That results in a power supply voltage of 6 V, and at a current limit of 20mA we need a series resistor of (6 - 3.5) / 0.02 = 125 Ω. Very annoying, because this resistor causes a power loss of 2.5 × 0.02 = 50 mW. Compared to the power consumption of the LED (3.5 × 0.02 = 70 mW) this would mean that nearly 42% of the energy would be wasted! By using an integrated pulse-width modulator from Siemens, the BTS629, this power loss can be limited to about 10%. In combination with two CR2025 cells with a capacity of 170mAh, the pocket torch will have an expected operating time of 15 hours.
Two CR2032 (230mAh) cells will last an astonishing 21 hours! Another advantage of the IC that has been used here, is that the pulse width, and hence the brightness of the LED, can be smoothly adjusted with P1. The compact printed circuit board for this pocket torch has been designed so that it fits exactly inside the key ring enclosure UM14 from KM. The PCB has a 20 mm diameter hole for the button cells. The negative terminal for the button cells is made with a piece of paperclip soldered to the bottom. For the positive terminal, on the topside of the PCB a flat terminal can be attached with the aid of an M3 bolt and nut, as can be seen in the photograph.
