Transistor Boost Converter for LED Flash Light (from 1.5V)

Finished circuit

Finished LED flashlight

Here in this blog I introduce a boot converter circuit for flash light using transistor. It is because, some days before I find an old filament bulb flash light. But it is not in working condition, So I decided to insert an led to it to give a new life for the beautiful flashlight.

But the problem faced in adding the LED is that the led need above 3V volt for its better working. But the small flash light work from a single or double 1.5V cells. So it is impossible to light an led from this low voltage. So for  inserting an led to the small flash light we need a special circuit which boost the input voltage. That is, its output has a voltage which is grater than the input voltage. Here I design a simple low cost boost converter by using a single transistor. Its efficiency is above 80 %. It is work very well with the flashlight.

The details about circuit is given below.

The above  circuit diagram indicate my boost converter.  The main components are the transistor and the inductor. Here 2 inductors are used. One is for the boost converter (L1) and other for the feedback for the oscillator circuit (L2). The transistors is the main component in this circuit. The transistor is used as an oscillator and driver transistor for the boost converter. The output diode and capacitor used for produce a steady DC voltage at the output. R1, C2, L2, Q1 are the oscillator part and the L1, D1, C1, Q1 are the boost converter parts. Here I am not explain more about the boost converter theory, here I plan to explain about the circuit working. If you like to know about the boost converter theory please visit my previous blog which is about boost converter theory. In that blog I explain about boost converter theory very well. Link is given below.

https://0creativeengineering0.blogspot.com/2019/04/5v-boost-converter.html

Circuit Working

The circuit is start working when the transistor is at ON position. For starting the circuit working R1 is used. When switch is ON the transistor base get a +ve voltage through the resistor R1. So the transistor get ON and current pass through the inductor L1 and it store some energy in its magnetic field. Now the output diode is at reverse bias and no voltage at output. When the current pass through the inductor L1, It induce a voltage at inductor L2 because the two inductors wind in  the same core. It has an opposite polarity with respect to the first inductor because it is wind in an opposite direction with the first one (shown in the circuit as dots). So a -ve voltage is appear at the transistor base due to the inductor L2. So the transistor get OFF. So the charge stored in the first inductor changes its polarity for discharging. So the diode is move to the forward bias condition and start to conduct. So effectively we get a sum voltages of inductor and the battery in the output (because inductor changes its polarity so two voltages are in series so added). So the capacitor charges and led is light up. In that instant the resistor R1 again switch ON the transistor and the process repeats. So we get a voltage which is higher than the input. The capacitor is used to supply a current for the led when the diode is in the reverse bias condition (transistor on condition). The capacitor C2 is used to control the timing of the transistor ON and OFF. Its value is important for getting a good efficiency. This is the working of this transistor boost converter circuit.

Circuit Making

I made the boost converter circuit in small PCB by using SMD components. It is because the flash light is a small one. Its images are given below. 

finished smd PCB

SMD transistor code is 1P

SMD diode code is M1

SMD capacitor 1 is tantalum and 2 is ceramic

For more details about full project and the circuit making, visit my instructables page. In there I explain very well about all the project steps with images. Please visit it, link given below,

https://www.instructables.com/id/-LED-FLASHLIGHT-From-Trash/

The inductor core is taken from an old CFL bulb. The using a thin enameled copper wire for making the inductor. The inductor winding direction and turns are given in the circuit diagram. The winding direction is indicated by the dot representation.

Finally my circuit is work very well from 1.5-3V supply voltage.

Hand made Inductor

Thank you.

Bye....