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Raspberry Pi’s 40-pin header combines fixed power and ground connections with 3.3V GPIO signals. A GPIO pin can be read as an input, driven high or low as an output, or switched to an alternate peripheral such as I2C, SPI, UART or PWM. The safe approach is to identify the exact board, verify the physical-to-BCM numbering, keep every GPIO signal at 3.3V or below, add current limiting for LEDs, and use a driver circuit for motors. Raspberry Pi’s official hardware reference and pin-mapping tables are the final authority for your model: Raspberry Pi computer hardware and its pin/GPIO mappings.
What the 40-pin GPIO header is
The header is a 2-by-20 row of pins on most current full-size Raspberry Pi boards, with a 0.1-inch (2.54 mm) pitch. It is not a block of 40 interchangeable outputs. Some contacts are fixed 5V supplies, some are fixed 3.3V supplies, several are ground returns, and the rest are configurable GPIO signals.
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GPIO means general-purpose input/output. In input mode, the processor samples whether a signal is logically low or high. In output mode, it drives the line low at approximately 0V or high at approximately 3.3V. A pin can also be assigned an alternate function implemented by the SoC, allowing a hardware peripheral to use that contact instead of ordinary GPIO. Changing a pin to an alternate function can make it unavailable for another use at the same time.
Fixed supply and return pins
The two 5V contacts are connected to the board’s 5V rail, and the two 3.3V contacts provide the board’s 3.3V rail. Ground contacts are the return path for circuits. None of these fixed pins can be changed into software-controlled GPIO. A 5V pin is a power connection, not a 5V-tolerant logic input.
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Configurable GPIO signals
General-purpose pins can be configured as inputs or outputs. Pull-up and pull-down resistors give an input a defined idle state instead of allowing it to float. GPIO2 and GPIO3 have fixed pull-ups in the documented common configuration; software can configure pulls on other GPIOs. Pull resistors are weak biasing devices, not substitutes for a proper power source or load driver.
GPIO numbering versus physical pin numbering
Every header contact has a physical pin number, counting from 1 through 40 on the connector. A signal also has a BCM GPIO number, referring to the SoC’s GPIO controller. These numbers identify different things. For example, the SPI0 MOSI signal is BCM GPIO10 on physical pin 19, while SPI0 SCLK is BCM GPIO11 on physical pin 23.
A wiring diagram that says “GPIO18” means a BCM number unless it explicitly says physical pin 12. If your code uses BCM numbering while you wire by physical numbering, the circuit will be connected to the wrong signal. Before inserting a jumper, run pinout in Raspberry Pi OS or consult the model-specific diagram in the official hardware documentation. Check the connector’s orientation as well; pin 1 is normally marked on the board, and reversing the header view mirrors the entire layout.
40-pin header reference
The following table shows the standard mapping used by the common 40-pin header. “GPIO” values are BCM numbers. Entries marked as general-purpose can also have additional alternate functions; the table highlights the commonly documented assignments rather than promising that every function is enabled by default on every board.
| Physical pin | BCM GPIO or fixed connection | Common documented role |
|---|---|---|
| 1 | 3.3V power | Fixed 3.3V supply |
| 2 | 5V power | Fixed 5V supply |
| 3 | GPIO2 | I2C SDA; fixed pull-up in the common mapping |
| 4 | 5V power | Fixed 5V supply |
| 5 | GPIO3 | I2C SCL; fixed pull-up in the common mapping |
| 6 | Ground | 0V return |
| 7 | GPIO4 | General-purpose or alternate function |
| 8 | GPIO14 | UART TX in the documented mapping |
| 9 | Ground | 0V return |
| 10 | GPIO15 | UART RX in the documented mapping |
| 11 | GPIO17 | General-purpose or alternate function |
| 12 | GPIO18 | Hardware PWM-capable pin |
| 13 | GPIO27 | General-purpose or alternate function |
| 14 | Ground | 0V return |
| 15 | GPIO22 | General-purpose or alternate function |
| 16 | GPIO23 | General-purpose or alternate function |
| 17 | 3.3V power | Fixed 3.3V supply |
| 18 | GPIO24 | General-purpose or alternate function |
| 19 | GPIO10 | SPI0 MOSI |
| 20 | Ground | 0V return |
| 21 | GPIO9 | SPI0 MISO |
| 22 | GPIO25 | General-purpose or alternate function |
| 23 | GPIO11 | SPI0 SCLK |
| 24 | GPIO8 | SPI0 CE0 |
| 25 | Ground | 0V return |
| 26 | GPIO7 | SPI0 CE1 |
| 27 | GPIO0 | Reserved for advanced use |
| 28 | GPIO1 | Reserved for advanced use |
| 29 | GPIO5 | General-purpose or alternate function |
| 30 | Ground | 0V return |
| 31 | GPIO6 | General-purpose or alternate function |
| 32 | GPIO12 | Hardware PWM-capable pin |
| 33 | GPIO13 | Hardware PWM-capable pin |
| 34 | Ground | 0V return |
| 35 | GPIO19 | Hardware PWM-capable pin |
| 36 | GPIO16 | General-purpose or alternate function |
| 37 | GPIO26 | General-purpose or alternate function |
| 38 | GPIO20 | General-purpose or alternate function |
| 39 | Ground | 0V return |
| 40 | GPIO21 | General-purpose or alternate function |
GPIO0 and GPIO1 on physical pins 27 and 28 are reserved for advanced uses in the general hardware documentation. Leave them alone unless you understand the interface that owns them and have checked the board documentation.
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Common peripheral functions
I2C: GPIO2 and GPIO3
The documented common I2C mapping uses GPIO2 on physical pin 3 for SDA (data) and GPIO3 on physical pin 5 for SCL (clock). I2C devices share those two lines, with each device selected by address. The lines normally need pull-up resistors to the logic supply; use the pull-ups specified by the peripheral or breakout board and keep the bus at 3.3V logic.
UART: GPIO14 and GPIO15
GPIO14 on physical pin 8 is the common UART transmit line, TX. GPIO15 on physical pin 10 is the receive line, RX. A serial connection crosses the signals: one device’s TX goes to the other device’s RX, and both devices need a common ground. Confirm the board’s serial-console and alternate-function configuration before using these pins for your own protocol.
SPI0: GPIO7 through GPIO11
The standard SPI0 mapping is:
- GPIO10, physical pin 19: MOSI (controller to peripheral data).
- GPIO9, physical pin 21: MISO (peripheral to controller data).
- GPIO11, physical pin 23: SCLK (clock).
- GPIO8, physical pin 24: CE0 (chip select 0).
- GPIO7, physical pin 26: CE1 (chip select 1).
SPI can support more peripherals by adding chip-select lines, but the available alternatives depend on the board and its pin-multiplexing configuration. The official pin/GPIO mapping reference lists standard and alternate mappings.
PWM
Software PWM can be generated on all GPIO pins, subject to timing and operating-system load. The documented hardware-PWM-capable pins are GPIO12, GPIO13, GPIO18 and GPIO19, on physical pins 32, 33, 12 and 35 respectively. Hardware PWM is preferable when stable pulse timing matters, but enabling one peripheral can constrain other alternate functions on the same pin.
Electrical levels and current limits
Raspberry Pi GPIO outputs are specified as approximately 3.3V high and 0V low. They are not 5V-tolerant inputs. Applying 5V to a GPIO can damage the SoC, even if the source is connected only briefly. The official warning is: “Do not use 5V for 3.3V components.” Treat sensors, displays and other modules as 3.3V logic unless their documentation explicitly provides safe level shifting.
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Do not interpret a drive-strength table as permission to draw that current continuously from a pin. For older BCM2835, BCM2836, BCM2837 and RP3A0-based products, the cited table lists a default drive strength of 8mA and a maximum drive strength of 16mA. For BCM2711-based 4-series devices, it lists a default of 4mA and a maximum of 8mA. Raspberry Pi explains that drive strength describes output behavior, not a simple per-pin current limit.
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How to connect an LED safely
An LED is a small load, but it still must have a series resistor. Without current limiting, the GPIO and LED can draw excessive current and fail. Raspberry Pi’s hardware guidance states: “LEDs should have resistors to limit the current passing through them.” It does not specify one universal resistor value because the correct value depends on the LED’s forward voltage, desired current and circuit supply.
- Choose a free GPIO after checking the model-specific pinout. Do not use a 5V, 3.3V or ground pin as the controlled signal.
- Connect the GPIO to one end of a suitable series resistor.
- Connect the resistor to the LED anode, normally the longer lead. Connect the LED cathode, normally the shorter lead or flat-edged side, to a ground pin.
- Check the LED polarity, resistor placement and ground connection before powering the board. The resistor can be on either side of the LED as long as it is in series.
- Configure the chosen GPIO as an output and start with it low. Set it high only after confirming that the circuit is wired correctly.
The GPIO high level is about 3.3V, so calculate the resistor from the available voltage, the LED’s specified forward voltage and the target current; then select a standard value that keeps the current conservative. Never assume a resistor that is suitable for one LED or supply is automatically suitable for another.
How to control a motor without damaging the Pi
A motor has a high startup current and generates voltage spikes when its winding current changes. A GPIO cannot supply that current or absorb those transients safely. Raspberry Pi’s explicit guidance is: “Do not connect motors directly to the GPIO pins, instead use an H-bridge circuit or a motor controller board.”
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- Use a motor-controller or H-bridge rated for the motor’s voltage and stall current.
- Power the motor from a suitable external supply through the driver, not from a GPIO output.
- Connect a Raspberry Pi GPIO to the driver’s logic input, enable or direction input as required by that driver’s documentation.
- Connect the Pi ground to the driver’s logic ground so both devices share a reference. Keep the motor-current path separate from the GPIO signal path.
- Use the driver’s built-in flyback protection or the protection components specified by its manufacturer.
- Test with the motor unloaded, then monitor for resets, hot components, noise or unexpected GPIO states before applying a mechanical load.
PWM can control speed or power through a driver’s enable input, while separate GPIO lines can select direction. The exact truth table and safe voltage range belong to the driver board, not to the Raspberry Pi header.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Using inputs, pull resistors and switches
A switch input needs a defined state when the contacts are open. Configure an internal pull-up or pull-down where the board and software support it, or add an external resistor sized for the circuit. With a pull-up, wire the switch to ground so an open switch reads high and a closed switch reads low; with a pull-down, wire the switch toward the positive logic rail so the opposite states result.
Always debounce a mechanical switch in software or with suitable hardware. Contact bounce can produce several rapid transitions from one press. Keep switch wiring short, route it away from motor leads, and use a common ground. Never connect a switch that can place 5V directly onto a GPIO input.
Model differences and alternate mappings
The 40-pin mechanical layout is shared by many current boards, but the SoC, boot configuration and enabled peripherals can differ. Alternate functions are multiplexed: assigning a peripheral to a pin can remove it from ordinary GPIO use or conflict with another peripheral. Some boards expose additional routing options, while a Compute Module may have a different carrier-board arrangement.
Do not copy an electrical limit from one family to another. The official documentation separates older BCM2835/BCM2836/BCM2837/RP3A0 products from BCM2711-based 4-series devices, and it directs Compute Module users to the relevant datasheet. Identify the exact model and revision, then verify the active function in software before connecting hardware.
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A safe wiring checklist
- Read the board-specific pinout and run
pinoutin Raspberry Pi OS before wiring. - Choose one numbering convention and use it consistently in both the diagram and code.
- Mark physical pin 1 and orient the header correctly.
- Keep every GPIO signal within the board’s 3.3V logic range; use a level shifter for 5V peripherals.
- Use a series resistor for every LED.
- Use a transistor, H-bridge or motor-controller board for motors, relays and other high-current or inductive loads.
- Connect grounds deliberately and avoid accidental shorts between supply rails.
- Do not treat a GPIO as a general-purpose power supply.
- Power down while changing jumper wires, then inspect for reversed polarity or loose strands.
- After powering up, measure suspicious nodes with a meter and stop if the board resets, a component heats rapidly or a signal is stuck at an unexpected level.
Troubleshooting common mistakes
The program controls the wrong pin
Usually the code uses BCM numbering while the wiring follows physical numbers, or the reverse. Compare the exact number in the code with the table and check the board in pinout. Rewire only after confirming the connector orientation.
An input changes randomly
The input may be floating, missing its pull resistor, or picking up interference from long wires. Enable an appropriate pull-up or pull-down, add an external resistor when required, shorten the wiring and debounce the switch.
An LED does not light
Check that the selected pin is a GPIO rather than power or ground, verify LED polarity, confirm the resistor is in series, and measure the voltage between the GPIO and ground. If the pin was previously exposed to 5V or a short, stop using it until the board has been checked.
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A motor resets the board
The motor may be drawing from the Pi’s supply, injecting noise, or lacking a proper driver and shared ground. Move motor power to an appropriate external supply, use an H-bridge or motor controller, follow its protection requirements and keep high-current wiring away from GPIO lines.
An I2C, SPI or UART device is silent
Confirm the documented pins, peripheral enablement and alternate-function selection for the exact board. Check common ground, logic voltage, signal direction and pull-ups. A pin already assigned to another peripheral may not be available for the interface you selected.
Choosing pins for a project
Start with the peripheral requirement, not with an arbitrary free-looking pin. Reserve GPIO2 and GPIO3 when you need the common I2C bus, GPIO14 and GPIO15 for the common UART mapping, and GPIO7 through GPIO11 for SPI0. Select GPIO12, GPIO13, GPIO18 or GPIO19 when a hardware-PWM channel is important. For a simple button or LED, use another ordinary GPIO and document both its BCM and physical numbers in the project notes.
Leave power and ground pins clearly labeled, keep reserved GPIO0 and GPIO1 for systems that explicitly require them, and record any alternate-function settings. That small amount of documentation prevents the most expensive GPIO mistake: wiring a correct circuit to the wrong contact.
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