1. Key Chain Light
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.

2. Caution. The white LED beam, when viewed directly, can harm the eyes.