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A phototransistor is a light-sensitive semiconductor device, a transistor that detects light and converts it into an electrical signal, amplifying it in the process. When light strikes the phototransistor's base (B), it allows current to flow more easily between the collector and emitter. Essentially, the more light that strikes the device, the greater the current flow.

Phototransistors are widely used in applications like light sensors, optical switches, remote controls and even in some safety devices. They're highly efficient for detecting and responding to varying light intensities.

A phototransistor light sensor (seen above) works by detecting light and converting it into an electrical signal, which triggers or controls other components in a circuit.

Power Supply: The circuit is powered by a 9V battery. When the switch is closed (ON), the circuit becomes active.

Light Detection: The phototransistor is the component that detects light. When light of sufficient intensity (e.g., 698 lux in this case) strikes the phototransistor, it allows current to flow through it.

Transistor Activation: Voltage can now flow to the base of the regular transistor, turning it on, allowing current to flow from its collector to its emitter.

Relay Activation: The current flowing through the regular transistor energises the relay coil. This causes the relay to activate and close its contacts.

Output Control: The closing of the relay contacts, can be used to control other devices or circuits, making the system light-sensitive. For example, it could turn on a light or activate an alarm, when the light intensity reaches a certain threshold.

Variable Resistor / Preset Resistor: Altering the resistance by turning the variable resistor up of down, increases or decreases the sensitivity of the phototransistor.

This type of circuit is commonly used in automatic lighting systems, security systems, and other applications where light detection is needed.

PCB LAYOUT PRODUCED WITH CIRCUIT WIZARD SOFTWARE
(please note the fixed resitor has been replaced with a prreset resistor)

The difference between a light-dependent resistor (LDR) and a phototransistor, is seen in their structure, the way they work, their sensitivity and practical applications.

Structure
An LDR is a passive component made of a photoconductive material like cadmium sulfide. Its resistance changes based on the intensity of light it receives.
A Phototransistor is a semiconductor device that functions like a transistor, but is triggered by light instead of an electric current.

Working Principle
As light intensity increases, the resistance of an LDR decreases.
Light striking the phototransistor's base generates charge, causing current to flow between the collector and emitter, with amplification occurring in the process.

Sensitivity
An LDR is less sensitive than a phototransistor and slower in response time. It detects changes in light levels, but cannot amplify signals.
A phototransistor is more sensitive and capable of amplifying the electrical signal generated by light. It also has a faster response time.

Output
An LDR produces a change in resistance that can be measured in a circuit.
A phototransistor produces a current or voltage output, depending on the circuit design.

Practical Applications
LDRs are used in applications like streetlights, light-activated alarms and devices requiring simple light sensing.
Phototransistors are found in more advanced applications, such as optical communication, infrared receivers and light-based switches.

Phototransistors are incredibly versatile and are used in a variety of applications.

Light sensors: Phototransistors are commonly used in devices that need to detect the presence or intensity of light, such as automatic lighting systems or solar tracking equipment.

Infrared receivers: In devices like remote controls, phototransistors detect infrared signals emitted by the transmitter to control functions on TVs, DVD players, and more.

Optical switches: They are used in optoelectronic circuits to trigger actions based on light changes. For example, they can serve as a sensor to count objects on a conveyor belt in industrial settings.

Safety devices: Some security systems use phototransistors to monitor light beams. If the beam is interrupted, the phototransistor detects the change and triggers an alarm.

Communication systems: Phototransistors are integral to fibre optic communication, where light is transmitted through cables and detected by the phototransistor.