1.       Introduction

The motivation for developing an automatic night light using a transistor lies in its simplicity and educational value. This project allows engineering students to apply fundamental concepts of electronics such as semiconductor devices, voltage dividers, transistor biasing, and sensor interfacing. At the same time, it addresses a real-world problem related to energy conservation and automation. The proposed system demonstrates how basic electronic components can be effectively utilized to design an intelligent and practical solution without the complexity of microcontrollers or advanced digital systems [1-3].

In recent years, the rapid growth of electrical and electronic systems has increased the demand for energy-efficient, reliable, and automated solutions in everyday applications. One such essential application is lighting, which plays a crucial role in residential, commercial, and public infrastructure. Conventional lighting systems rely heavily on manual operation, requiring human intervention to switch lights ON and OFF according to environmental conditions. This approach is often inefficient, as lights may remain ON unnecessarily during daylight hours or remain OFF during darkness due to negligence or absence of users, resulting in energy wastage and safety concerns. Automatic lighting systems provide an effective solution to these challenges by intelligently controlling illumination based on ambient light conditions. An automatic night light is a simple yet practical electronic system that automatically turns a light source ON when the surrounding light intensity falls below a certain level and switches it OFF when sufficient light is available. Such systems are widely used in street lighting, garden illumination, staircases, corridors, and household night lamps. The automation not only enhances user convenience but also significantly reduces energy consumption and operational costs [4-6].

The core principle behind automatic night lighting systems is light sensing and electronic switching. Light Dependent Resistors (LDRs) are commonly employed as light sensors due to their simplicity, low cost, and high sensitivity to changes in light intensity. An LDR exhibits high resistance in darkness and low resistance under bright light conditions. By integrating an LDR with a transistor-based switching circuit, the variation in ambient light can be effectively converted into an electrical control signal that governs the operation of a lamp or LED. Transistors play a vital role in electronic control systems, acting as switches or amplifiers depending on the circuit configuration. In automatic night light applications, a transistor operates as an electronic switch, enabling or disabling current flow to the load based on the input signal from the LDR. The use of a transistor eliminates the need for mechanical switches, improving system reliability and response time. The transistor-based circuits are compact, cost-effective, and easy to implement, making them ideal for educational and small-scale automation projects [7-10].

This work presents the design and experimental validation of a low-cost and energy-efficient automatic night light using a transistor-based switching circuit. The proposed system employs a Light Dependent Resistor (LDR) for ambient light sensing and an NPN transistor operating in cut-off and saturation regions to achieve reliable automatic ON–OFF control of a light source. The design eliminates the need for microcontrollers or complex digital circuitry, thereby reducing system complexity and cost. Experimental results confirm stable and consistent switching performance under varying light conditions. In addition to practical implementation, the work reinforces fundamental concepts of sensor interfacing, transistor biasing, and electronic switching, making it suitable for educational and small-scale automation applications. The proposed approach demonstrates an effective analog solution for intelligent lighting systems with potential for further enhancement.

2.       Circuit description and Mathematical Modelling

A basic automatic night light circuit using an LDR and an NPN transistor (BC547), as shown in Figure 1(a). The circuit operates from a 9 V DC supply. The LDR and resistor (100 kΩ) form a voltage divider network that provides a control voltage to the base of the transistor. The LED, along with the current-limiting resistor (470 Ω), is connected in the collector circuit of the transistor. The resistance of the LDR decreases significantly, pulling the base voltage of the transistor below the base–emitter threshold voltage during bright light conditions. As a result, the transistor remains in the cut-off region, and the LED remains OFF. In dark conditions, the LDR resistance increases, raising the base voltage above the threshold level, thereby driving the transistor into saturation. This allows current to flow through the LED, turning it ON automatically.

Fig. 1 Basic LDR–transistor night light circuit

An enhanced version of the automatic night light circuit using an operational amplifier (LM358) configured as a comparator, as shown in Figure 1(b). The LDR and resistor network generate a voltage proportional to ambient light intensity, which is compared with a reference voltage set by a potentiometer (RP1). The comparator output drives a BC547 transistor through a base resistor, which in turn energizes a relay to control a high-power AC bulb. A diode (1N4148) is connected across the relay coil for protection against back electromotive force (EMF).

Fig. 1 Op-amp-assisted night light with relay output

The mathematical model of the system is based on the behavior of the LDR, voltage divider action, and transistor switching characteristics. The resistance of the LDR varies inversely with the ambient light intensity and can be approximated by equation (1).

                             (1)

Where R_LDR is the LDR resistance (Ω). is the light intensity (lux), and are sensor-dependent constants. As light intensity decreases, increases sharply. The base voltage of the transistor is determined by the voltage divider formed by and  by equation (2).

                           (2)

Where is the base voltage of the transistor, is the supply voltage. In darkness, , resulting in a higher base voltage. The transistor turns ON when the base voltage satisfies equation (3).

                             (3)

The base current is illustrated by equation (4).

                                       (4)

The collector current is given by equation (5).

                                               (5)                                                                                                                                             

Where is the current gain of the transistor.  When the transistor is in saturation, the LED or relay current is given by equation (6).

                               (6)                                                                                                 

Where is the LED forward voltage or the relay coil voltage. In bright light conditions, decreases, causing , which forces the transistor into cut-off and switches the load OFF.

3.       Methodology

The methodology followed in this work focuses on the systematic design, implementation, and validation of an automatic night light using a transistor-based switching circuit. The approach integrates theoretical analysis with practical experimentation to ensure reliable and energy-efficient operation, as shown in Figure 2.

Fig. 2 Experimental setup for an automatic night light using a transistor

The proposed methodology begins with defining system requirements for automatic ambient light detection and autonomous light switching. An LDR was selected as the sensing element and combined with a fixed resistor to form a voltage divider that produces a control signal proportional to illumination. This signal drives an NPN transistor (BC547) configured as a switch, operating in cut-off during daylight and saturation under low-light conditions to control an LED load. To enhance threshold accuracy and load capability, an advanced design employing an LM358 comparator with a potentiometer-defined reference was implemented, enabling relay-based control of high-power loads. Circuit behavior was verified through simulation before hardware implementation, followed by experimental validation, which confirmed stable and reliable automatic operation.

Fig. 3 Flow chart

The flowchart describes the operation of an automatic night light system that functions based on ambient light conditions. Once the power supply is switched ON, the circuit becomes active and the Light Dependent Resistor (LDR) continuously senses the surrounding light intensity. The sensed light level is compared with a predefined threshold value. When the ambient light intensity falls below or equals the threshold, indicating darkness, the resistance of the LDR increases, providing sufficient base current to the transistor, which then operates in the saturation region and turns the light ON. Conversely, when the ambient light intensity exceeds the threshold under bright conditions, the LDR resistance decreases, the transistor remains in the cut-off region, and the light remains OFF. This process ensures automatic and energy-efficient lighting without human intervention.

4.       Conclusion

This work successfully demonstrates the design, implementation, and validation of a low-cost automatic night light system using simple analog components. By employing an LDR for ambient light sensing and a transistor-based switching mechanism, the proposed circuit achieves reliable automatic ON–OFF operation without the need for microcontrollers or complex digital control. The system exhibits stable performance under varying light conditions and effectively reinforces fundamental concepts such as sensor interfacing, voltage divider operation, and transistor biasing. The enhanced configuration using an operational amplifier and relay further extends the applicability of the design to higher-power loads. Overall, the proposed approach offers an energy-efficient, economical, and educational solution for intelligent lighting applications, with potential for future improvements through sensitivity tuning and integration with advanced control techniques.