Tripwire

Two robots work together. One stands still as a tripwire and, when the other passes, cues it to do a trick.

Factories put sensors beside conveyor belts: when a box passes, the next machine is told to act. Shops use beams at the door to tell a speaker to chime. One device notices, another acts.

An optional challenge: two robots working as a team. One robot stands beside the track as a tripwire: its distance sensor watches the line. The other robot patrols. When the patrol robot passes in front of the tripwire, the tripwire sends a cue by radio, and the patrol robot does a trick.

Both robots run the same program. Press A+B on a micro:bit to make its robot the tripwire, then drag that robot beside the line, facing it. The simulator's distance sensor sees other robots as well as boxes.

Predict: Read the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.
Run: Run it and compare the result with your prediction.
Investigate: Trace how each line of code connects to what you see.
Modify: Change the code on purpose and check the result.
Make: Build something new using what you have learned. Expect a few tries; that is normal.

Explain it to me

A tripwire notices; another robot acts.

Arm it, fire once, disarm: a true/false variable.
The robot that hears the cue sets its mode; forever does the trick, then goes back to patrolling.

Show as code

function followLine () {
    if (robot.detectLine(RobotLineDetector.Left) && robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 40)
    } else if (robot.detectLine(RobotLineDetector.Left)) {
        robot.motorTank(10, 40)
    } else if (robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 10)
    } else {
        robot.motorStop()
    }
}
input.onButtonPressed(Button.AB, function () {
    role = "tripwire"
    robot.motorStop()
    basic.showIcon(IconNames.Target)
})
let role = ""
robot.elecfreaksCuteBot.start()
radio.setGroup(1)
role = "robot"
basic.forever(function () {
    if (role == "tripwire") {
        if (robot.obstacleDistance() < 20) {
            basic.showIcon(IconNames.Surprised)
        } else {
            basic.showIcon(IconNames.Target)
        }
    } else {
        followLine()
    }
})

The example, as blocks

PredictRead the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.

Predict

Show as code

tripwire.ts

function followLine () {
    if (robot.detectLine(RobotLineDetector.Left) && robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 40)
    } else if (robot.detectLine(RobotLineDetector.Left)) {
        robot.motorTank(10, 40)
    } else if (robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 10)
    } else {
        robot.motorStop()
    }
}
input.onButtonPressed(Button.AB, function () {
    role = "tripwire"
    robot.motorStop()
    basic.showIcon(IconNames.Target)
})
let role = ""
robot.elecfreaksCuteBot.start()
radio.setGroup(1)
role = "robot"
basic.forever(function () {
    if (role == "tripwire") {
        if (robot.obstacleDistance() < 20) {
            basic.showIcon(IconNames.Surprised)
        } else {
            basic.showIcon(IconNames.Target)
        }
    } else {
        followLine()
    }
})

The second robot is the tripwire, beside the line and facing it. What does the tripwire's micro:bit show as the first robot passes?A surprised face that stays for everA surprised face while the robot is in front, then the target againThe first robot stops at the tripwireNothing: a tripwire cannot see robots

RunRun it and compare the result with your prediction.

Run

Press A+B on the second micro:bit. Drag its robot next to the line, pointing at it, about a hand's width away in the simulator. Watch its micro:bit as the first robot goes past.

Show as code

tripwire.ts

function followLine () {
    if (robot.detectLine(RobotLineDetector.Left) && robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 40)
    } else if (robot.detectLine(RobotLineDetector.Left)) {
        robot.motorTank(10, 40)
    } else if (robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 10)
    } else {
        robot.motorStop()
    }
}
input.onButtonPressed(Button.AB, function () {
    role = "tripwire"
    robot.motorStop()
    basic.showIcon(IconNames.Target)
})
let role = ""
robot.elecfreaksCuteBot.start()
radio.setGroup(1)
role = "robot"
basic.forever(function () {
    if (role == "tripwire") {
        if (robot.obstacleDistance() < 20) {
            basic.showIcon(IconNames.Surprised)
        } else {
            basic.showIcon(IconNames.Target)
        }
    } else {
        followLine()
    }
})

Try it yourselfYour edits are saved in this browser.

It opens full screen, so the robot simulator has room, and takes a few seconds to load. Esc comes back. Your work in this step is kept.

I made one robot the tripwire, put it beside the line, and watched it notice the other robot.

InvestigateTrace how each line of code connects to what you see.

Investigate

Show as code

tripwire.ts

function followLine () {
    if (robot.detectLine(RobotLineDetector.Left) && robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 40)
    } else if (robot.detectLine(RobotLineDetector.Left)) {
        robot.motorTank(10, 40)
    } else if (robot.detectLine(RobotLineDetector.Right)) {
        robot.motorTank(40, 10)
    } else {
        robot.motorStop()
    }
}
input.onButtonPressed(Button.AB, function () {
    role = "tripwire"
    robot.motorStop()
    basic.showIcon(IconNames.Target)
})
let role = ""
robot.elecfreaksCuteBot.start()
radio.setGroup(1)
role = "robot"
basic.forever(function () {
    if (role == "tripwire") {
        if (robot.obstacleDistance() < 20) {
            basic.showIcon(IconNames.Surprised)
        } else {
            basic.showIcon(IconNames.Target)
        }
    } else {
        followLine()
    }
})

Ivy's tripwire, in forever:

Show as code

basic.forever(function () {
    if (role == "tripwire") {
        if (robot.obstacleDistance() < 20) {
            radio.sendString("trick")
        }
    }
})

Think about what forever is doing while the message arrives.

Ivy's tripwire sends 'trick' whenever something is closer than 20 cm. Her patrol robot starts its trick again and again as it passes. Why?Tripwires only work onceThe robot is in front of the tripwire for many times round forever, so 'trick' is sent many timesThe trick is too longHer radio group is wrong

When the patrol robot hears 'trick', why should the handler only set mode to trick, instead of doing the moves itself?The handler runs too lateIt saves batteryHandlers cannot use motor blocksforever is calling followLine again and again, which would undo the trick's moves straight away

ModifyChange the code on purpose and check the result.

Milestone 1: arm and fire

Turn the tripwire into a one-shot trigger:

Pressing A on the tripwire arms it (show a tick).
When something passes while it is armed, it sends "trick", disarms itself, and shows a cross.

Try it yourselfYour edits are saved in this browser.

It opens full screen, so the robot simulator has room, and takes a few seconds to load. Esc comes back. Your work in this step is kept.

Need a hint?

Make a variable armed. On button A: if role = tripwire, set armed to true and show a tick.

Another hint

In the tripwire's part of forever: if armed and distance < 20, set armed to false, send the word trick, and show a cross.

A arms the tripwire; when the robot passes, it sends 'trick' once and disarms.

This ticks by itself when your blocks do what is asked.

ModifyChange the code on purpose and check the result.

Milestone 2: the trick

Make the patrol robot do a trick when it is cued: lights on, a quick wiggle, lights off, then back to patrolling. Keep the trick short and finish facing the same way, so the robot can find the line again.

Try it yourselfYour edits are saved in this browser.

It opens full screen, so the robot simulator has room, and takes a few seconds to load. Esc comes back. Your work in this step is kept.

Need a hint?

Make a variable mode, set to patrol in on start. on radio received: if role = robot and the word is trick, set mode to trick.

Another hint

In the robot's part of forever: if mode = trick, do the trick (lights and a wiggle), then set mode back to patrol; else follow the line.

When the patrol robot hears 'trick', it does a trick with its lights, then carries on patrolling.

This ticks by itself when your blocks do what is asked.

MakeBuild something new using what you have learned. Expect a few tries; that is normal.

Milestone 3: a self-resetting tripwire

Make the team run by itself:

The tripwire counts the passes and shows the count.
It re-arms itself a few seconds after it fires, ready for the next lap.
The trick plays a sound as well.

Try it yourselfYour edits are saved in this browser.

It opens full screen, so the robot simulator has room, and takes a few seconds to load. Esc comes back. Your work in this step is kept.

Need a hint?

After the tripwire fires: count the pass (a variable passes, change by 1), show it, pause 5 seconds, then arm again.

Another hint

Add a sound to the trick with play tone … in background, so the robot keeps moving.

The tripwire counts the passes, shows the count and re-arms itself; the trick plays a sound.

This ticks by itself when your blocks do what is asked.

Info

On a real robot

With real robots, the tripwire can be a robot or just a micro:bit with an ultrasonic sensor. Place it a short distance from the track, facing across it. Real readings jump about, so you may need a range (for example between 5 and 20 cm) to ignore false readings near zero.

For teachers

An optional challenge for students who finish early. Two robots work as a team; one stands still as a tripwire and, armed with a true/false variable, cues the patrolling robot once per pass; the patrol robot does a trick through its mode, then carries on. About 50 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can turn a sensor into a one-shot trigger with a true/false variable.
I can make one robot cue another by radio.
I can design two devices that work together as a system.

Before class

Optional: the high ceiling of the course, not needed for Challenge complete. It is based on a common sensor pattern: one device notices, another acts. The tripwire is a robot that has been switched (A+B) to watch instead of drive; its distance sensor sees the other robot (the simulator treats robots as obstacles). Students drag the tripwire robot beside the line, facing it.

Two ideas from earlier come back: fire once (armed, like alerted in lesson 9) and handlers set the mode, forever does the job (lessons 6 and 11). The trick is a wiggle so the robot finishes facing along the line and can find it again; a full spin may leave it off the line.

Real ultrasonic sensors give false readings near zero, so a real tripwire often checks a range (for example 5 to 20 cm).

Running the lesson

Predict and Run: placing the tripwire is fiddly. Show it once on the projector.
Investigate: Ivy's tripwire is lesson 9's repeat-alert slip; the second question is the mode rule from Patrol and alert.
Milestone 1: arm with A, fire once, disarm.
Milestone 2: the trick lives in forever, chosen by mode.
Milestone 3: re-arming after a pause makes it work lap after lap.

The hard part (on purpose)

Firing once per pass with arm and disarm, and the trick in forever. Ask when the tripwire is armed and where the trick's moves are.

When students get stuck

The tripwire never fires. Ask: Is it armed? Is it facing the line, close enough? Fix: Press A on the tripwire; drag it nearer and point it at the line.
The trick is cut short or never shows. Ask: Where are the trick's moves: in the handler or in forever? Fix: Handler: set mode to trick. forever: if mode = trick, do it, then set mode to patrol.
After the trick the robot stops for good. Ask: Which way is it facing after the trick? Fix: Make the trick end facing the same way (a wiggle), or drag it back onto the line.

Stretch and support

Stretch: Two tripwires at different places on the track, each sending a different word, and a different trick for each.

Support: Give Milestone 1 as a worked example to read; students build Milestones 2 and 3.

Where it leads

Real-world links: conveyor-belt sensors, shop door chimes, lap timers at races, automatic doors.

What each step is for

Investigate: A tripwire must fire once per pass. Arming and disarming with a true/false variable gives one cue per pass; the robot that gets the cue does its trick in forever, not in the handler.
Milestone 1: arm and fire: Arm, fire once, disarm: a true/false variable turns a sensor into a one-shot trigger.
Milestone 2: the trick: The handler changes the mode; forever does the trick, then goes back to patrolling.
Milestone 3: a self-resetting tripwire: Two devices, each with its own job and memory, working together over radio: that is a small system.

Now you can

Turn a sensor into a one-shot trigger with a true/false variable.
Make one robot cue another by radio.
Design two devices that work together as a system.

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Source: content/code/robotics/robot-tripwire/lesson.md