graph show the percentage of charge remain-
ing in the battery and correspond to the fig-
ure indicated by the microcontroller. The mea-
surement resolution is approximately
±
30 mV,
which corresponds to a worse case measure-
ment error of about 4%.
This method of charge measurement is not
particularly accurate but is acceptable for our
application here. The cell self-discharge is
typically less than 5% per year may so the
torch will still be ready for use even if it has
not been used for some time.
A time-out feature ensures that if the torch
is accidentally left on it will not drain the bat-
tery. The time-out will occur (independent of
power) after approximately 3% of the maxi-
mum cell capacity of 5 Wh (watt-hour) is
used. When the charge in the cell falls below
10% the microcontroller will reduce power to
the LEDs to the next lowest level. The cell
voltage will rise slightly. When the remaining
charge falls below a threshold the LEDs will
be turned off. In this case it is still possible to
switch the torch back on at a low setting for a
short period.
With just 1% charge left in the cell the
torch will function for 5 hrs at brightness level
4 or more than 12 hrs at level 5 so you should
never be left completely in the dark.
The battery and coil
The battery specified in the parts list has a
capacity of 1.3 Ah (approximately 5 Wh). This
is a little bit generous for the torch but the
cell size is widely used for Laptops
so it is relatively economical. Mobile
phone batteries are more difficult to
obtain and are therefore more
expensive.
More information on Li-Ion cells
can be found on the Web at
www.sanyo.com/industrial/batter-
ies/industrial_liion.html
or
www.panasonic.com/industrial/bat
tery/oem/chem/lithion/index.html
Coil L1 has an inductance of 220 to
250
µ
H with I
MAX
= 0.5 A and can
be purchased off the shelf or alterna-
tively hand-wound using the EPCOS
core specified together with 0.2 to
0.22 mm diameter enamelled copper
GENERAL
INTEREST
62
Elektor Electronics
11/2002
2.6
20
22
24
26
28
30
32
34
36
38
40
3.5
012019 - 13
4.0
4.5
5.0
5.5
6.0
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
U
batt
[V]
I
batt
[mA]
012019
T1
C1
C2
C3
D1
IC1
IC2
L1
R1
+
0
S1
T2
A
B
20.9
19.9
20.4
19.4
18.4
7
3
23
3.5
25.5
5
2
68
29.5
9
19
21
17.7
8
012019-14
49
Figure 4. Current consumption of the LED torch.
Figure 5. Mechanical layout for a splashproof case.
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