Low cost LED automatic emergency light circuit

Currently, there are many simple and cost-effective LED emergency lighting solutions. In this article, microcontroller development engineers share an ultra-low-cost emergency light solution that can also be charged from the main power supply and activated when the main power supply is turned off. Moreover, in order to save the power of the emergency lights and increase the duration, light-sensitive electronic components are added.

The principle of emergency lights is that when the power is turned off, the circuit senses daylight and lights up the LED according to the light. Even if the power fails, if the light remains on, the circuit turns off the LED. The LDR ( Light Dependent Resistor ) here is used to sense light and this emergency light solution can work for more than 8 hours (depending on the battery capacity and the power consumed by the LED).

1. Principle of automatic emergency light circuit

When power is available, the battery is charged via the battery charging circuit. When power is interrupted, a white LED connected to the MOSFET emits light depending on the lighting conditions until the battery dies or mains power is connected.

When the LDR (Light Dependent Resistor ) lights up, the resistance of the LDR is very low. As a result, the base of transistor Q2 goes high. As a result, the white LED connected to the MOSFET goes out. When the circuit is in darkness, the resistance of the LDR is very high. The base of the transistor goes low, resulting in transistor Q2 switching the white LED to the on state.

2. Automatic emergency light circuit diagram

The entire emergency light circuit is divided into two parts: the first is the battery charging circuit, which can also be used as an indication circuit if the mains power is turned off. The second circuit is an emergency light circuit using LEDs. Depending on the main power supply and lighting conditions, the emergency LED lights up or goes out.

1.Battery charging circuit diagram

2.LED emergency light circuit

3. Automatic LED emergency light circuit components

(1) 7805 voltage regulator;

(2) Photoresistor – 2MΩ;

(3)I RF 540MOSFET;

(4) BC548NPN transistor;

(5)Pot-10KΩ;

(6) High brightness LED – 3V15mA;

(7) Red LED;

(8) 10KΩ resistors – 3 pcs;

(9) 1KΩ resistor.

4. Working principle of automatic emergency light circuit

Initially, when the main power is turned on, the battery charging circuit will charge the battery. If utility power is shut off, the battery charger circuit indicates the emergency light circuit on utility power and activates the emergency light circuit through the battery.

It is worth noting that this solution does not turn on the LED immediately, but first reads the ambient lighting through the LDR, and then turns on the LED if the lighting is insufficient.

5. How to operate the automatic emergency light circuit?

1. Make connections according to the circuit diagram.

2. When making connections, please note that there is no common connection between the AC and DC power supplies.

3. Connect the main power supply to the circuit, now you can observe that the LED indicator light is not glowing and the battery will be charged.

4. Disconnect the AC power supply and place the circuit in the dark, then the LED will glow.

5. If you put the circuit under a light, the LED goes out.