This is the circuit from Chan's website (http://elm-chan.org/works/led1/report_e.html) which I modified and hived off two components. The modified circuit is seen here. The circuit is very robust and it worked with a wide variety of transistors; I tried BC547, 2N2222 and 2n3904. The coil I used had 25 turns each, of 26 gauge wire. Resistor R1 changes the intensity of the light. I tried values between 2KOhm to 220Ohm and I settled for 220Ohm. The light is quite bright at this value.
There are not many AM transmitters that are easier to build than this one because the inductor is not tapped and has a single winding. There is no need to wind the inductor as it is a readily available RF choke (eg, Jaycar Cat LF-1536). To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors used instead. To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency. Q1 is biased with a 1MO resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.
A key chain with a built-in white LED comes in handy to help you at your front door or search your valuables in the dark. The intensity of white LED is 4000 to 5600 mcd (millicandela) at forward voltage of 3.6V and forward current of 20 mA.
Fig. 1: Circuit for key chain light
Here’s such an LED light circuit for key chains. It comprises a toroidal transformer and two complementary transistors, and is powered by a single AAA cell. Transistors T1 (BC547) and T2 (BC558) form a relaxation oscillator with capacitor C2 (0.01 µF) in the feedback loop. The feedback is controlled by the time constant of timing components R1 and C2, which controls the frequency of operation.
The toroidal transformer steps up the oscillator output to a sufficient value to flash the white LED. The values of R1 and C1 need not be precise. Use of surface mount devices will make the unit more compact.
Fig. 2: Suggested enclosure for key chain light
A single 1.5V AAA cell gives enough brightness. For more brightness, connect two such cells in series. A good-quality white LED from a reputed manufacturer is highly recommended.
Caution. The white LED beam, when viewed directly, can harm the eyes.
