Saturday, March 15, 2025

Entry 2: Pull-up and pull-down resistors

In entry 1, we briefly discussed the fundamental logic gates:

  • NOT gate
  • AND gate
  • OR gate
  • NAND gate
  • NOR gate
I asked Grok how to demonstrate the workings of an logic gate. The answer was very clear, with the exception of "pull-up" and "pull-down" resistors, so let's begin there.

Pull-up and pull-down resistors.

I asked Grok "Explain "pull-up" and "pull-down" resistors and how I can demonstrate these using buttons, resistors and led."

Explanation of Pull-Up and Pull-Down Resistors

Pull-up and pull-down resistors are used in digital circuits to ensure that an input pin (e.g., on a microcontroller, logic gate, or IC) has a defined voltage level (HIGH or LOW) when it’s not actively driven by a switch, button, or another circuit. Without them, inputs can "float," picking up noise or random voltages, leading to unpredictable behavior.

Pull-Up Resistor

  • Definition: A resistor connected between a pin and the positive supply voltage (e.g., +5V or VCC).
  • Purpose: Pulls the pin’s voltage to HIGH (logical 1) when no other signal overrides it.
  • Behavior:
    • When a switch connects the pin to ground (0V), the pin goes LOW (0).
    • When the switch is open, the resistor "pulls up" the pin to VCC (HIGH).
  • Typical Value: 1kΩ to 10kΩ (depends on current and speed requirements).
  • Use Case: Common in active-low switches (pressed = 0, released = 1).

Pull-Down Resistor

  • Definition: A resistor connected between a pin and ground (0V).
  • Purpose: Pulls the pin’s voltage to LOW (logical 0) when no other signal overrides it.
  • Behavior:
    • When a switch connects the pin to VCC (e.g., +5V), the pin goes HIGH (1).
    • When the switch is open, the resistor "pulls down" the pin to ground (LOW).
  • Typical Value: 1kΩ to 10kΩ.
  • Use Case: Common in active-high switches (pressed = 1, released = 0)

Demonstration Using Buttons, Resistors, and an LED

To demonstrate pull-up and pull-down resistors, we’ll build two simple circuits using a button, resistors, and an LED. We won’t use a logic gate IC here (to focus purely on the resistor effect), instead directly observing the voltage state with the LED.

Materials Needed

  • Breadboard
  • Power supply: 5V (e.g., USB or battery).
  • Push button (momentary switch).
  • LED (e.g., red).
  • Resistors:
    • 220Ω or 330Ω (for the LED).
    • 10kΩ (for pull-up or pull-down).
  • Jumper wires.

1. Pull-Down Resistor Demo

Goal: Show that the input defaults to LOW (LED off) and goes HIGH (LED on) when the button is pressed.

Circuit Setup:

  1. Connect +5V to one side of the button.
  2. Connect the other side of the button to a breadboard row (call this the "input node").
  3. Connect a 10kΩ resistor from the input node to ground (pull-down).
  4. Connect the input node to the LED’s anode.
  5. Connect the LED’s cathode to ground through a 220Ω resistor.






2. Pull-Up Resistor Demo

Goal: Show that the input defaults to HIGH (LED on) and goes LOW (LED off) when the button is pressed.

Circuit Setup:

  1. Connect +5V to one side of a 10kΩ resistor.
  2. Connect the other side of the 10kΩ resistor to a breadboard row (input node).
  3. Connect one side of the button from the input node to ground.
  4. Connect the input node to the LED’s anode.
  5. Connect the LED’s cathode to ground through a 220Ω resistor.





I guess a point here is to avoid something like


with no pull-up or pull-down resistor. When the button is pressed, we do have 5V or HIGH to digital input but we do not have LOW or 0V when it is not pressed. In this case it is floating and the digital input will randomly detect a 1 or a 0.

In the next entry, I will start working on the fundamental logic gates.



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