E00c072.PDF

COM PUTER

Multi-Purpose

For ‘Running Text Display’

Design by F. Wohlrabe

This universal IR receiver

works with the IR key-

board transmitter for the

Running Text Display

described in the February

2000 edition of Elektor

Electronics. Its versatility

allows it to perform simple

switching tasks or complex

control of test equipment

remotely from a standard

PC keyboard.

+5V

IC3

KEYBOARD

DATA

DIN 5

RESET

rear view

IC1

CLK

ZSM560

G0/INT

JP1

GND

COP8782

1 DATA

3 GND

4 +5V

5 CLK

G4/SO

KEYBOARD

G5/SK

D 1

PS-2

G6/SI

rear view

TSUS

5201

G3/T10

IC2

G7/CKO

CKI

7 805

100n

220µ

16V

33p

10MHz

33p

ZTX603

see text

990090 - 12

Figure 1. The IR keyboard transmitter from the February edition.

There are two basic methods used for send-

ing data over an infrared link. The simplest is

to switch the transmitting diode on and off to

represent 1‘s and 0‘s in the data stream. To

save energy, these bits are made as short as

possible. This however makes it more difficult

for the receiver to detect the signal, especially

if there are high levels of ambient light. The

accepted method of overcoming this problem

is to switch the transmitter diode with a carrier

frequency that is modulated by the data. The

receiver electronics are relatively simple and

can be easily integrated along with the IR

receiving diode to produce a compact device

with good sensitivity and immunity to ambi-

ent light levels.

There are many integrated receivers avail-

able and they generally operate with a carrier

frequency in the range of 30 kHz to

40 kHz. Much remotely controlled

household equipment employs the

RC5 encoding standard, this has a

carrier of 36 kHz. We will also use

this frequency in our design.

The circuit diagram for the IR key-

board transmitter is reproduced here

as Figure 1 . It was fully described in

the February 2000 edition of Elek-

tor Electronics , but just to re-cap; a

36 kHz carrier frequency is gener-

ated by the microcontroller, this is

modulated by the code of the

pressed key. This signal then drives

the transmitting diode D1. Using a

standard receiver, it is possible to

achieve a line of sight range of about

10 m. Most walls and ceilings offer

good reﬂectivity to Infrared so a line

of sight link is not strictly necessary.

The communications protocol

used for this project does not comply

with any IR communication stan-

dard. It was chosen as a compromise

between low current consumption,

good data security and simplicity of

decoder design. Each keypress gen-

erates a message comprising 1 start

bit, 8 data bits, 1 parity bit and 1

stop bit. The parity bit ensures that

the number of 1’s in the data (includ-

ing the parity bit) is always an even

count. A typical message generated

by a keypress is shown in Figure 2.

The infrared transmitting diode is

Elektor Electronics

12/2000

IR Receiver

COM PUTER

driven by Darlington transistor T1.

This transistor produces pulses of

current to drive the transmitting

diode from the control signal. To

increase the range of the unit, it is

possible to reduce the value of the

current limiting resistor R1 and

replace D1 with a high intensity IR

LED. In principle, any IR LED can be

used for this transmitter. The short

pulses of current are not sufficient to

generate excessive power dissipa-

tion in the transistor.

start bit 6 ms H,

6 ms L

data bit "1"

2 ms H, 4 ms L

data bit "0"

2 ms H, 1 ms L

stop bit

2 ms H

990090 - 13

Figure 2. Modulation signal format.

output lines. These lines can be used

to control or switch external equip-

ment. Alternatively the receiver can

be conﬁgured to output the hex code

for each key press of the keyboard.

Applications for this design range

from simple control of remote lamps

or devices to a remote front panel for

some test equipment. Building keypads into

test equipment can often make the equip-

ment design too complicated. This remote

keyboard could also help to simplify testing

procedures.

The IR signal is received by a sensitive IR

detector (IC4). This device includes an IR

photodiode, ﬁlter, ampliﬁer and demodulator.

The output signal is fed directly to an input of

the microcontroller (IC2).

A low-pass filter formed by R1 and C1

removes noise and ripple from the supply

voltage.

In order to make the receiver as versatile

as possible it can be conﬁgured to output the

key presses in two ways:

Receiver

The receiver is shown in Figure 3 .It

decodes the IR signal and outputs

the corresponding data to its eight

Microcontroller

The OTP-Microcontroller from National Semiconductor is particularly suited for

this application and is used in the transmitter as well as in the receiver:

- 4096 x 8 OTP EPROM

- 128 Bytes RAM

- 1 µs cycle at 10 MHz

- 16 Bit Timer with the operational modes: Timer with Auto-Reload, External

Event Counter and with Capture Function

- 16 I/O, of these 14 can be individually programmed as input or output

- Selectable Pin conﬁguration: TriState, push-pull or pull-up

- Microwire interface

- Interrupt source: External Interrupt with selectable edge, Timer Interrupt, Soft-

ware Interrupt

Mode 1

This configuration is used for direct control.

The eight output lines L0 to L7 are switched

when one of the 1 to 8 keys is pressed on the

keyboard. For example, one press of key 1 will

set output L0 high. Pressing key 1 again will

cause L0 to be reset low. LED’s D1 to D8 will

switch on when the corresponding output is

IC1

1N4148

35ms

trigger

G3/T10

RESET

IC1

IC2

1D1

470 Ω

IC3

470 Ω

93C06

G5/SK

G4/SO

G6/SI

CB1 DIN

1D2

470 Ω

100µ

DOUT

COP8782C

IC4 2

G0/INT

R11

G7/CKO

CKI

EN1

EN2

TFMS

74LS244

5360

JP1

100n

33p

10MHz

33p

coding

004055 - 11

Figure 3. The IR receiver also makes use of the COP8-Microcontroller from National Semiconductor.

12/2000

Elektor Electronics

COM PUTER

high. When the Enter key is pressed all eight

outputs will be reset low.

Project Software

The software (source code) for both COP8 controllers may be found on a

diskette with order code 004055-11.

Ready-programmed controllers are also available from Elektor Electronics:

996527-1 (transmitter)

004055-41 (receiver)

Mode 2

This conﬁguration will cause the controller to

output the hexadecimal code for the pressed

key. The output code corresponds to the scan

code of the pressed key as explained in the

February 2000 edition of Elektor Electronics.

For example:

Key Code

Esc 08H

Return 5AH

Space 29H

1CH

decoding in the receiver.

Pin G1 of the microcontroller is

used to switch between the two

operating modes of the receiver. If no

jumper is ﬁtted the pin will be pulled

high by an internal pull-up resistor,

and the receiver will operate in

mode 1. A jumper fitted to G1 will

pull this pin low and this will signal

the receiver controller to operate in

mode 2.

Output G3 provides a trigger sig-

nal whenever a key is pressed on the

transmitter keypad. This can be

used by external equipment to cap-

ture and latch the new key value. It

has a pulse width of approximately

35 ms and is output in both modes of

operation.

R2, C2 and D9 generate a power-

up reset for the microcontroller.

Bus driver chip IC1 buffers the

output from the controller and can

supply a useful 15 mA from each out-

put pin. The controller itself can only

supply 3 mA per pin, but in applica-

tions where this is sufficient, IC1 can

be omitted. In this case the output

LEDs will not be ﬁtted either.

The receiver also includes a small,

economical serial EEPROM that can

store up to 32 bytes of information.

Only one byte of this memory is

used. When the receiver is switched

off, it will store the state of its output

pins L0 to L7 in this EEPROM and

read them out at power-up to ensure

that the output pins remember their

last state. This device can also be

omitted if this feature is not impor-

tant in you application

1BH

This hexadecimal code will appear on the

output pins L0 to L7 and can be used for your

chosen application. Each time a new key is

pressed, it will simply overwrite the last code

at the output.

The article ‘PC Keyboard Encoding’ in the

February 2000 edition of Elector Electronics

gives an insight into how versatile a PC key-

board can be and how it can be reconﬁgured

for different uses. In our application, the con-

troller in the IR transmitter conﬁgures the key-

board to use Scan Code Set 3. The keyboard

will therefore output only one keycode when

a key is pressed, for example, pressing key 1

will output the value 16hex. This simplifies

(004055-1)

Elektor Electronics

12/2000

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