High Efficiency 9-volt LED Flashlight With Touch Control




Using only 10 off-the-shelf parts, this simple circuit converts power from a 9-volt battery to run 2 white LEDs at 20mA, while only using 13mA on the battery - which is over 90% efficient!

Step 1: What You Need:

2 x White or blue LEDs (25mA or better)
1 x Silicon diode (choose a schottky like the 1N5819 for better performance)
1 x BC327-25 or MPS751 PNP transistor (Do not substitute)
1 x BC337-25, MPS651, 2N4401 or PN2222/MPS2222 NPN Transistor (Do not substitute)
1 x 220uH coil inductor (see text)
1 x 10uF 10-volt (or better) capacitor.
1 x 1000pF (1nF, 102) capacitor.
1 x 10k resistor
1 x 1M resistor
9-volt battery and mounting hardware.

Most of these parts are available at AllElectronics Surplus and on eBay, here and here

If you want to experiment, you can try and make your own coil by starting with 15 turns on a toroid. If the circuit stays on by itself, add more turns; if the LEDs are dim, REMOVE turns.

Step 2: Assembly

With only 10 parts, placement is very simple.

The first picture below shows how I arranged the components. The next image shows the wiring on the underside. The third image is the 2 superimposed together.

Step 3: How It Works

The efficiency of this circuit comes from the coil (inductor) and the small 1000pF capacitor. NOTE: The 1000pF capacitor is critical to the circuit! Do not change its value and defnitely, do not omit it!

The circuit begins operation when moisture from your finger turns transistor Q1 on, which switches Q2. Current from the battery is drawn through the 2 LEDs, the coil, through Q2 and to Ground.

C, the 1000pF capacitor forces Q1 and Q2 to saturate - and improve efficiency. Since the 2 LEDs only require 6.5v to light up, the coil builds up its charge with the excess of (9 - 6.5 =) 2.5-volts.

Capacitor C cuts off the transistors when the coil saturates and its magnetic field collapses. This supplies voltage which flows through the diode to keep the LEDs lit.

The oscilloscope trace shows how this cycle repeats over and over, almost 1/4-MILLION times a second!

As a result, power is only drawn from the battery about 1/2 the time, while the coil 'recycles' the excess power the other times. So the LEDs maintain full brightness without draining the battery continuously.

Step 4: Conclusion

This is a very simple example of a switch-mode circuit.

Unlike series regulators (like the 7805 and LM317), which burns up extra power as heat, this circuit stores the excessive energy in a coil, to be re-cycled periodically, to maximize battery life.

I hope you will try and build one - as 'green' as LEDs are, their efficiency can still be improved by limiting the energy wasted in resistors and series regulators.

More information on this and other circuits can be found on my website.



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    18 Discussions


    4 years ago on Introduction

    hey,can i skip the "TOUCH" part?????I mean, can i use just slide switch???

    thanks in advance


    Reply 6 years ago on Introduction

    Thank you for your comments!

    Even though it acts as a buck converter, strictly speaking, the circuit uses a V-boost topology, one which is almost identical to one frequently used as a Joule Thief. The main difference is that the LEDs are placed in series to drop the voltage, and the output of the coil is 'folded' back through the diode to supply power to the capacitor across the LEDs.


    Reply 4 years ago on Introduction

    hey,can i skip the "TOUCH" part?????I mean, can i use just slide switch???

    thanks in advance


    Reply 9 years ago on Introduction

    The parts came to under US$2 - all or most of them are available from eBay, for even less.


    Reply 9 years ago on Introduction

    I've been collecting, and experimenting with electronics for a number of years, but most, if not all of these parts are obtainable on eBay. In the USA, you can also try All Electronics.


    Reply 9 years ago on Introduction

    I have a question about the switch. when you touch it, it comes on, but does it stay on only when it is being touched, or does it stay on after, and needs to be touched again to make it turn off?


    Reply 9 years ago on Introduction

    Such a feature here would double the parts needed.

    Having said that, the 'magic' of this circuit is that it measures skin resistance to operate the light - in other words, a moistened piece of cloth held over the wires will fool it into turning on, and keep it turned on without being touched. That's one way you can make a 'switch' for it!

    However, there is an even simpler MOSFet circuit on my website (scroll down to the bottom) that has the touch-on and off features.


    Reply 9 years ago on Step 3

    The image is from a DSO-2090 USB scope - more details are here.