3

SL490B

Fig. 5 Infra-red application circuit

OPERATING NOTES

Fig. 5 shows the circuit for a simple infra-red transmitter where

the PPM output pulses from pin 2 of the SL490B are differentated

by C3 and R3 and amplified by TR1 to produce current pulses

about 15

µs wide.These pulses are further amplified by TR2 and

applied to the infra-red diodes D1 and D2.

The current in the diodes and the infra-red output is controlled

by the quantity, type, and connection method of the diodes and

also by the gain, at high currents, of the transistors.

The most common solution where cost is important is to use

two single-chip diodes, such as the CQY99 connected in series.

Improved output can be obtained by using four CQY99

diodes in a series/parallel arrangement, but it is usually simpler

to use two multi-chip diodes such as the CQX47 connected in

parallel or a single CQX19, which gives similar results.

A significant increase in range can be obtained by using

diodes such as the CQY99 in conjunction with a plated plastic

parabolic reflector.

When building the transmitter, care should be taken with the

choice of the capacitor C4 and with the circuit layout, particularly

when multi-chip diodes are being used, as the current pulses can

be as high as 6 to 8A.

Transistor choice is also important and any substitutes should

have high current gain characteristics and switching speeds

compatible with the application.

An increase in output can be obtained by connecting TR2 in

common emitter configuration, but care should be taken not to

exceed the rating of the diodes.

Choice of PPM Frequency

When the transmitter is being used with an infra-red link, with

high current pulses fed to the diodes as in Fig. 5, power

consumption will increase with frequency. It is thus advisable

that, with a battery power supply, the slowest PPM rate consistent

with adequate response time should be chosen.

9V

(PP3)

10

11

12

13

14

15

16

17

18

9

8

7

6

5

4

3

2

1

SL490B

C2

4·7

µ, 6V

C1

0·22

µ, 5%

2 3 CQY99

OR

1 3 CQX47

D1

D2

15k

R2

2·2k

R3

100

C3

68n

8 3 4

KEYPAD

47k

TR1

BC327

TR2

BD437

150

µ

10V

1

2











R1