Limit switches

A limit switch reports whether a mechanism has reached a physical location. It can establish a repeatable reference for an encoder or prevent further travel toward a stop. Mounting, wiring, and a measured stopping distance matter as much as the boolean in code.

Sensor reading

For a sensor supported by the SDK's TouchSensor interface, isPressed() supplies the logical pressed state. Configure it as homeSwitch, then save this complete file as LimitSwitchReader.java:

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.LinearOpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.TouchSensor;

@TeleOp(name = "Limit Switch Reader", group = "Practice")
public class LimitSwitchReader extends LinearOpMode {
    @Override
    public void runOpMode() {
        TouchSensor homeSwitch = hardwareMap.get(TouchSensor.class, "homeSwitch");
        waitForStart();
        while (opModeIsActive()) {
            telemetry.addData("Home switch pressed", homeSwitch.isPressed());
            telemetry.update();
            idle();
        }
    }
}

Press and release the switch by hand with mechanism power disabled. Confirm the displayed state matches the physical state before using it to allow movement.

Digital wiring

A raw DigitalChannel reports an electrical high/low state. Set its mode to INPUT and use the actual sensor's wiring specification. Some switches are active-low, so a low electrical state means pressed. That polarity is not universal, and TouchSensor.isPressed() already interprets the state for its driver.

A broken wire can look like a released switch in some circuits. Choose wiring and fault detection deliberately; software cannot assume that false proves the mechanism is free to travel indefinitely. Follow the hub and sensor manufacturer's port/pin instructions.

Homing

A homing routine moves slowly toward a reference, detects the switch, stops, and only then establishes an encoder zero. It needs:

• A verified direction and low speed for that mechanism.
• An active-OpMode guard and a maximum time or travel limit.
• A pressed-state check before beginning movement.
• A defined failure result when the switch does not activate.

Do not set the encoder to zero merely because a timeout expired. That would convert an unknown position into a false known position. If switch bounce affects the result, require a measured period of consistent state without blocking the main loop. Test repeatability, approach direction, and stopping distance with the actual load.

Travel limits

A pressed lower switch should block commands farther downward while allowing a safe move away from it. A second upper switch or calibrated soft limit may restrict the other direction. Soft limits depend on a valid home reference, encoder sign, and counts-to-distance scale; resetting an encoder at an arbitrary location invalidates them.

For mounting and failure testing, see Sensor integration. For responsive state-based behavior, see Nonblocking code.