Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Bedienungsanleitung PDF herunterladen (Seite 66)

LOW

-BATTERY

ALARM

Free

yourself

from

dead

-battery

hassles

with

a

circuit

that

monitors

the voltage

of your

back -up

batteries

and

tells

you

when

they

start to run

down

before

its

too late

By

Jan Axelson

IN THEORY,

BATTERY

BACK

-UP

IS

GREAT.

WHEN

AC POWER

fails,

battery

power

kicks in

and

keeps

your

equipment

running

as if

nothing

had

happened.

With

clocks

and

timers,

that

means

that

you

don't have

to

tediously step

through

the hours

and minutes

to

reset the

correct

time when

power

is

restored.

But

the system

isn't

foolproof.

Because

batteries

have

a

limited

shelf -life,

you

have

to remember

to check

them

periodically.

Otherwise,

chances

are

that when

power

goes

out, the

batteries

will

be dead

and

you'll

end up without

your

back -up power

anyway.

Our

Solution

A low-

battery indicator

light

can help

to warn

you of

failing

batteries,

but

an audible

alarm is harder

to

miss.

The circuit

presented

here

gives just

such

a warning.

It continuously

monitors

battery voltage

and turns

on

a

warning

buzzer

when

the voltage

has dropped

to

5

volts.

The

circuit

receives

its power

from the

battery

or batteries

being

monitored,

so

no complicated

modifications

to the

original

equipment

are required.

By selecting

different

re-

sistors,

you

can change the

trip

voltage

to suit

your needs.

The

circuit

can be

used

with

any clock,

timer,

or other

device

that's battery

powered

or has battery

back -up,

using

batteries

of 3 to 16 volts.

Circuit

design is made

easy

due to

an IC

designed

specifically

for

voltage

monitoring

-the

In-

tersil

8211 programmable

voltage

detector.

While

monitor-

ing,

the low-

battery

alarm

draws well

under

100

microamperes

and

so has little

effect

on battery life.

In

alarm

mode,

the

8211 provides

current to

operate a

555 timer.

And

the

timer

in turn

activates

a piezoelectric

warning

buzzer.

Let's now

look

at the

circuit in

more detail.

Circuit

Details

The

schematic

diagram

for

the circuit

is shown

in Fig.

I.

The battery voltage

is monitored

by UI, which

contains its

own internal voltage

reference

of

1.15

volts.

A voltage

divider

made

up

of Rl, R2,

and R3

determines when

Ul's output

at

pin

4 turns

on

and off.

When

the

voltage

at pin

3 of

UI falls to

about I volt,

pin 4

turns

on

and provides

current

to operate

U2. The resistor

values

are chosen

so that

this happens

when the

battery

voltage

is

5

volts.

The hysteresis

connection

at pin

2 causes pin

4 of

Ul to

remain on

unless the battery voltage

rises to

5.5

volts

(when

fresh

batteries

are inserted

and

pin

3 rises to

at least 1.15

volts).

That

0.5

-volt

hysteresis,

or difference between

the

"on" and

"off" voltages,

guarantees

that the

output will

snap

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