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With only 3 parts other than the LED and battery, the basic blocking oscillator is probably the simplest circuit to convert a single NiCad or Alkaline battery source to light one or more LEDs.
  Current through the resistor biases the transistor on and current flows through the coil, building a magnetic field. When the battery cannot increase the field, it chokes off the driving current to the base and the transistor switches off.
  The magnetic field collapses, creating a potential across itself which is opposite to the charging source. This also forces the Base bias towards -ve to ensure the transistor is OFF.
  This voltage, in series with the battery, is used to light the LED. As the coil's field dissipates, current can again flow through to the base of the transistor, and the cycle repeats.
NOTE: It is important to choose a transistor with a low Vce(on) because we have so little battery voltage to work with.  Common audio transistors may work, but will not be able to drive the LEDs to maximum brightness.

  The BC337 is an adequate device; but I have had the best results with the 2SC2500, also available as FSC2500 from Fairchild; the 2SD965 is also acceptable (Fairchild FJN965), as are most transistors designed for 1.5v disposible flash cameras - but be careful of polarity and Vce limits.
  These are all available in standard TO-92 form, which, because of the low Vce, can accomodate peaks of 1 Amp or more.


Making the coils
The secret of this circuit is in the coil. Along with the Primary section used to store electrical energy, we need a Secondary, or base-driver segment on the same core to control the circuit.
  For best performance and small size, toroids are recommended. You can take a commercial 20-100uH coil (left & top) and add the same number of windings to make the Base-driver leg of the circuit. Or you can roll your own with surplus ferrite cores. The coil on the bottom-right uses a form from All Electronics. Fifteen turns of #28 wire will give you a 100uH coil.
   Alternatively, you can make your own air-core inductor by winding 2 lengths of wire-wrap (awg 30 or 32) wire simultaneously around a 1" dowel (or any form about that size). 20 turns will be about 20uH, while 25 turns will be 50uH.
Wind both coils in the same direction, and cover the entire surface as evenly as you can. In some of the circuits here, there is a "+" marking to indicate the same end (start or finish) of each coil. This polarity is critical for proper operation. Use as thick a wire as you are comfortable with for the Primary (#20-28), while the secondary can be much lighter (#26 or thinner) since it is not current carrying.
WARNING! Ferrite material is specially made from powdered iron-oxide, it is not necessarily magnetic! Do not attempt to use any solid metals as the core! IT DOES NOT WORK!

The circuit is surprisingly versatile, and will work for a rather wide variety of components, but a couple of additions will make it perform even better.

  Putting a 470pF cap across the bias resistor will allow a more complete charge/discharge of the coil.

  To keep the Internal AC resistance of the battery supply low, parallel a small (10uF) capacitor to it. This way, the circuit will work until the battery voltage is down around 0.6V
  A lower value for R (try 1K5) will drive 5 LEDs in series, or 3 x 2-LEDs chains in parallel with impressive brightness.

A General Rule of thumb is to use a coil value of N / Io (in uH) where Io is the desired output current, in Amps, and N is 2 for Single cell and 11 for 2-cells. Put a 30K pot in place of R and adjust for the proper performance.
  While the circuit will drive up to 6 white LEDs in series, best overall performance is with no more than 2-LEDs in series. Parallel them and adjust R for additional output.
 
And now, are you ready to MYSTIFY and IMPRESS your friends with

 

If you don't find custom-making your own coils interesting, it is possible to use standard inductors with the circuit on the right. It uses a BC337 to power the light while a BC327 PNP transistor is used as a timing switch. The coil can be any value from 20uH to 500uH, and choose a capacitor with a value of 5-10pF per uH of inductance (e.g. 100pF to 200pF for a 20uH coil).
  A resistor of 100K will drive the LED at 20mA. Lowering the value of R will drive the LED with more current. With a high-gain BC337 (the BC337-40), this circuit will work for up to 80mA.

A rather extreme demonstration of the flexibility of the circuit: the coil is made up of 2-ft (1' for each leg) of #30 enamel-coated wire random wound over the smoothing input capacitor. Although the reactance is under 15uH, the circuit will light a 80mA 10mm LED to full brightness at about 70% efficiency.
  If you are wondering about the diode in the circuit (just in front of the LED), it is used, along with a 10uF cap as a output rectifying circuit. I find it increases light output and quality. And it also brings increased versatility that we will discuss in the NEXT section, More LED circuits

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