Lesson 11.1: Reading Touch Sensor Binary States via DigitalChannel

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Why Mechanisms Need Limit Switches

A motor that does not know where it is will keep driving until something stops it physically. For a linear slide or a rotating arm, that physical stop is the robot's own frame, and the moment the mechanism hits it, the motor is stalling against a hard wall. Stall current is the highest current a motor can draw, and sustained stall causes heat buildup, gear damage, and can trip the protection circuitry on the Control Hub port. A touch sensor mounted at the mechanism's physical limit solves this problem before it becomes a hardware failure.

The principle is simple: the robot checks the sensor before applying power in a dangerous direction. If the sensor says the mechanism has reached its limit, the code blocks movement in that direction even if the driver is still holding the joystick.

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How the SDK Represents a Digital Signal

The DigitalChannel class is the SDK's interface for any sensor that operates on a binary electrical signal. The pin is either HIGH (the circuit is open) or LOW (the circuit is closed). For the REV Touch Sensor, pressing the button shorts the internal circuit to ground, pulling the signal LOW.

This creates what engineers call active-low logic: getState() returns true when the sensor is not pressed (circuit open, pin pulled HIGH by a resistor), and false when it is pressed (circuit closed, pin driven LOW). Because this is the inverse of what most programmers expect, the standard practice is to invert the result using the ! operator. Wrapping the call as !limitSwitch.getState() gives you a boolean where true means "the sensor is physically pressed," which is far easier to reason about inside an if statement.

One additional required step separates digital sensors from analog ones: the digital pin on the Control Hub is bidirectional. It can either send a signal as an output or receive one as an input. You must declare the intended direction by calling setMode(DigitalChannel.Mode.INPUT) immediately after mapping the sensor. Skipping this step can leave the pin misconfigured, causing the sensor to appear frozen at a single value.

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Annotated Code

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DigitalChannel;

@TeleOp(name="Touch_Sensor_Demo")
public class TouchSensorDemo extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor        liftMotor    = hardwareMap.get(DcMotor.class,     "lift_motor");
        DigitalChannel limitSwitch  = hardwareMap.get(DigitalChannel.class, "limit_bottom");

        // Required: configure the pin direction before the match starts
        limitSwitch.setMode(DigitalChannel.Mode.INPUT);
        liftMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        while (opModeIsActive()) {
            double power = -gamepad1.left_stick_y;

            // Active-low: getState() is false when pressed, so ! gives us
            // a true "is pressed" boolean that reads naturally in code
            boolean isAtBottom = !limitSwitch.getState();

            // Block downward movement if the bottom limit has been reached
            if (isAtBottom && power < 0) {
                power = 0.0;
                telemetry.addData("Safety", "BOTTOM LIMIT REACHED");
            }

            liftMotor.setPower(power);
            telemetry.addData("At Bottom", isAtBottom);
            telemetry.addData("Power",     power);
            telemetry.update();
        }
    }
}

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Fill-in-the-Blank Practice

1. To configure a digital sensor pin to receive incoming signals, you must call sensor.setMode(DigitalChannel.Mode.__________); during initialization.
2. The REV Touch Sensor uses active-low logic, meaning getState() returns __________ when the button is physically pressed.
3. To obtain an intuitive "is pressed" boolean from a touch sensor, you apply the __________ operator to the getState() return value.

Show answers

1. INPUT
2. false
3. ! (logical NOT)

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Template Challenge

Robot Scenario: You are programming a lift mechanism. The lift should allow upward movement freely, but the motor must be set to zero power the moment the bottom touch sensor named "limit" is triggered and the driver commands downward movement.

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DigitalChannel;

@TeleOp(name="Safety_Lift_Challenge")
public class SafetyLift extends LinearOpMode {

    @Override
    public void runOpMode() {
        DcMotor        lift  = hardwareMap.get(DcMotor.class,     "lift");
        DigitalChannel limit = hardwareMap.get(DigitalChannel.class, "limit");

        // INSERT CODE HERE: Set limit to INPUT mode
        lift.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        waitForStart();

        while (opModeIsActive()) {
            double power = -gamepad1.left_stick_y;

            // INSERT CODE HERE: Read limit and invert for active-low logic
            boolean pressed = // INSERT VALUE HERE

            // INSERT CODE HERE: If pressed and power is negative (downward), set power to 0
            
            lift.setPower(power);
            telemetry.addData("Limit Pressed", pressed);
            telemetry.addData("Power",         power);
            telemetry.update();
        }
    }
}

Show answer

limit.setMode(DigitalChannel.Mode.INPUT);

// Inside the while loop:
boolean pressed = !limit.getState();

if (pressed && power < 0) {
    power = 0.0;
}

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