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.

Comments

[yoyology]

 Elegant device clearly described.  5* and subbed!

[qs]

Thank you! 

[adnimo]

what scope / program is that?

[qs]

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

[zoone]

how much did you pay for all the parts?

[qs]

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

[Davidl3]

where did you buy it?

[qs]

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.

[lasermaster3531]

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?

[qs]

The light stays on only as long as the wires are touched.

[lasermaster3531]

is there any way to make it stay on? like touch-on touch-off?

[qs]

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.

[zoone]

i think u need a button for that

[basher345]

There's an App for that!


I'm sorry I had to say it xD