e033018.pdf

MICRO CONTROLLER

Two-Colour

Running Light

with microprocessor control

Design by A. Köhler

Lighting effects are always popular and we frequently publish such projects

in the pages of Elektor Electronics . Here is a circuit that uses a

microcontroller to drive up to 96 bicolour LEDs in a wide range of

patterns. A particular feature of the design is the way it exploits the

persistence of human vision.

Instead of `regular’ LEDs this running light

uses so-called bicolour LEDs which can emit

two different colours, usually red and green.

These are available in versions with three

connections (a common cathode and sepa-

rate anodes) or with two connections. We

use the second type in this circuit, where the

two LEDs are connected in antipar-

allel so that one LED or the other

lights according to the direction of

current ﬂow.

A special circuit is required to

drive the bicolour LEDs. Since the

LEDs generally only require a current

of between 5 mA and 20 mA to light

with sufficient brightness, the circuit

does not have to switch a particu-

larly large current. HCT logic devices,

which can sink or source up to 8 mA,

can therefore safely be used. The

bicolour LEDs are connected

Elektor Electronics

3/2003

MICRO CONTROLLER

between two such outputs. If the

two outputs are at the same logic

level, both LEDs will remain dark; if

the levels are different either the red

or the green LED will light, according

to the direction of current ﬂow.

Since every LED will require two

outputs, they cannot be driven

directly from the microcontroller’s

outputs. A serial drive circuit is

therefore used that requires just two

signals: with just a data signal and

a clock signal we can control a large

number of LEDs. The serial-to-paral-

lel conversions are done by HCT

shift registers. In the first phase of

operation the data bits are shifted

rapidly through the shift registers: so

rapidly, in fact, that this operation in

not visible to the naked eye. In the

second phase of operation the data

remains at the outputs of the shift

registers for a longer period and thus

appears stable. After a brief pause a

new pattern is loaded into the shift

registers. With a suitable physical

arrangement of the LEDs the effect

can be improved still further.

+5V

20mA

16V

100n

4x 10k

RST

IC1

2345

RXD/P3.0

TXD/P3.1

P1.0

P1.1

INT0/P3.2

INT1/P3.3

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

T0/P3.4

T1/P3.5

89C2051

P3.7

22k

547B

234

33p

1N4148

7.3728MHz

010134 - 11

IC2

7805

+5V

The microcontroller...

The circuit consists of a central

microcontroller unit and up to four

bicolour LED modules. The arrange-

ment shown in Figure 1 is a

straightforward microcontroller cir-

cuit employing an Atmel 89C2051.

The data sheet for this device can be

found at www.atmel.com/atmel/

acrobat/doc1045.pdf . Components

C3 and R4 form a power-on reset

network. A primitive RS232 interface

is provided via pin 2 for receiving

data only; the transmit function via

pin 3 is not implemented. Transistor

T1 performs the voltage level con-

version. D1 limits negative input

voltages, while R3 limits the base

current of T1 as well as the current

through the diode. This circuit

removes the need for the charge

pump circuit that would otherwise

be required.

The oscillator connected to pins 4

and 5 uses a 7.3728 MHz crystal

from the junk box. This oscillator fre-

quency restricts the choice of avail-

able RS232 baud rates, but simpliﬁes

the programming of longer delay

loops. Any other crystal could of

course be used, although a few delay

constants in the software would then

1N4001

470

16V

100

10V

Figure 1. This minimal microcontroller circuit can control up to 96 bicolour LEDs.

have to be adjusted to suit.

Pins 6 to 9 are connected to a DIP

switch with pull-up resistors.

Depending on the software in the

microcontroller, these can be used,

for example, to select between light

patterns, or to set the baud rate of

the RS232 interface. Neither of these

options is yet implemented in the

software.

The LED modules are controlled

via port 1. Each has a data signal

(P1.0, P1.2, P1.4 and P1.6) and a

clock signal (P1.1, P1.3, P1.5 and

P1.7) to control a group of LEDs via

their shift registers. Since the con-

nections are short no protection cir-

cuitry is necessary. A point to note is

the provision of pull-up resistors on

port pins P1.0 and P1.1. These pins

serve as non-inverting and inverting

inputs (respectively) to an internal compara-

tor. Without pull-up resistors of less than 2

the LEDs would not be driven correctly:

the other pins of port 1 have internal pull-ups.

The current drawn remains under the 15 able

carrying data and clock as well as power.

This is derived from a mains power supply

which must have an output voltage of at least

9V and be able to deliver a current of 200

mA per LED module plus 20 mA for the con-

troller board. That can add up to a sizeable

total current, and so a heatsink must be ﬁtted

to voltage regulator IC2.

... and the four LED modules

As can be seen from Figure 2 each LED mod-

ule consists of 24 bicolour LEDs and six

74HCT164 shift registers. The data bits are

presented to the data inputs (pins 1 and 2)

of IC1, the ﬁrst shift register, and then taken

3/2003

Elektor Electronics

MICRO CONTROLLER

+5V

74HC164

9 8

IC2

IC4

IC6

SRG8

IC4

IC5

IC6

+5V

R10

R11

R12

R13

R14

R15

R16

R17

R18

R19

R20

R21

R22

R23

R24

D10

D12

D14

D16

D18

D20

D22

D24

100n

D11

D13

D15

D17

D19

D21

D23

+5V

IC1

IC3

IC5

IC1

SRG8

IC2

SRG8

IC3

SRG8

+5V

200mA

74HC164

+5V

010134 - 12

Figure 2. Two shift register outputs are used per bicolour LED.

from the ﬁnal output (pin 13) to the inputs of

the second shift register. In this way a chain

is formed of the six shift registers, which are

all clocked together (by pin 8).

The LEDs are connected to the outputs of

the shift registers with series current-limiting

resistors. A value of 390

Insulated wire should be used for

the links, since in places they run

close alongside one another, have

bends or pass under IC sockets.

Apart from that, there is little else to

say about the circuit board.

following routines.

TAKT 1 to TAKT 4 (clock 1 to clock 4)

These routines generate the clock

signals for the shift registers. In

order to allow the modules to be

updated at different times, each

module is provided with its own rou-

tine.

gives a current in

the red LED of 9 mA, in the green of 7 mA.

This is at the limit of what the logic ICs can

supply, and does indeed give adequate

brightness.

Populating the circuit boards should not

present any difficulties, as long as the `small-

est components ﬁrst’ principle is adhered to.

Watch out for the three wire links. The micro-

controller on the control circuit board ( Figure

3 ) should be fitted in a socket. If connection

to a PC will not be required, the sub-D con-

nector and level shifting circuit can be dis-

pensed with. Also, if the DIP switch will not

be used, it too can be dispensed with, along

with its pull-up resistors. Unused inputs

should however be tied to a fixed voltage

level, RxD to +5 V.

The single-sided printed circuit board for

the LED module could not be routed without

wire links, as clearly shown in Figure 4 .

Software variants

Now that we have seen that the

hardware does not contain any big

surprises, the key to the circuit must

lie in the software. The software can

be downloaded free of charge from

the Elektor Electronics website

(under this month’s Free Downloads:

look for number 010134-11 ). The soft-

ware is available not just as `ready

to burn’ hex code, but also as assem-

bler source code.

The basic version of the software

does not use the DIP switch. It

could, for example, be used to select

between a number of different pat-

terns; this could also be done via the

serial port.

The software is built around the

MUAU 1 to MUAU 4

These routines emit the patterns,

and are designed for LED modules

with six shift registers, the number

of shift registers being given in R2.

R1 is used as a bit counter. A byte is

fetched from the pattern table and

split into individual bits via the carry

ﬂag. When a complete byte has been

transmitted, the next byte is fetched

from the table.

ZEIKO (time correction)

To allow all modules to use the same

pattern, the pattern table pointer

must be adjusted after each module

has been processed.

Elektor Electronics

3/2003

MICRO CONTROLLER

COMPONENTS LIST

Controller board

010134-1

IC2

Resistors:

R1 = 4-way10k

SIL array

R2 = 2k Ω

R3 = 22k

R4 = 10k

Ω

R5,R6 = 1k Ω

Capacitors:

C1,C2 = 33pF

C3 = 10 µ F 16V radial

C4 = 470

F 16V radial

C5 = 100

F 16V radial

C6 = 100nF

Semiconductors:

D1 = 1N4148

T1 = BC547B

IC1 = AT89C2051-12PC, programmed,

order code 010134-41

IC2 = 7805

Figure 3. The microcontroller printed circuit board.

Miscellaneous:

K1 = 9-way sub-D socket, angled pins, PCB

mount

K2-K5 = 5-way SIL pinheader

K6 = mains adaptor socket, PCB mount

PCB, order code 010134-1

Disk, order code 010134-11

LOE1, LOE2 (clear 1, clear 2)

These routines shifts zeros into all

the shift registers of one module,

thus extinguishing all the LEDs.

ZEIT (time)

This is a small delay loop that slows the dis-

play down to a reasonable speed. This speed

can easily be adjusted.

The largest part of the program consists of

the nine pattern tables. With the help of the

documentation it should be possible to

replace the contents of the tables with your

own patterns.

(010134-1)

COMPONENTS LIST

LED board

Resistors:

R1-R24 = 390 Ω

Capacitors:

C1,C2,C3 = 100nF

Semiconductors:

D1-D24 = bicolour LED, 2 pins, 3mm dia.

IC1-IC6 = 74HC164

Miscellaneous:

K1 = 5-way SIL pinheader

PCB, order code 010134-2

Figure 4. Beware! There are many wire links on the LED module printed circuit

board.

3/2003

Elektor Electronics

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