e01ch01.PDF

SMALL CIRCUITS COLLECTION

001

Voltage Tester for Model Batteries

With a suitable load, the terminal voltage of a

NiCd or lithium-ion battery is proportional to

the amount of stored energy. This relationship,

which is linear over a wide range, can be used

to build a simple battery capacity meter.

This model battery tester has two functions: it

provides a load for the battery, and at the same

time it measures the terminal voltage. In addi-

tion, both functions can be switched on or off

via a model remote-control receiver, to avoid

draining the battery when it is not necessary to

make a measurement.

The load network, which consists of a BC517

Darlington transistor (T2) and load resistor R11

(15

/5W), is readily evident. When the load

is active, the base of T1 lies practically at

ground level. Consequently, T1 conducts and

allows one of the LEDs to be illuminated.

The thoroughly familiar voltmeter circuit,

which is based on the LM3914 LED driver,

determines which LED is lit. The values of R6

and R7 depend on the type and number of

cells in the battery. The objective here is not to

measure the entire voltage range from 0 V, but

rather to display the portion of the range

between the fully charged voltage and the fully

discharged voltage. Since a total of ten LEDs

are used, the display is very precise. For a

NiCd battery with four cells, the scale runs

from 4.8 V to 5.5 V when R6 = R7 = 2 k

+5V

100k

R11

IC1

100n

RX/CX

JP1

10n

RX/CX

IC1.B

IC1.A

JP2

SIGNAL

. The

measurement scale for a lithium-ion battery

with two cells ranges from 7.2 V to 8.0 V if R6

= 2 k

IC1 = 74HC123

BC517

For remote-control operation, both jumpers

should be placed in the upper position

(between pin 1 and the middle pin). In this

configuration, either a positive or negative sig-

nal edge will start the measurement process.

A positive edge triggers IC1a, whose output

goes High and triggers IC1b. A negative edge

has no effect on IC1a, but it triggers IC1b

directly. In any case, the load will be activated

for the duration of the pulse from monostable

IC1b. Use P12 to set the pulse width of IC1a to

an adequate value, taking care that it is shorter

than the pulse width of IC1b.

If the voltage tester is fitted into a remote-con-

trolled model, you can replace the jumpers

with simple wire bridges. However, if you want

to use it for other purposes, such as measuring

the amount of charge left in a video camera

and R7 = 1 k

R12

+5V

D10

BC557

MODE

SIG

RHI

L10

IC2

R10

REFOUT

RLO

LM3914

REFADJ

014071 - 11

Elektor Electronics

12/2001

SMALL CIRCUITS COLLECTION

014071-1

D10

HOEK2

SIGNAL

JP2

JP1

R10

HOEK4

014071-1 (C) ELEKTOR

R12

R11

COMPONENTS LIST

tors!)

R8 = 1k

Capacitors:

C1 = 10nF

C2 = 100nF

T2 = BC517

IC1 = 74HC123

IC2 = LM3914AN

R9 = 1k Ω 2

R10 = 330

Resistors:

R1,R2 = 47k Ω

R3 = 100k

Semiconductors:

D1-D10 = LED, red, high effi-

ciency

T1 = BC557

Miscellaneous:

PC1,PC2,PC3 = solder pin

JP1,JP2 = jumper or pushbut-

ton

R11 = 15 Ω

R12 = 15k

R4 = 500k Ω

R5 = 1k

P1 = 100k Ω

preset

Ω

R6,R7 = see text (1% resis-

battery, it is recommended to connect double-throw push-

button switches in place of JP1 and JP2. The normally

closed contact corresponds to the upper jumper position,

while the normally open contact corresponds to the lower

position.

(014071-1)

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