Adopting, and maybe improving on the idea of the PAK-LITE (available here), a small flashlight which attaches atop a 9-volt (smoke alarm) battery, we proudly present the HAK-LITE.    An important consideration is the choice of LEDs. The sample here uses two 10mm (T3-sized) white LEDs from here. With a tight beam of +/-6 degrees, it casts a very useful light with only 20mA draw.    A most useful improvement over the original light is replacing the switch with a simple Touch-control. This allows the circuit to be completely sealed, with 2 small metal touch plates at opposite ends acting as the switch itself.    Performance is very good; average draw is around 12mA to drive both LEDs to 20mA, which translates to over 90% efficiency and an alkaline battery will run the light brightly for 30 hours. The light will continue to run at diminished brightness until the battery is below 6-volts. This means the light will work with earlier rechargeable batteries with only 7.2-volt output. For slightly less light, but double the run-time, change R2 to 180Kohm.    To further increase its usefulness, glow-powder from Glow Inc was added to the epoxy potting compound. Side spill from the LEDs and exposure to daylight will keep the unit visible throughout the night.
The circuit uses the coil as an active resistor. When on, current from the battery flows through the LEDs and excess power is used to charge up the coil through transistor Q2. This current is controlled by resistor R2 and can be calculated by (145-I)[k-ohms], where I is the desired LED current in milliAmps.    When the coil saturates, the transistor switches off, and the coil discharges through diode D1 and the LEDs. The cycle then repeats itself.    The 5uF capacitor smooths out the charge/discharge pulses to provide a steady current to the LEDs.    Q1, a general purpose PNP transistor (2N3906, 2N4403, BC327 etc), can be used for the touch-control. Shorting or touching the end of R1 to ground will allow a small current to flow to Q2 to activate the circuit.   You can use any diode for D1 rated for more than 250mA and 20-volts. Any hi-gain NPN transistor designed for disposible flash camera use will work for Q2, as will a BC337, but with lowered efficiency.