LCD-LED Display

Automatic LED Emergency Light Circuit

Here is an uncomplicated and cost-effective automatic LED emergency light circuit that relies on light sensitivity. This setup is connected to the mains power supply and activates when the mains power is cut off. The emergency light can remain operational for more than 8 hours (subject to the battery’s capacity and the power consumption of the LEDs).

When the main power source is interrupted, the circuit identifies the presence of daylight and activates the LEDs accordingly. It also ensures that the LEDs are turned off if there is ambient light, even in the absence of power. This light detection process relies on an LDR (Light Dependent Resistor).

Automatic LED Emergency Light


  • Automatic Emergency Light Circuit Principle
    • Automatic Emergency Light Circuit Diagram
    • Components for Automatic LED Emergency Light Circuit
    • Working of Automatic Emergency Light Circuit
    • How to Operate Automatic Emergency Light Circuit?
    • Advantages of Automatic Emergency Light Circuit
    • Automatic Emergency Light Circuit Applications
  • Emergency Light With Battery Charger
    • Circuit Diagram
    • Circuit Components
    • Components Description
    • Working of Mobile Phone Battery Charger Circuit

Automatic Emergency Light Circuit Principle

The battery undergoes recharging through the battery charging mechanism during periods of electrical availability. In the event of a power outage, the white LEDs linked to the MOSFET illuminate in accordance with the ambient light conditions, continuing to operate until the battery is depleted.

When the LDR (Light Dependent Resistor) is exposed to light, its resistance significantly decreases, leading to an increase in the base voltage of transistor Q2. Consequently, the white LEDs linked to the MOSFET are switched off.

In contrast, when the circuit experiences darkness, the LDR’s resistance climbs to the mega-ohm range. This results in a decrease in voltage at the base of the transistor, leading to the activation of transistor Q2 and subsequently, the illumination of the white LEDs.

Automatic Emergency Light Circuit Diagram

I’ve divided the circuit into two distinct sections. The first part functions as both a battery charging circuit and an indicator circuit for situations when the mains power is disrupted. The second section of the circuit is designed for the LED emergency lights, determining their activation or deactivation based on the availability of mains power and ambient lighting conditions.

Battery Charging Circuit

Coming to the Automatic LED Emergency Light Circuit, the following is the circuit diagram.

Components for Automatic LED Emergency Light Circuit

  • 7805 voltage regulator
  • Light Dependent Resistor – 2MΩ
  • BC548 NPN Transistor
  • Pot – 10KΩ
  • High bright LEDs – 3V@15mA
  • Red LED – 1
  • 10KΩ Resistors – 3
  • 1KΩ Resistor – 1

Working of Automatic Emergency Light Circuit

At the outset, the battery charging circuit is responsible for replenishing the battery when the mains power supply is active. In case of a mains power failure, the battery charger circuit serves as a communicator to the Emergency Light circuit, enabling it to initiate the emergency lighting system through the battery.

Rather than immediately illuminating the LEDs, it commences by assessing the ambient lighting conditions through the LDR. If the ambient lighting falls below a certain threshold, the LEDs are then activated.

How to Operate Automatic Emergency Light Circuit?

  1. Connect the wires as shown in the circuit diagram.
  2. Make sure there are no common connections between the AC and DC supply while making the connections.
  3. When you connect the circuit to the mains, you’ll see that the LEDs are no longer glowing and the battery is charging.
  4. Remove the AC power and set the circuit in the dark; the LEDs should now illuminate.
  5. When the circuit is turned on, the LEDs turn off.

Advantages of Automatic Emergency Light Circuit

  • This is very simple circuit and the cost is also very less.
  • Power is saved because the circuit switches the LEDs based on light conditions

Automatic Emergency Light Circuit Applications

  • Used in child’s study rooms to avoid the sudden power failure.
  • As an emergency lamp in homes.
  • Used in security systems to switch ON the lights automatically during the power failure.

Emergency Light With Battery Charger

This is a relatively straightforward circuit for a “versatile power supply and charger.” It proves to be quite practical not only in emergency power outage situations but also as a primary power source. This circuit serves as a valuable tool for assessing or experimenting with electronic projects in your workshop. Additionally, it can be employed for charging mobile phone batteries. Furthermore, it can serve as a backup lighting source in case of power disruptions.

Circuit Diagram

Circuit Components

  • LM317 – 1
  • Resistor
  • R1 (220E) – 1
  • R2-R12 (220E) – 11
  • R13 (470E)
  • VR1 (100K) – 1
  • C1 (100uF) – 1
  • C2 (. 1uF) – 1
  • D1-D4 (1N4007) – 4
  • S1-S5 (on/off switch) – 5
  • LED1-LED12 – 12
  • Transformer – 1
  • Battery – 1
  • Zener diode (3.3) – 1

Components Description

  1. The LM317 is a variable voltage power supply. It’s a device with three inputs and outputs. It operates at a current of 1.5 amps and a voltage range of 1.25 V to 37 V.
  2. The resistor is in charge of controlling the flow of current in any circuit. It is, in essence, a passive gadget. There are two types of resistors on the marketVariable
  1. Fixed Resistor — a resistor with a fixed resistance value.
  2. Resistor — a resistor with a variable resistance value.

3. Capacitor is a device that stores electrical charges. It’s also a passive gadget, and there are two varieties on the market. i.e.

  1. Capacitors with polarity, i.e. capacitors with + and – terminals, such as electrolytic capacitors, are referred to as polarised capacitors.

2. Non-Polarized Capacitor — A capacitor that does not have any polarity, such as a ceramic or paper capacitor.

4. Diode – A diode is a device that allows current to flow in only one direction. It is a two-terminal passive gadget.
5. Switches – A switch is defined as a “change of state” in its literal definition. ON and OFF are the two conditions in electrical logic, and switches help to change the state of an electrical equipment from ON to OFF or reverse. To put it another way, it doesn’t switch the machine on or off; it just makes or breaks contact.

6. LED (light-emitting diode) — A semiconductor device that emits a variety of beams at its output. The forward state of a p-n junction emits a restricted spectrum of light when it is electrically biassed. LEDs are readily available in a variety of hues, including red, yellow, green, and white, orange, and others.

7. A transformer is an apparatus utilized for altering the flow of current from one circuit to another. During this conversion process, the characteristics of the AC signal undergo modification. For instance, low voltage AC can be transformed into high voltage AC, and conversely. The transformer’s functionality relies on the creation of a magnetic field around a conductor when electric current passes through it. This principle is referred to as electromagnetic mutual induction. In a transformer, two wire coils are wound around a core.

8. Battery – A battery is primarily a collection of one or more electrochemical cells in which chemical energy is converted into electrical energy. The fundamentals of operation have remained unchanged since Volta’s time. The battery’s cells are made up of two halves of cells connected in series by an electrolytic solution. While the anode and cathode make up 1/3 of the cell, positive ions from the anode travel from the electrolyte to the cathode.

9. Zener Diode – This diode operates in reverse bias mode and begins to conduct when the voltage reaches the break point. All you need to do to establish a constant voltage is connect a resistor across it so that current flow may be controlled.

Working of Mobile Phone Battery Charger Circuit

You can easily obtain the desired output from the circuit by toggling the appropriate switches (S3, S4, and S5) based on your requirements.

To use the variable power supply function, switch S3 should be turned on. The circuit employs the LM317, a three-terminal positive voltage regulator, to deliver variable power. The LM317 provides an output voltage range from 1.2V to 37V. You can achieve different voltage ranges by adjusting the variable resistor within the circuit. By using a multimeter, you can monitor the output and set it to your desired voltage. The power supply range can be adjusted between 1.5V and 12V.

For charging Li-ion batteries commonly found in mobile phones, you can switch to S5 in the circuit, using your mobile connectors. The charging current in the circuit can be fine-tuned with the help of resistor R13. If you want to activate the emergency light function, switch to S5. You can enhance the light’s brightness by incorporating reflectors into the circuit.

S1 and S2 are the two switches in the circuit that enable you to power your circuit either directly from the AC supply or using a battery.

To utilize an AC power source, activate switch S1. If you prefer a battery power source, turn on switch S2. Solar panels can be employed instead of AC power, and rechargeable batteries can be used to store the charge. This not only reduces your electricity costs but also provides a helpful backup.


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