touch sensor, reading an LED as a photodiode, or reading an analog sensor with a scheme such as RCTime." By the way, I have an Uno R4 Wifi.

[image] numnum1: your background Several jobs, 50 years, all in industrial electronics. Including 35 years in the oil/gas and electricity power utilities. PLC and SCADA.

Oh, OK cool. That is a cool resume! I did electronics but mainly in tuning things like 3.5 inch floppy systems, with oscilloscopes, etc. I tuned some circuits associated with motor speeds on VHS loading lines. That is why I believe that the power transistors were possibly more for motor control speed, rather than switching. This is 90s though, so it is hard to remember all details.

Lots of us old timers here sharing what we have learned and giving back to this community.

Excellent community, might I add! This is one of the best I have ever come across.

[image] Wawa: [image] numnum1: the input resistance is massive, as in about 100 Megohms. Not realy a fixed resistance, but more like a temp dependent leak current. Could be more than 1000 Megohm at room temp, In another topic, asking a similar question to yours: would-i-need-an-current-limiting-resistor-in-series-when-looping-back-the-digital-io-pins I did a test where I measured the current in to / out of an Arduino digital input. The current was around 10nA, with spikes of 100nA as the input changed state.

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[image] numnum1: This means it only needs INPUT_PULLUP configuration on Arduino. That depends.

With ATmega INPUT pins, the pin samples such a tiny amount of current per read that being connected to VCC (like through a button or transistor) with no resistor is not a problem. With newer devices, I expect that the samples are even smaller. What to be concerned about is voltage, not current.

You can safely connect a 3.3V power supply directly to an input pin on the Arduino Uno R4 WiFi, as long as that pin is set as an input. The massive input resistance (about 100 MΩ) means the pin draws virtually no current, so there's no risk of damaging your board or the power supply.

[image] ahsrabrifat: You can safely connect a 3.3V power supply directly to an input pin on the Arduino Uno R4 WiFi Only if the R4 is powered on before you make the connection and the connection is removed before the R4 is powered off, otherwise you risk destroying the board. In general this is not a good idea as you can see that a simple mistake can damage the board.

Are the input pins so high resistance that resistors are not required?

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LarryD June 24, 2025, 9:30pm 38

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Are the input pins so high resistance that resistors are not required?

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