Simple LED Emergency Light Circuit

This article shows you how to build a simple LED emergency light that does not need a bulky transformer.

This makes the design smaller, cheaper and easier to build.

What is a LED Emergency Light Circuit:

An LED emergency light circuit is a system designed to provide illumination during power outages or emergencies using LEDs.

LEDs are energy efficient, compact, and have a longer lifespan compared to traditional light sources making them suitable for emergency lighting applications.

Circuit Working:

Parts List:

The transformer, bridge and capacitor collaborate to create a standard power supply.

Central to this design is a single PNP transistor functioning as a switch.

PNP devices, referencing positive potentials act as a ground.

Connecting a positive supply to the base of a PNP transistor grounds its base keeping it switched off when mains power is ON.

Operation during Mains Power Presence

While mains power is active the positive supply to the transistors base keeps it switched off.

Consequently, the battery voltage cannot reach the LED bank ensuring the LEDs remain switched off.

The battery undergoes trickle charging through the power supply.

Operation during Mains Power Disruption

When mains power is disrupted the positive supply at the transistors base disappears.

The transistor becomes forward biased through the 1k resistor switching ON instantly and illuminating the LEDs.

Diodes in the voltage path gradually bypass as the LEDs dim.

The Transformerless Emergency Light Circuit

Despite similarities to previous designs the removal of the transformer enhances the circuits utility.

Its main features include:

• The absence of a transformer makes the circuit extremely compact.
• Reduced component count leads to a lower cost.
• The simplified design facilitates easy assembly.

Safety Considerations

It is crucial to note that the circuit being directly linked to AC mains poses a significant safety risk.

Therefore, the constructor must implement all necessary safety measures when assembling it.

Circuit Operation Details

PNP transistor T1 remains switched OFF during AC mains presence creating a safe condition.

The transformer is replaced by a configuration involving C1, R1, Z1, D1 and C2 forming a compact transformerless power supply.

The transistor switches to a biased condition when AC power fails allowing the battery to power the LEDs.

Formulas:

Here are some relevant formulas for the components commonly used in electronic circuits:

Ohms Law:

Ohms Law defines the relationship between voltage V, current I and resistance R:

V = I * R

Power in a Circuit:

The power P in a circuit can be calculated using the following formula:

P = V * I

Alternatively, using ohms law: P = I² * R

P = V²/ R

Resistor Color Code:

For a resistor with colored bands, you can use the color code to determine its resistance value.

The formula is: R = (10^first color * 10^{second color}) Ω

here,

• The third color represents the multiplier power of 10 and the fourth color represents the tolerance.

Capacitance:

The formula for the capacitance C of a capacitor is:

C = Q / V

where,

• Q is the charge stored on the capacitor and V is the voltage across it.

Time Constant in RC Circuits:

For an RC circuit, the time constant (τ) is given by the product of resistance R and capacitance C:

τ = R * C

LED Current Limiting Resistor:

To calculate the resistance R_LED​ for limiting current through an LED, you can use:

R_LED = V_source − V_LED / I_LED

where,

• Vsource is the source voltage
• VLED is the LED voltage drop, and
• ILED​ is the desired LED current.

Zener Diode Current Limiting Resistor:

For a Zener diode, the resistor value RZ to limit current is given by:

RZ = V_source −VZ / IZ

where,

• VZ​ is the Zener voltage
• IZ is the Zener current.

Power in a Transformerless Power Supply:

The power P_supply in a transformerless power supply is given by:

P_supply = V²_rms / R

​​ where,

• V_rms​ is the root mean square voltage of the AC supply and R is the load resistance.

Note:

These formulas can help in designing and understanding electronic circuits.

Ensure units are consistent throughout calculations.

Always verify and adapt these formulas based on the specific characteristics of your components and circuit.

How to Build:

Building the LED Emergency Light Circuit follow the below mentioned steps:

• Ensure you have all the specified components ready.
• Before starting disconnect any power sources.
• Use proper safety equipment, such as gloves and safety glasses.
• Design the circuit layout on the circuit board following the provided schematic diagram.
• Connect the PNP transistor T1 as per the schematic.
• Create the transformerless power supply using components C1, R1, Z1, D1 and C2.
• Connect the LEDs to the circuit, ensuring correct polarity.
• If using a 6 volt battery modify the circuit as mentioned earlier.
• Integrate resistors R1 and R2 into their designated positions.
• Insert diodes D1, D2 and D3 into the circuit according to the schematic.
• Connect the bridge rectifier to the circuit as specified.
• Connect the battery to the circuit, ensuring proper polarity.

Final Checks:

• Double check all connections for accuracy.
• Inspect for soldering errors and correct if necessary.

Power On Test:

• Connect the circuit to a power source and observe its operation.
• Verify that the LEDs illuminate when mains power is disrupted.

Safety Precautions:

• Ensure the circuit is insulated and housed in a protective casing.
• Avoid touching the circuit when connected to AC mains.

Conclusion:

Building this transformerless simple LED emergency light circuit involves careful component placement, soldering, and attention to safety.

If you encounter difficulties or uncertainties seek guidance from experienced individuals or professionals in electronics.

Always prioritize safety throughout the construction process.

References

Emergency light

Design and development of smart emergency light