Pilot Devices and Control Circuits — Electrical

Objective. Students connect and test a variety of pilot devices—limit switches, pressure switches, float switches, and photoelectric sensors—as discrete inputs that control a motor starter coil.

What Are Pilot Devices?

A pilot device is any component that senses a physical condition—position, pressure, liquid level, or light—and opens or closes a set of electrical contacts in response. Those contacts sit inside a control circuit and tell a motor starter coil when to energize or de-energize. The motor itself never runs through the pilot device; only low-voltage control current does. That distinction keeps the sensing device small, inexpensive, and safe to handle.

Four pilot devices appear in most industrial and commercial motor applications: limit switches, pressure switches, float switches, and photoelectric sensors. Each converts a different physical signal into the same electrical language—open contact or closed contact.

Limit Switches

A limit switch has a mechanical actuator (roller, plunger, or lever arm) that moves when an object contacts it. Conveyors, machine slides, and garage doors all use limit switches to detect position. The switch body contains normally open (NO) and normally closed (NC) contacts. When the actuator is at rest, NO contacts are open and NC contacts are closed. When the actuator is depressed or tripped, NO contacts close and NC contacts open.

Contact selection depends on the application's fail-safe requirement. A conveyor stop circuit often uses a NC limit switch wired in series with the starter coil so that losing the wire or the switch itself de-energizes the coil and stops the motor—a safe failure mode.

Pressure Switches

A pressure switch responds to fluid or air pressure. Inside, a diaphragm or piston moves against a spring. When system pressure rises above the set point, the mechanism shifts and changes contact state. Air compressors, hydraulic presses, and pump systems commonly use pressure switches.

A compressor motor, for example, uses a NC pressure switch wired in series with the starter coil. While pressure is low, contacts are closed and the motor runs. When pressure reaches the cut-out setting, contacts open and the motor stops. If the switch fails open, the motor stops—another safe failure mode.

Float Switches

A float switch monitors liquid level. A buoyant float rises and falls with the liquid surface and mechanically actuates a set of contacts. Sump pumps, cooling towers, and sewage lift stations all rely on float switches.

Contact selection is critical here. A sump pump must run when liquid is high and stop when liquid is low. The correct choice is a normally open float switch wired in series with the starter coil. When liquid is low, the float is down and contacts are open—motor is off. When liquid rises, the float rises, contacts close, and the motor starts. If wired with a NC contact instead, the motor would run continuously and only stop when liquid was high—the opposite of the intended behavior.

Photoelectric Sensors

A photoelectric sensor uses a light beam (visible or infrared) to detect the presence or absence of an object. In a retroreflective arrangement, the sensor and a reflector face each other. The sensor transmits a beam; the reflector bounces it back. When an object interrupts the beam, the sensor's output changes state.

Photoelectric sensors produce either a discrete dry-contact output or a solid-state transistor output. When wired into a relay-logic control circuit, the dry-contact type is simplest. Choose NO contacts to energize the coil when an object is detected; choose NC contacts to de-energize the coil (stop the motor) when an object is detected—for instance, a safety gate that stops a conveyor when a worker enters a hazard zone.

NO vs. NC: The Decision Rule

Ask two questions for any application:

1. What should happen when the device is at rest (unactuated)? If the motor should be off at rest, use NO contacts. If the motor should run at rest, use NC contacts.

2. What is the safe failure mode? Broken wire, failed sensor, or lost power should put the machine in a safe state. NC contacts wired in series with the coil ensure that any open-circuit fault stops the motor.

Control Circuit Safety

Before wiring any station:

Lockout/tagout (LOTO) the training panel following your site's written procedure. Verify the circuit is de-energized with a meter before touching terminals.
Wear safety glasses whenever conductors are under tension or terminals are being tightened.
Keep control circuit voltage at the level specified by the trainer (typically 120 V AC or 24 V DC). Never exceed the rated voltage of the pilot device.
Do not bypass the overload relay in the motor starter to make a circuit "work." The overload protects the motor; removing it is a code violation and a fire hazard.
When testing, keep hands clear of rotating parts and mechanical actuators before energizing.

Verifying the Circuit

After wiring and before energizing, use a multimeter set to continuity or resistance mode to trace the control circuit path. Confirm that actuating the pilot device produces the expected contact state change. Only then restore power and observe motor response. If the motor does not respond correctly, de-energize, apply LOTO, and re-check wiring against the diagram before touching any terminal.

Key terms9

Pilot deviceA sensing component that opens or closes low-voltage control circuit contacts in response to a physical condition such as position, pressure, liquid level, or light.

Normally open (NO) contactA contact pair that is open (circuit broken) when the device is at rest and closes when the device is actuated.

Normally closed (NC) contactA contact pair that is closed (circuit complete) when the device is at rest and opens when the device is actuated.

Limit switchA pilot device with a mechanical actuator that changes contact state when a moving object physically trips it; used to detect position.

Pressure switchA pilot device that changes contact state when fluid or air pressure crosses a set point; commonly used to start or stop compressor and pump motors.

Float switchA pilot device whose contacts are actuated by a buoyant float that rises and falls with liquid level; used in sump pump and liquid-level control applications.

Photoelectric sensorA pilot device that uses a transmitted light beam and a receiver or reflector to detect the presence or absence of an object, producing a discrete contact or transistor output.

Motor starter coilThe electromagnetic coil inside a contactor or motor starter that, when energized by the control circuit, closes the power contacts to connect the motor to line voltage.

Fail-safe wiringA wiring strategy in which any open-circuit fault—broken wire, failed device, or lost power—drives the system to a safe (usually stopped) state.

In class: Instructor demonstrates wiring each pilot device type into a control circuit, explaining NO vs. NC contact selection based on the application (e.g., normally open float switch for pump motor). · Students rotate through four stations (one device type each), wiring the pilot device into a control circuit trainer and verifying correct motor start/stop response by actuating the device. · Application matching exercise: students receive five real-world motor control scenarios and select the correct pilot device type and contact configuration, justifying their choice in writing.
Materials: Control circuit trainers (one per station); limit switch; pressure switch (pneumatic simulator); float switch; photoelectric sensor and reflector; NCCER Electrical Level 2 motor controls module; wiring materials; multimeters
Check: Instructor observes each station; motor must respond correctly to pilot device actuation; application matching exercise scored at 80% minimum (4 of 5 correct).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.