Basic Introduction to Photocell

Catalog

Ⅰ What is a Photocell?

Photocells, also known as photoresistors or LDRs, are a type of light-sensitive resistor, but the term can also include photovoltaic cells that generate electricity from light. Elprocus (2019)

A photocell can be described as a module that is light-sensitive. In a wide variety of uses, such as sunset to sunrise illumination, this may be used by connecting to an electrical or electronic circuit that mechanically turns on when light intensity is low. These are also found in other applications, such as automatic doors and intrusion detectors.

photocell

Photocells are lightweight, cheap, low-power, user-friendly, and do not wear out. They also feature dolls, gadgets, and appliances for that purpose. CdS cells (made of Cadmium-Sulfide), light-dependent resistors (LDR), and photoresistors are commonly referred to.

For most light-sensitive applications such as "is it bright or dark out," "is there anything in front of the sensor (that will block light)," "is there anything interrupting a laser beam" (break-beam sensors), or "which is the most light entering it from various sensors," photocells would be a strong option!

One type of sensor is the photocell, which can be used to help you to detect light. These are very thin, low-power, economical, very easy to use, and are the key features of photo-cells. For these purposes, they are widely found in gadgets, games, and appliances. Cadmium-Sulfide (CdS) cells are often referred to as these sensors. Photoresistors and LDRs are made up of these.

These sensors are ideal for devices that are light-sensitive, such as light that would otherwise blackout. If the block light is in front of the sensor if there is something that disturbs a laser light, sensors that touch much of the light.

Ⅱ Working of Photocells 

An evacuated glass tube that contains two electrodes such as the collector and emitter can be used to create a Photocell. The shape of the terminal of the emitter will take the form of a semi-hollow cylinder. At a negative potential, it is still planned. The outline of the terminal of the collector can be in the form of a metal that can be arranged at the partly cylindrical emitter axis. This can be maintained consistently at a positive terminal. It is possible to patch the evacuated glass tube over a non-metallic base & pins are provided for external attachment at the base.

photocell’s internal structure

A photocell's working theory will depend on the phenomenon of electrical resistance & the photoelectric effect. This can be used to transform electrical energy into light energy.

The positive (+ve) terminal of a battery is connected while the emitter terminal is connected to the negative (-ve) terminal & collector terminal. The frequency of radiation in the emitter will be higher than the threshold frequency of the material, and so photo ton leakage will occur. The photon electrons are engaged in the collector's path. Here, with respect to the emitter terminal, the collector terminal is the positive terminal. The movement of current inside the circuit would therefore be there. If the strength of radiation is improved, then the photoelectric current is increased.

The photoelectric effect occurs when photons strike a semiconductor, freeing electrons to flow, reducing resistance and converting light energy into electrical energy. For example, in a photoresistor, resistance can drop from several megaohms in the dark to a few hundred ohms in bright light, as noted in scientific literature. ScienceDirect (n.d.) for detailed explanations, "A photocell is a light-to-electrical transducer, with resistance decreasing as light intensity increases, sensitive to wavelengths 400-700 nm, similar to the human eye."

Ⅲ Circuit Diagram of Photocells 

The photocell used in the circuit is otherwise called the transistor switched circuit as a dark sensing circuit. Breadboard, jumper wires, battery-9V, transistor 2N222A, photocell, resistors-22 kilo-ohm, 47 ohms, and LEDs are the necessary components to construct the circuit.

This circuit uses the photocell to detect darkness, triggering the transistor to switch on the LED, useful for automatic night lights. In light, the photocell’s low resistance keeps the LED off.

In two conditions, such as when there is light and when it is dark, the above photocell circuit runs.

The photocell resistance is smaller in the first example, and then there will be a current flow into the second resistor, such as 22Kilo Ohms & photocell. The transistor 2N222A here acts as an insulator. The lane that contains the LED1, R1 & transistor would also be off.

The resistance of the photocell is high in the second example, and then the circuit lane will change. So the low resistance to the base of the transistor or through the photocell would be there.

Whenever the transistor's base terminal gets power, then the transistor 2N222A works like a conductor. ON will be the lane including the Lead, R1 & 2N222A resistor, and the LED will blink. But, if the transistor's base terminal gets power, then the transistor would behave like a conductor, then the LED will turn on.

Ⅳ Types of Photocells

The main types of photocells are Photoemissive, Photovoltaic, and Photoconductive. Specialized devices like CCDs (used in scientific imagery), Golay Cells (for IR sensing), and Photomultipliers (highly sensitive, multiplying light by 100 million) are advanced variants.

Ⅴ Applications of Photocells

Photocells have extensive applications across various fields. In automatic lighting systems, they detect darkness to activate lights at night, such as streetlights automatically turning on at sunset in "dusk-to-dawn" systems. In sporting events, they function as timers to measure runners' speeds. For traffic monitoring, photocells count vehicles on roads by sensing shadows or interruptions. They're also used in measuring instruments like light meters to determine light intensity, and in security systems as components of burglar alarms. In robotics, photocells enable robots to hide in darkness, follow light beacons, or track paths by detecting contrast differences. In photography and cinematography, photocells are applied in exposure meters to determine optimal settings, while also assisting with sound reproduction in film audio systems. Compared to variable resistors and photovoltaic cells, photocells serve uniquely as both switches and sensors, offering distinct advantages in these applications.

Reference

Elprocus. (2019). Photocell: Circuit Diagram, Working, Types and Its Applications. Retrieved from https://www.elprocus.com/photocell-working-and-its-applications/

Vedantu. (2023). Overview of Photocells: Learn Important Terms and Concepts. Retrieved from https://www.vedantu.com/evs/overview-of-photocells

ScienceDirect Topics. (n.d.). Photocell: An Overview. Retrieved from https://www.sciencedirect.com/topics/engineering/photocell

Wikipedia contributors. (2024). Photoresistor: Definition and Working Principle. In Wikipedia. Retrieved from https://en.wikipedia.org/wiki/Photoresistor