Lesson 5 of 5

A ROS 2 drive node with an e-stop

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# Publish /cmd_vel at 10 Hz until the e-stop is pressed import rclpy from rclpy.node import Node from geometry_msgs.msg import Twist from gpiozero import Button, LED class Drive(Node): def __init__(self): super().__init__('drive') self.pub = self.create_publisher(Twist, 'cmd_vel', 10) self.timer = self.create_timer(0.1, self.tick) # 0.1 s period: 10 Hz self.estop = Button(27) # GPIO27, pressed joins it to GND self.estop.when_pressed = self.halt self.status = LED(17) self.stopped = False self.count = 0 def tick(self): msg = Twist() if not self.stopped: msg.linear.x = 0.2 msg.angular.z = 0.5 self.pub.publish(msg) self.status.value = 0 if self.stopped else 1 self.count += 1 if self.count % 10 == 0: self.get_logger().info(f'sent {self.count} commands, linear.x={msg.linear.x:.2f}') def halt(self): if self.stopped: return self.stopped = True # stays stopped until the node restarts self.status.off() self.get_logger().warn('e-stop pressed: stopping') def main(): rclpy.init() node = Drive() try: rclpy.spin(node) except KeyboardInterrupt: pass finally: node.destroy_node() rclpy.try_shutdown() if __name__ == '__main__': main()

Stopped · Your code isn't running yet. Until you press Run, the board runs the program already on it.

Nothing printed yet. Press Run.

Turn the Pi into a robot's brain: a ROS 2 node publishes velocity commands on /cmd_vel ten times a second, and a button on GPIO27 stops it for good.

A publisher on a timer
gpiozero inside a ROS 2 node
A latching software e-stop

Pin	Connects to	What it does
`GPIO27`	E-stop button, other side to GND	Digital input with pull-up
`GPIO17`	330 Ω resistor and status LED	Digital output
`GND`	Button and LED	Return

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 3.3 V
Anode (long leg) to GPIO17 on pin 11 through a 330 Ω resistor, cathode to GND on pin 9: about 4 mA from 3.3 V. gpiozero calls it LED(17).
Part pin Board pin
A GPIO17Wire colour orange
K GNDWire colour black
Careful
Never connect an LED without its resistor: the pin would try to source far more than 16 mA.
Push buttonSupply 3.3 V
One leg to GPIO27 on pin 13, the other to GND on pin 14. gpiozero's Button turns on the internal pull-up, so no resistor is needed: the pin reads high until you press.
Part pin Board pin
1 GPIO27Wire colour yellow
3 GNDWire colour black

All 7 parts on the Raspberry Pi 5 / Raspberry Pi 4 page

Connects to

What it does

GPIO27

E-stop button, other side to GND

Digital input with pull-up

GPIO17

330 Ω resistor and status LED

Digital output

GND

Button and LED

Return

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 3.3 V

Anode (long leg) to GPIO17 on pin 11 through a 330 Ω resistor, cathode to GND on pin 9: about 4 mA from 3.3 V. gpiozero calls it LED(17).

Part pin	Board pin
A	GPIO17Wire colour orange
K	GNDWire colour black

Careful

Never connect an LED without its resistor: the pin would try to source far more than 16 mA.

Push buttonSupply 3.3 V

One leg to GPIO27 on pin 13, the other to GND on pin 14. gpiozero's Button turns on the internal pull-up, so no resistor is needed: the pin reads high until you press.

Part pin	Board pin
1	GPIO27Wire colour yellow
3	GNDWire colour black

All 7 parts on the Raspberry Pi 5 / Raspberry Pi 4 page

A ROS 2 drive node with an e-stop

Try it yourself

Serial monitor

Checks

Break it

Inputs

Parameters

Wire it in

A ROS 2 drive node with an e-stop

Try it yourself

Serial monitor

Checks

Break it

Inputs

Parameters

Wire it in