Rechargeable LED Torch

Rechargeable LED Torch is a DIY pocket size LED torch light using single AA rechargeable battery, super bright white LED and rechargeable using standard USB port. This is a great DIY project where you can learn about voltage booster and USB charger for AA rechargeable battery.

Step by step guide

First of all, you have to gather all the components required.

Component list:

  1. 1 x 3.9K Ohm 1/4W resistor
  2. 1 x 10 Ohm 1/4W resistor
  3. 1 x 1.5K Ohm 1/4W resistor
  4. 1 x 470 Ohm 1/4W resistor
  5. 1 x 6.8K Ohm 1/4W resistor
  6. 1 x 10K Ohm 1/4W resistor
  7. 1 x 100K Ohm 1/4W resistor
  8. 1 x 150pF Ceramic capacitor
  9. 1 x 16V 100uF Electrolyte capacitor
  10. 1 x 1N5819 diode
  11. 1 x USB-Type B female connector
  12. 1 x 5mm super bright white LED
  13. 1 x 3mm red LED
  14. 1 x 22uH inductor
  15. 1 x BC557 PNP transistor
  16. 1 x 2N2222 NPN transistor
  17. 1 x Single Pole Double Throw slide switch
  18. 1 x Push button
  19. 1 x LM358 IC
  20. 1 x AA rechargeable battery
  21. 1 x 7cm x 3cm donut board

Normal electronic store doesn’t sell 7cm x 3cm donut board, so, maybe you have to cut the donut board from a larger donut board.

Divide the donut board into 3 parts: battery, charger, and LED.

Start by soldering the USB Charger circuit. You may need to drill a 2mm hole at the donut board to insert the USB B-type connector.

Place the USB B-type at the end of the board so that it is easier to plug in the USB cable to charge the torch. Also try to organize the board, like how much spaces will the battery take, slide switch, and also components.

To reduce the size of the board, you may try to put the resistor at “standing” position.

Finish doing the charging circuit.

Continue with the voltage booster circuit.

Complete the circuit, and use double sided tape to hold the battery to the donut board.

Place the battery to the donut board and solder wires to the positive and negative terminal of the battery.

Switch to charging operating mode, and connect to USB cable to charge the battery. Make sure the charging red LED is turned on.

After finish charging, switch the slide switch back to normal operating mode, and have fun!

Circuit description

The description of the circuit will be divided into 2 parts, USB charger and voltage booster.

USB Charger

The USB AA battery charger is basically constructed by 2 main blocks, one is the comparator, and another one is the transistor charging circuitry. LM358 consist of 2 voltage comparator and its design to operate from a single supply.

In the charger circuit, we use comparator to compare the voltage of the battery with a fixed voltage from 3 resistor voltage divider. Once the battery is fully charge, the comparator will turn off the charging circuit.

R1, R4 and R6 are voltage divider and the output voltage of the nod A and nod B can be calculate by following equation:

These Va and Vb will be connect to the negative input of the comparator 1 and comparator 2 which will be compared with the voltage of the battery. Once the voltage of the battery reach 1.386V, the Out 2 will equal to one, and the red led will be turned off. Once the voltage of the battery reach 1.396V, the Out 1 will equal to one, and the transistor charging circuitry will be turned off.

The transistor charging circuitry is mainly build by a PNP transistor, BC557 which will be turn on when Out 1 equal to zero. The typical charging current for the setup is around 80mA.

Voltage Booster

The voltage booster is basically a boost converter where the output voltage can be greater than the input voltage. Since the super bright white LED needs around 3V to operate and the output of rechargeable AA battery is only 1.2V, then this boost converter is suitable for our application.

In general, a boost converter consists of an inductor, a switching circuit and a capacitor storing circuit. When the switching circuit turns on the inductor, the current flow through the inductor will increase. After a certain period, the switching circuit will turn off the inductor. As we know the current flowing in an inductor must be continuous, and at this time, the current is being force through the diode into the storing capacitor.

Overall Circuit

To separate the charging and booster circuit, the battery is being connect to a slide switch where it determines the operating mode, either charging or normal operation.


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63 thoughts on “Rechargeable LED Torch

  1. i’ve tried soldering the whole circuit…the recharge works but the voltage booster doesn’t really boost… is it because of the inductor? help needed

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