Understanding different electronic components is crucial for electronics students. This question asks us to identify a specific device based on its function: sensing light levels and controlling current flow between its emitter and collector terminals.

Analysing the Question: Identifying the Light-Sensing Device

The question describes a device that:

1. Senses light levels.
2. Alters the current flowing between emitter and collector.
3. The alteration of current depends on the level of light received.

This description points to a component that uses light to control electrical current flow, specifically within a transistor structure (as indicated by 'emitter' and 'collector').

Evaluating the Options

Let's examine each provided option:

• Option 1: Resistor
  A resistor is a passive component that opposes the flow of current. Its resistance value is generally fixed (though some types are variable or temperature-dependent), and it does not sense light to alter current flow in the way described.
• Option 2: Phototransistor
  A phototransistor is a semiconductor device similar to a regular transistor, but its base current is controlled by light instead of a direct electrical signal. Light striking the base-collector junction generates base current, which in turn controls a larger current flow between the collector and emitter. This perfectly matches the description given in the question.
• Option 3: Power transistor
  A power transistor is designed to handle high currents and voltages. While it is a transistor with emitter and collector terminals, its primary function is switching or amplifying high power signals, not sensing light.
• Option 4: IR LED
  An IR LED (Infrared Light Emitting Diode) is a device that emits infrared light when current flows through it. It is a light source, not a light sensor that controls current based on incoming light.

The Correct Device: The Phototransistor

Based on the analysis, the device that senses light and alters the current between its emitter and collector terminals according to the light level is the phototransistor.

In a phototransistor, the base terminal is often left unconnected or biased differently than a standard transistor. Light photons hitting the sensitive area (typically the base region) create electron-hole pairs, generating a base current proportional to the light intensity. This base current is then amplified, allowing a larger current to flow from the collector to the emitter (or vice-versa, depending on the type and biasing). Thus, the current flow is directly controlled by the amount of light.

Detailed Explanation of Phototransistor Operation

A phototransistor works on the principle of photoconductivity combined with transistor amplification. The structure is similar to a bipolar junction transistor (BJT), typically NPN or PNP. The key difference is how the base current is generated.

In a standard BJT, a current injected into the base controls the collector current (\(I_C = \beta I_B\)). In a phototransistor, light acts as the input. When light shines on the base region, it generates charge carriers. This light-induced current acts as the base current (\(I_{\text{light}}\)). The collector current becomes \(I_C \approx \beta I_{\text{light}}\).

This means the collector current is proportional to the intensity of the incident light, amplified by the transistor's current gain (\(\beta\)).

Applications of Phototransistors

Phototransistors are used in various applications where light detection and conversion into an electrical signal are required, such as:

• Light sensors in electronic circuits
• Automatic lighting control systems
• Optical switches and counters
• Remote controls (as detectors)
• Light-activated relays

Conclusion

The device that senses light levels and alters the current flowing between its emitter and collector based on the light received is the phototransistor. This unique characteristic makes it ideal for light-sensing applications.

Revision Table: Key Electronic Components

Additional Information on Phototransistors and Light Sensors

Phototransistors are part of a larger family of photosensitive semiconductor devices. Other devices include photoresistors (LDR - Light Dependent Resistor), photodiodes, and photo Darlington transistors.

• Photoresistor (LDR): Resistance changes with light intensity. Higher light means lower resistance. Simple but slower response than photodiodes or phototransistors.
• Photodiode: Operates in reverse bias. Light striking the junction creates current flow proportional to light intensity. Faster than phototransistors but typically produces a smaller current, often requiring external amplification.
• Phototransistor: Combines photodiode (base-collector junction) sensitivity with transistor amplification. Provides higher current output than a photodiode for the same light level.
• Photo Darlington: Two transistors connected in a Darlington configuration, where the first transistor is a phototransistor. Offers very high current gain and sensitivity to light, suitable for detecting very low light levels.

Understanding the differences helps in selecting the appropriate light sensor for a specific application requirement.