Rechargeable LED Flashlight
This is a simple LED flashlight with built in Li-ion battery with charge circuit and and under voltage lock out using famous programmable voltage reference TL431
Supplies
1x schottky diode (1N5819 or similar)
2x 680Ω resistor
1x 47Ω resistor
1x 22Ω resistor
2x 100kΩ resistor
1x 220kΩ resistor
2x 470kΩ resistor
1x 3.3kΩ resistor
1x 10Ω resistor
1x 1W power LED (any color you like I've used warm white)
2x TL431 adjustable voltage reference
1x 2N3906 PNP transistor (You can P channel MOSFET if you like then remove base resistor)
1x push button
1x rechargeable Li-ion battery (I've used 70mAh you can any value, higher the capacity longer the charge time with this circuit)
The schematic consists of 3 parts under voltage lockout, LED and charge circuit.
Charge circuit consist of a TL431 and some resistors.470kΩ resistor has 2.5V. The Schottky diode protects circuit from reverse polarity. LED and series resistor shows charging and correct polarity. 47 and 22Ω resistors connected in parallel to limit the current for the battery 100kΩ, 220kΩ and 470kΩ sets end of charge voltage which is 4.25V in my case. The charging current is about 55mA which drops as the cell charges up.The charge circuit draw 22microamps from battery while in idle.
The second part consist of PNP transistor and TL431 which cuts the the battery at 3.03V according to given formula. Since this circuit draws noticable amount of current from battery about 1mA while LED is off this part enabled with push button.
LED is driven with 10Ω resistor and with PNP transistor. Top LED current is abou 80-85 mA when the battery is charged to 4.2V and drops as the battery discharges. Since battery is quite small i ve used push button to momentarily use the light.With this arrangement the flashlight has a run time little over 1 hour. It is quite compact and can carried in a keychain easily