Q&A
S1
1N4001
+ Battery
Under
Test
9.1 ohms
3
10
1k
11
LED10
LED9
PN2222
2.21k
5
6
12
LED8
1N4001
7
13
LED7
LM3914
14
LED6
2.21k
301
15
LED5
4
16
LED4
1.21k
17
LED3
8
4-Cell Battery
Pack Monitor
18
LED2
1
1.07k
LED1
Bar
9
2
Dot
+
2.2uF
Figure 8
er, a silence stop record, and the
ability to resize it to fit into any form
design. It’s shareware with a $35
price tag. A demo download is free.
els of 3, 3. 5, 4, 4. 5, 5, 5. 5, and 6 volts.
Doc
tion from C. Brent Dane called the
HOTCHEK ( www.hem.passagen.
se/rasmus-modellflyg/elektron
ik/hotchek.pdf). He tests the charge
in an RC battery pack using a novel
method of monitoring the battery voltage under a heavy load (closing S1)
over time. Basically, the output voltage of a charged pack will drop
slightly over about 10 seconds and
stabilize. In a battery pack near the
end of life, the output voltage will
continue to decline without leveling
off. Of course, trying to figure the
amount of remaining charge during
the middle area of charge is no better
than an educated guess based on
past battery performance. Dane
explains it in his website, along with a
detailed chart of battery pack types.
Meanwhile, here is his schematic —
Figure 8 — almost. I removed one
resistor (pin 1) and added a capacitor.
Editor note: When I received
my copy of the July 2005 issue, I
discovered a feature called “Battery
Analyzer for RC Power.” It’s a little
complex, but an ideal way to
profile your battery pack. — TJ
Power to RC Cars
Q. I run RC cars, and after a couple of runs, I have no idea of the
state of my battery pack charge. I
would like to build an LED 3- to 6-volt
monitor with low current drain that
would indicate how much charge is
left. I’m thinking in terms of a row of
seven LEDs that indicate voltage lev-
A. If I’m not mistaken, most RC
cars use NiCd cells in their battery packs. The problem with your
approach is that the voltage of a
NiCd cell sags a mere 0.3 volts over
useful battery life. This makes it
almost impossible to interpret the
amount of charge using a voltmeter.
In a four-pack like yours, that means
the voltage would read between 4. 9
and 5.0 volts for 60% of the charge.
Fortunately, I ran across a solu-
READER CIRCUIT:
Multi-Purpose Tester
F
o
r
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
E
v
e
r
y
t
h
i
n
g
Figure 9
I was given this schematic
(Figure 9) over 30 years ago for a
combination voltage and continuity
tester. I built it and found it so useful
I’d like to pass it along. It uses a
Sonalert for continuity. It uses neon
lights and the Sonalert to signal
voltage from 110-480V AC and DC.
Nothing has to be done to go from
continuity to voltage.
Richard E. Woods
Cleveland, OH
Cool Website!
Penton Media’s magazine Electronics
Design has linked up with Discover
Circuits’ website to offer more than
11,000 schematics cross-referenced
into 500 categories. There are also
design and resource links, product
reviews, and more.
www.discovercircuits.com
16
SEPTEMBER 2005
