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Relay

Introduction​

We know that the GPIO output level of the Walnut Pi development board is 3.3V, which cannot directly control high-voltage devices such as light bulbs (220V). In this case, we can use a commonly used low-voltage control high-voltage module — the relay.

Experiment Objective​

Use a button to control relay on/off.

Experiment Explanation​

The image below shows a commonly used relay module. The left low-voltage control interface mainly includes power pins and signal control pins (supply voltage is generally 3.3V, subject to manufacturer specifications). The right blue section is the high-voltage part, which can connect to 220V appliances.

tip

It is recommended to use a relay with 3.3V level control, because the Walnut Pi GPIO output is 3.3V. Using a 5V-controlled relay improperly may damage the Walnut Pi development board.

relay1

The image below shows the appliance connection diagram — left side is the low-voltage control part, right side is the high-voltage control part (pay attention to electrical safety when wiring):

relay2

The relay operates similarly to the LED in the previous GPIO chapters. Simply connect the module signal pin to the Walnut Pi, then program to control the high/low level changes of the Walnut Pi GPIO pin to control the relay, thereby controlling the high-voltage switch on/off.

digitalio Object​

In CircuitPython, you can directly use the digitalio module to program I/O input for detecting the input level. Details are as follows:

Constructor​

relay=digitalio.DigitalInOut(pin)

Parameter description:

  • pin Development board pin number. Example: board.PB6

Usage​

relay.direction = value

Defines pin as input/output. Value options:

  • digitalio.Direction.INPUT : Input.
  • digitalio.Direction.OUTPUT : Output.

relay.pull = value

Set pull-up/pull-down resistor. Value options:

  • digitalio.Pull.UP : Pull-up.
  • digitalio.Pull.DOWN : Pull-down.

relay.value = value

GPIO output value. Value options:

  • True or 1 : High level.
  • False or 0 : Low level.

For more usage, please refer to the official documentation:
https://docs.circuitpython.org/en/latest/shared-bindings/digitalio/index.html

In this experiment, we use a button to control relay on/off: press once to turn on, press again to turn off, and repeat. The code flow chart is as follows:

Reference Code​

'''
Experiment Name: Relay
Experiment Platform: Walnut Pi 2B
'''

# Import related modules
import board,time
from digitalio import DigitalInOut, Direction, Pull

# Build relay object and initialize
relay = DigitalInOut(board.PB6) # Define pin number
relay.direction = Direction.OUTPUT  # IO as output
relay.value = 1 # Initialize relay to off

# Build key object and initialize
switch = DigitalInOut(board.KEY) # Define pin number
switch.direction = Direction.INPUT # IO as input

state = 1 # Relay initial state, high level means off

while True:

    if switch.value == 0: # Button pressed
        time.sleep(0.01) # Delay 10ms debounce
        if switch.value == 0: # Button pressed
            state = not state # Toggle state
            relay.value = state # Change relay state
            
            # Wait for button release
            while switch.value ==0:
                pass

Experiment Results​

Here we use Thonny to remotely run the above Python code on the Walnut Pi. For instructions on running Python code on the Walnut Pi, please refer to: Running Python Code

relay3

When the sensor detects a person, the blue LED lights up.

relay4

When no person is detected, the blue LED turns off.

relay5