This mini pocket torch
combines the advantages of Lithium button cells with a super-bright
white LED. The lithium cells are small, have a long shelf life and have very little self-discharge. The LED
has very high efficiency, an extremely long life expectancy and a
modest current consumption (20mA). In combination this results in an
exceptionally long-life pocket torch. It is unfortunate that a Lithium
cell has a voltage of only 3 V, while a super-LED has a forward voltage of 3.5 V. One cell will not suffice and we therefore will have to connect two in series.
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.