Here is a tried and tested design of an ultra-simple emergency torch built around the inexpensive dc-dc converter chip MC34063. Beyond a handful of simple components, a lithium coin cell and a small key fob case no additional parts are needed!
The MC34063 (IC1) is a monolithic control circuit containing the primary functions required for dc−to−dc converters. MC34063 consists of an internal temperature compensated reference, comparator, controlled duty cycle oscillator with an active current limit circuit, driver and high current output switch.
This chip was specifically designed to be incorporated in buck and boost (and voltage−inverting) applications with a minimum number of external components. Here, the IC-MC34063 is used to drive three high-efficiency white LEDs using 3V dc supply available from the built-in CR2032 lithium coin cell.
In this circuit, designed to drive ultrabright white LEDs at constant current, IC1 (MC34063) compares the external voltage at its pin 5 with an internal reference voltage of 1.2 V in order to stabilize its output. Here, resistor R3 (connected to pin 5 of IC1) sets the operating current of the series-connected white LEDs (LED1-LED3). Note that the circuit can provide a variable intensity by making R3 to be a variable resistor. If R3 is linear then the perceived light intensity will be approximately linearly variable with the variable resistor’s shaft position.