Signal Input

What Is a Photoelectric Sensor?

Before building a control circuit with relays, let's first confirm how to use sensor signals to operate a relay.

We will be using a photoelectric sensor. The sensor emits light, and when that light is interrupted, the sensor responds and outputs a signal. This signal is then used to activate the relay. Photoelectric sensors come in two types: through-beam and reflective.

Through-Beam Sensor

A through-beam sensor is a paired sensor with a light emitter on one side and a light receiver on the other. When an object interrupts the beam between the two units, the receiver can no longer detect the light, and the sensor responds.

By installing sensors in a passage or aisle, the sensor will respond every time a person or object passes through.

Reflective Sensor

A reflective sensor has the light emitter and receiver integrated in one unit. Light emitted by the sensor hits an object in front of it and reflects back to the sensor, causing it to respond.

Dark-ON and Light-ON

Sensors have two operating modes: "Dark-ON (D/O)" and "Light-ON (L/O)." Some sensors allow switching between these modes.

• Dark-ON: The sensor output turns ON when the receiver is no longer receiving light (becomes dark). For a through-beam sensor, this means ON when the beam is blocked; for a reflective sensor, ON when there is no object in front.
• Light-ON: The sensor output turns ON when the receiver is receiving light (is bright). This is the opposite of Dark-ON.

Note that the ON/OFF conditions are also reversed depending on whether you use a through-beam or reflective sensor, so be careful when selecting and wiring.

Sensor Wiring Method

Now that you understand sensor basics, let's explain the actual wiring method. Color-coded wires extend from the sensor.

• Brown: DC+ side (connect to DC 24V+ in industrial applications)
• Blue: DC− side (connect to DC 24V−)
• Black: Signal wire (drops to minus when the sensor responds)

Brown and blue are power supply wires, so multiple sensors can share the same power supply (common plus). Connect the black signal wire to the minus side of the relay — when the sensor responds, the relay will operate.

※ On older equipment, wire colors may differ. Refer to: Red → Brown (DC+), Black → Blue (DC−), White → Black (signal wire).

Next, let's also confirm the connection for a cylinder sensor. A magnet is built into the piston inside the cylinder. When the cylinder moves, a sensor mounted on the cylinder body responds to the magnet and outputs a signal, allowing detection of the cylinder's extended and retracted positions.

If you choose a 2-wire type cylinder sensor, it can be connected simply by inserting it in series between the relay and the power supply, just like a switch. Also, the type explained here is the NPN type. Since most Japanese industrial equipment uses NPN, when you need to choose a type, NPN is the safe choice.

Now that you have mastered sensor basics, let's move on to relay control.

Frequently Asked Questions (FAQ)