How to Illuminate 18 LEDs from a 3.7V Battery

Powering 18 LEDs from a 3.7V battery is like having a party with lots of tiny light bulbs!

To make sure all the bulbs light up nicely, we need a special plan.

This plan involves putting the LEDs in smaller groups and connecting them in a smart way.

By connecting them in lines series and rows parallel, we can adjust the voltage and current so all the LEDs get exactly what they need to shine bright without burning out.

Circuit Working:

Parts List:

This circuit operate 18 white LEDs using a 3.7V Li-Ion cell and incorporates components from an old Compact Fluorescent Lamp.

An intriguing feature of this circuit is its use of two transistors to maintain constant brightness with the BC547 providing feedback to sustain oscillation.

Despite the seemingly high 10k base resistor the circuit was tested for 12 hours on a 1200mA Hr cell, and the brightness remained consistent.

Brightness is controlled by the drive strength of the 2N2222 transistor.

The transistor is activated by the 10k resistor delivering minimal current.

However, due to the transistors gain of 100 to 300 the collector current can reach up to about 100mA.

Essentially, the circuit drives the LEDs beyond their capacity and the BC547 regulates the current to the necessary brightness level.

The BC547 acts to switch off the 2N2222 at a specific point in each cycle reducing its overall on time.

This switching is timed by the 10k and 1k resistors which form a voltage divider that produces a base voltage.

When the 2N2222 turns on a voltage develop across the 10 ohm resistor that when added to the base voltage slightly delays the transistors activation.

The 1nF capacitor determine the circuits oscillation frequency.

Through experimentation with these four components the desired brightness can be achieved and maintained throughout the cells lifespan.

All LEDs are connected in series in each string and the brightness depends on matching each string.

By swapping some LEDs you can adjust the brightness to ensure they all emit equally.

Formulas:

Using a boost converter, you may raise the voltage from 3.7V to a higher voltage that can be used to drive 18 LEDs in series or parallel arrangement from a 3.7V battery.

The boost converters important formulae is provided below:

Output Voltage Vout:

A boost converters output voltage is determined by:

V_out ​= V_in​​ / 1−D

where,

• Vin is the input voltage (3.7V in this circuit), and D is the switches duty cycle.

Inductor Current IL:

I_{L ​}= V_out ​* D * T / ​ L

where,

T is the switching cycle period, while L is the inductance.

Duty Cycle D:

The duty cycle, which is determined by dictates how long the switch is closed as opposed to open.

D = V_out​−V_in​​ / V_out​

Choose Inductor L:

Based on the intended output current and the converters operating frequency, choose an inductor.

The following formula may be used to estimate inductor value:

L ≥ ​(V_out​−V_in​) * V_out​​ / f * ΔI_L

where,

• The peak to peak ripple current via the inductor is represented by ΔIL.

This formulas offers a foundation for building an efficient boost converter circuit to light up to eighteen LEDs using a 3.7V battery.

The precise LED forward voltage requirements and intended operation circumstances will determine the component selections and detailed calculations.

How to Build:

How to Illuminate 18 LEDs from a 3.7V Battery find out the below mentioned steps for connection of components.

• Connect positive side of 3.7V li-ion battery to positive supply through ON/OFF switch, and negative side of battery to ground.
• Connect collector of transistor Q1 to positive supply through resistor R1.
• Connect base of transistor Q1 between resistor R2 and capacitor C2.
• Connect emitter of transistor Q1 to ground.
• Connect a capacitor C1 from positive supply to negative supply to ground.
• Connect a 1mH inductor through a positive supply between R1 resistor and collector of Q2 transistor.
• Connect collector of Q2 transistor to positive supply.
• Connect base of transistor Q2 between R1 resistor and collector of transistor Q1.
• Connect emitter of transistor Q2 to ground through R4 resistor.
• Connect R3 resistor between R2 resistor and C2 capacitor.
• Connect 18 straw hat white LEDs in series from positive supply to negative supply as in diagram.

Note:

• When working with LEDs and batteries ensure that the circuit is properly insulated and components are connected to prevent short circuits.
• Additionally, use appropriate current limiting resistors to protect the LEDs from excessive current, which can cause overheating and damage.

Conclusion:

Illuminating 18 LEDs from a 3.7V battery requires a well thought circuit design that consider the voltage and current requirements of the LEDs.

Overall, the goal is to create a circuit that efficiently utilize the battery power to provide adequate illumination from the LEDs.

References:

Is 3.7V LiPo suitable to power arduino+led strip?