Elenco Electronics MO-1251 Bedienungsanleitung PDF herunterladen (Seite 42)

100

IR transmitter

As shown in Fig. 2,

current

through

IR -LEDI and IR -LED2 is switched on

and off by Q2.

Resistors R6 and R7

limit the peak current to approximate-

ly 1.4 amperes. ICI is configured as

an astable

oscillator,

and

R1, R2, and

set the frequency to about 1500

Hz. In the astable configuration, R1

controls the length of

time that pin 3 is

low- approximately

microse-

conds. During that

low, Q1

and Q2

are

switched on,

allowing

current

to flow

through both

IR- LED's. By limiting

the on time to

microseconds,

the

power dissipation in the two IR

-LED's

is approximately

128 milliwatts. Re-

sistors R3-R5 limit the base current

of Q1 and Q2, and C2 provides

low

impedance bypassing

of the power

supply.

Receiver

/transmitter

The schematic for the IR receiver/

RF transmitter

is shown in Fig. 3, and

we'll

discuss the

receiver

section

first.

IC1 -a,

Q2,

and R1, in parallel

with LDR1, a light- dependent re-

sistor, create a current source. Be-

cause phototransistor

QI and LDRI

are both

exposed

to the same ambient

light, LDR1 automatically adjusts

the

current that

is feeding into Q1, in

order to maximize the sensitivity of

the photodetector circuit.

When

Ql faces the pulsing infrared

beam from the transmitter,

Q1's

emit-

ter-

collector

voltage fluctuates in step

with

the pulses. Capacitor C14

cou-

ples Ql's

output to

op -amp ICI -b. R4

and R5 set

ICl -b's gain to about 51

dB.

Resistors R2, R3, RI7, R18, and

capacitor C13 establish

a DC

voltage

offset that is approximately two- thirds

PARTS LIST

INFRARED TRANSMITTER

All resistors

are '/4

watt,

5 %, un-

less

otherwise

noted.

R1 -3300

ohms

R2-

82,000 ohms

R3- 33,000

ohms

R4 -1300

ohms

R5- 10,000

ohms

R6, R7 -2.2

ohms,

'/2

watt

Capacitors

C1-0.01 µF, metalized film, 16 volts

C2 -100

µF

16 volts,

electrolytic

Semiconductors

I01 -7555

CMOS timer

IR -LED1, IR-

LED2- VT1261 Vactec

infrared light-

emitting diode

Q1-

2N4403 PNP transistor

Q2- TIP110 NPN

transistor

Miscellaneous:

SPST switch, PC

board, LED mounting rings, hook-

wire,

enclosure, etc.

Note: The following items

are

available

from

the source men-

tioned in the Sources Box. A PC

board (TS5.1),

$3.95;

IR -LED1

and

IR -LED2 (kit IR5

-1),

$7.95;

All

components

including

semicon-

ductors,

resistors,

capacitors,

optoelectronic devices, IC

sock-

et, LED mounting rings, and

board (IR5 -2

kit), $17.95.

of the power-

supply

voltage,

Vcc, or

about 8

volts.

That allows the ampli-

fier to operate from a

single

-ended

power supply.

The AC signal and the 8 -volt DC

offset are fed to pin 2 of IC2. As long

as the

voltage

on pin 2

is greater

than

two- thirds

Vcc,

IC2 operates as a

monostable multivibrator

whose time

delay

determined by

R6 and

C2.

IC2

and

function as

missing-

80mA

2S2 2.2f 2

IR-LED2

vn2s1

)IR-LED1

VT1261

1.3K

TIP110

2N4403

33K

10K

ICI

7555

82K

3.3K

FIG. 2 -THE IR TRANSMITTER, one component

the three -part system,

continuous beam of

infrared light to the

IR receiver RF

transmitter.

transmits

pulse detector.

As long as pini 2 is held

above 8

volts, Q3 is

biased off, and

has no effect on the operation

the

monostable's circuit. Once a timing

cycle is completed, pin 3 goes to-

and remains at- ground potential.

However, with an infrared signal pres-

ent, the base of Q3 and pin 2 of IC2

are repeatedly triggered by negative -

going pulses from pin 7 of ICI -b.

Consequently, the timing cycle of IC2

is continuously interrupted before it

has a chance to complete one time -

delay

cycle.

That

causes

re-

main high.

With

pin 3 high, the RF

transmitter is turned off;

with pin

low,

the RF transmitter

turned

on.

The RF-transmitter section consists

of a crystal oscillator and an audio -

tone modulator. In the crystal -os-

cillator

circuit,

R14 -R16

establish

DC bias of approximately 7 mA. The

circuit is tuned to

49.890

MHz by

L1,

C9, and C10.

Tl provides

imped-

ance match between the oscillator's

output

and the antenna.

The

antenna

coil,

L2,

tunes a

one -meter

whip to

resonance at

49.890 MHz.

Amplitude modulation

achieved

connecting R16 to pin 3 of astable -

timer IC3. Components

R11 -R13 and

set the astable's frequency to

490

Hz. Pin 3 of IC3 goes low once during

each

490

-Hz cycle,

which turns on

Q5, allowing it to generate and trans-

mit an RF signal.

When pin

3 of

IC3 is

high, Q5 is off, and RF oscillation

stops.

Because IC3 is set to oscillate

the

audio

rate of 490 Hz, a 490 -Hz

amplitude -modulated

carrier

generated by the

circuit.

Figure

shows the envelope

of the

transmitted RF signal as

seen

on a

spectrum analyzer.

The bandwidth is

less than

10 kHz, as required by

Part 15

of the FCC's

Rules and Reg-

ulations.

The RF transmitter is turned on or

off by Q4 and its associated compo-

nents (see Fig. 3). To do that, Q4 turns

the

modulator, IC3, on or off. Q4's

drain is connected through L4, a

ra-

dio- frequency choke, to pins

2 and 6

of IC3. When

is on, pins 2 and 6

are

held below four

volts (one -third of

Vcc),

causing pin 3 to go high; that

stops

the RF oscillation because Q5

turned off.

When Q4

off, pins

2 and

6 of IC3 are unaffected by Q4's

very

high off

-state impedance. Con-

sequently, RF transmission occurs.

Transistor Q4 is switched on and

off by pin

of IC2. C4 charges

through R7

when

pin 3 goes

high. The

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