Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Manual de usuario descargar pdf (Pagina 59)

Vref

B

TIME.

Fig.

3 -The

charted

relationship

between

the comparator's

input and output

vol ages is shown

here.

The upper

graph (A)

shows

inputs to the comparators

and circuits

diagrammed

in Fig. 2. The

lower grEph (B) shows

the circuit's

output

response. qV

A

B

C

+9V

Vrel

Vin

V

RI

10K

R2

3905!

LEO

+9V

nt

2N2222

+9V

R1

68012

LED

Fig. 4 -Here are

th ee ways of using

a light- emitting

diode

(LED)

to indicate the electrical

output state

of a comparator.

In Fig. 4A,

when

Vin

is greater than

Vref, pin 2 goes high,

transistor

Q1 turns

on through pull

-up resistor R1, and the

collector

current through

Ql lights the LED. When

Vin

is

less

than

Vref, pin

2 is low and

Q1 is cut off, turning

the LED

off. Resistor

R2 limits the current

through the LED.

The circuit in Fig.

4B is similar to Fig.

4A, but this time

the comparator

controls a PNP transistor.

When

pin 2 goes

low, Q 1 turns

on and lights the

LED,

giving

the opposite

effect of the NPN

circuit in Fig.

4A.

Figure 4C shows

yet another option for connecting

an

LED.

Typical current -sink

capability of the LM339

is 16

milliamperes.

This is enough

current to light a

high -

efficiency

LED directly,

without using a

drive transistor.

The circuits

in Fig.

4

are

shown using one

of

the

four

identical comparators

in the LM339.

In

these

and the circuits

that

follow, inputs and

outputs to unused comparators

on

the chip

should be tied to ground.

Power supplies are

not

shown, but

should be connected at pins 3

and 12 as shown

in Fig. 2.

Achieving Snap -Action

The

circuits shown

so far all have limitations. If

Vin

has

noise riding on

it,

the

output may chatter

high and low as

Vin

approaches

V

ref

. And

a

slowly changing

input

may per-

mit the

output to oscillate as

Vin

nears the

trip

voltage.

Add-

ing a little positive

feedback can take care of both of

those

problems.

Figure 5 shows a temperature-

monitoring circuit

with posi-

tive feedback added

through resistor R6. The trip

voltage

is set with potentiometer

R4. The sensed voltage is taken

from a

voltage

divider

containing a thermistor

(temperature -

dependent

resistor) and resistor R2. As the temperature

of

thermistor

R I increases, its resistance decreases

because it

has a

negative temperature coefficient.

The resulting drop

in the network's (R1/R2)

resistance increases the current

through

R2, raising the

voltage

at pin

4 of the LM339.

Here's

how the positive

feedback

works.

When

the output

at pin

2 is high, a

small part

of

the

output

voltage feeds back

through R6 to

pin 5. This

raises the

voltage at pin 5 slightly

higher

than the

level set at R4.

When

rising temperatures

cause pin

4 to go higher

than pin 5, pin

2 goes low,

buzzer

BZI is energized,

and the

voltage at pin 5

drops, this time

to

a level slightly

lower than that

at R4.

The

buzzer remains

on until the

temperature falls enough

so that pin

4 is less than

pin 5 again.

Because the turn

-on

trip

voltage is higher than the

turn-off

voltage, the buzzer

snaps on decisively

at the desired

temperature

and remains

on until the temperature

drops. ,yy

R1

THERMISTOR

10K

AT

25'C

+9v

BZ1

PIEZOELECTRIC

BUZZER

Fig. 5 -The

piezoelectric

buzzer sounds

at and above

a temp-

erature selected

by

R4. Positive

feedback through

R6 ensures

that the buzzer

snaps on decisively

at

the trip voltage.

63

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