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  • Any-Battery LED Flashlight - General Schematic

  • Created: Aug 16, 2020

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LEDs need a minimum voltage to operate due their own potential drop. Also, their lighting depends directly of current flow through them. LED AC bulbs normally are configured with series LEDs to reduce the current consumption at the expense of needing a high input voltage to operate. This behaviour is explained from Kirchhoff and Ohm's Laws.

In portable devices, like flashlights, they are powered by low voltage batteries and is difficult have a great amount of series LEDs. Due that, portable LEDs devices have a few LEDs, normally between 2 to 6, instead of AC LED bulbs that can have tens.

One but expensive and elaborated solution is to implement a DC-DC boost converter to raise battery's low voltage to one higger. But, another simpler and cheaper solutions is one based on transistors that allows to configuration the LED circuit as serie circuit or as parallel circuit as wish.

The LED circuit are organized in pairs of two LEDs plus transistor but it can be configured all LEDs in parallel. Series circuit works for input voltages greater than 5 V but lows the current consumption. Instead parallel configuration works for low voltages smaller than or equal to 5 V and increase the current consumption. But, this last point, current, can be regulated by a potentiometer, no matter the configuration.

Thus, no matters the input voltage, the LED circuit function selecting his configuration (series or parallel) and regulating current through potentiometer

More specific, electrical functioning of whole circuit is:

  1. J1 can be whatever input DC connector but keep in mind you need to make a multi connector in order to conect different batteries and power sources. I recommend you J1 as female 2.1 mm DC plug-in and externally make adaptable connectors to it.
  2. J2 is for current measure. You can disconnect jumper and connect and multimeter/ammeter. Thus, you can control the current flowing through circuit.
  3. S1 is to On/Off the circuit.
  4. S2 allows you to select series configuration (S) activating Q1, Q2 and Q3 (Q4 and Q5 are off), and parallel configuration (P) activating Q4 and Q5 (Q1, Q2 and Q3 are off)
  5. S3 is optional, but allows you to transmit morse code
  6. R1, R2 and R3 are current limiters for transistors. If you know the equations used in transistors circuits, you can calculate appropiated values. But its important these resistor to be high value (More than 1k) to avoid extra current to LEDs.
  7. R4 allows to regulate the current flowing throught LEDs. It's important to calculate or know power limits of components, especially LEDs and potentiometer, to know how much current can flow in the circuit
  8. Q1 to Q3 are transistors to configure circuit in series
  9. Q4 to Q5 are transistors to configurre circuit in parallel
  10. L1 to L6 are LEDs.


The circuit is fully customizable. You can select different values from components but keep in mind the electrical limits and operating values (voltage, current, resistance, power) from every component

For example, if LEDs are white color, 3mm (2.8 V drop - nominal 20 mA current), the total current its recommend never exceed 120mA to 180m in both configurations but the potentiometer and transistor must be able to operate with that total current value.

For that is J2, although is optional, allows a easy way to connect a multimeter/ammeter to measure total current

Mandatory requirements are:

  • The series configuration is for input voltages greater than 5 V, and the parallel configuration is for input voltages smaller than or equal to 5 V.
  • If you want to add more LEDs, follow the series configuration LED-Transistor-LED.
  • It isn't necessary add series resistor for each LED. Potentiometer acts a general resistor for all LEDs.
  • The maximum input voltage depends of how much power can dissipate (how much current can flow) through potentiometer, transistors and LEDs. With commons THT LEDs, it's recommended maximum input voltage be 12 V and minimum input voltage be 3 V

If you have any question, feel free to add a comment.


A simply and transistor based circuit to allow use of any battery and DC Source to power on the flashlight.

You can try simulations on Tinkercad and Multisim Live:



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