Simple 12V Mini Emergency Light Circuit with LDR

A Simple 12V Mini Emergency Light Circuit that features a Light Dependent Resistor (LDR) is a easy and efficient solution for lighting up spaces when the power goes out.

This circuit activates a bright white LED automatically when the surrounding light dims.

Running on a 12V battery, it is perfect for various applications, including homes, garages or emergency situations.

Circuit Working:

Parts List:

The above circuit responds to changes in light.

The Light Dependent Resistor LDR measures the amount of light present.

When it gets darker like during a blackout or at night the LDRs resistance increases.

This change activates the transistors which then turn on the LED.

This is how the working of circuit helps:

The LDRs resistance drops when there is light and rises when it is dark.

Variable Resistor VR1 50k adjusts how sensitive the circuit is to changes in light.

Transistor Q1 BD140 functions like a switch that controls what the second transistor does.

Transistor Q2 BD139 powers the 1W LED by allowing electricity to flow through it.

Resistor R3 33Ω limits the current to keep the LED safe from damage.

12V battery supplies the necessary power for the circuit to operate.

The switch S1 allows you to turn the circuit ON or OFF manually.

Formulas with Calculations:

Below are the formulas with calculations for Simple 12V Mini Emergency Light Circuit with LDR:

Transistor base current calculation for BD139 Q2:

IB2​ = IC2​​ / β

where,

• IC2​ is the LED current 0.3A 300m
• β gain of BD139 is 40 to 100
• Taking β is 50 average value for BD139

IB2​ = 0.3A​ / 50 = 6mA

Base current IB2 for BD139​ is 6mA

LED Current Calculation:

ILED​ = VR3​​ / R3

where,

• Supply Voltage Vsupply​ is 12V
• LED forward voltage VLED​ is 3.5V
• Resistor R3 limits the current to the LED.

Voltage drop across R3:

VR3​ = Vsupply​ − VLED​

VR3 = 12V − 3.5V = 8.5V

ILED = 8.5V/ R3

ILED​= 8.5V / 33Ω​ = 0.257A = 257mA

LED Current ILED = 257mA (with 33Ω resistor).

Power Dissipation in Resistor R3

P = I2 × R

where,

I is LED Current that is 0.257A (257mA)

R3 = 33Ω

P = (0.257A)² × 33Ω

P = 0.0661A2 × 33Ω

P = 2.18W

A 3W resistor used for R3 resistor is good choice to handle heat dissipation.

How to Build:

To build a Simple 12V Mini Emergency Light Circuit with LDR follow the below mentioned steps for connection and assembling:

• Gather all the components as mentioned in the above circuit diagram
• Connect the emitter of transistor Q1 to positive supply of the circuit
• Connect the base of transistor Q1 between VR1 preset and LDR
• Connect collector of transistor Q1 to the junction of resistor R2 and base of transistor Q2
• Connect the emitter of transistor Q2 to GND of the circuit
• Connect the collector of transistor Q2 to cathode of LED1
• Connect the anode of LED1 to one terminal of resistor R3 and other end connect to positive supply of the circuit
• Connect in series LDR , VR1 preset and resistor R1 from positive supply of the circuit to GND of the circuit
• Connect S1 switch one end from positive supply of the circuit and the other end to positive of the +12V battery and negative of battery connect to GND

Conclusion:

This Simple 12V Mini Emergency Light Circuit with LDR is a smart and affordable solution for providing light when the power goes out.

You can change the sensitivity of the circuit by tweaking the variable resistor.

Thanks to the transistors, it uses very little power while still being able to run a 1W LED.

This circuit is easy to assemble and is perfect for use in homes, offices and garages.

References:

Datasheet transistor BD140

Datasheet transistor BD139