Build this simple
12/24V light chaser
Looking for a simple circuit to sink
your teeth into? This 12V light chaser
has four separate channels, variable
chase rate, fuse protection and uses
just two ICs.
By DARREN YATES
Light chasers using light-emitting
diodes (LEDs) or low voltage light
bulbs have been around for ages. You
don't have to look far for examples. If
you go down to your local video store,
chances are you'll see a line of flashing lights in the window. Believe it or
not, our august publisher recently
travelled in a "stretch-limo" which
had little LED chasers running down
the interior of the car.
We wanted something a bit more
potent than a LED chaser, so we designed this circuit to drive 12V light
bulbs. There are four channels and
each channel can handle up to 36W
with only a modest amount of heatsinking. A single rotary control lets
you vary the chase rate from about
one flash every two seconds to four
flashes per second.
To make the circuit as versatile as
possible, we also designed it to run
from a 12V to 24V DC supply. This
means that you can power it from the
mains via a 12V DC plugpack supply,
or you can use a 12V or 24V battery if
no mains supply is available.
A small PC board holds all the parts except for the external pot. If you don't
need to vary the flash rate, the pot can be replaced by a fixed value resistor.
So, if you want to jazz up the interior of your purple Monaro or panel
van, this circuit is the one to go for.
For 12V supplies, we recommend that
you use the MES (miniature Edison
screw) lamps which are rated at 3W.
This means that you can use 12 lamps
per channel, or a total of 48 lamps for
the entire chaser.
How it works
If you look at the circuit diagram of
Fig.1, you'll see that there are only a
handful of components to the 12/24V
Light Chaser. In fact, the circuit is
based on two very common ICs - a
555 timer and a 4017 CMOS decade
counter. Let's see how it works.
ICl is the 555 timer and is connected as a simple variable frequency
oscillator. Its output frequency is determined by the l0kQ and 1.8kQ fixed
resistors, the lO0kQ potentiometer
(VRl) and the l0µF electrolytic capacitor. The lO0kQ pot varies the frequency from about 0.5Hz to 4Hz.
The output at pin 3 is a pulse waveform which is fed directly into the
clock input (pin 14) of IC2 , a 4017
CMOS decade counter.
On each rising edge of the incom ing clock pulses, one of 10 outputs of
IC2 goes high in turn, starting at pin
3. Since we only need the first four
outputs and not all 10, we connected
output 4 (which is actually the fifth
output since we count from zero) to
the reset input.
When this output goes high, it resets the chip and the first output at
pin 3 goes high again. There is a finite
delay as output 4 resets the IC but
because we are operating at such a
low frequency, it is almost instantaneous in effect.
So the sequence of events is this: at
switch on, the Q0 output (pin 3) is
high. When the next clock pulse from
ICl arrives, Q0 switches low and Ql
switches high. At the next clock pulse,
Ql switches low again and Q2
APRIL 1991
53
F1
5A
100U
10
+
16VWJ
16
IC1
555
14
i : : - - - - -""1 CLK
36W
MAX
IC2
4017
Fig.1: the circuit is
based on two low-cost
ICs. ICl is a 555 timer
wired as an astable
oscillator. This clocks
decade counter IC2
which switches its
outputs high in
succession. These
outputs then drive
transistors Q1-Q4 to
switch the lamps on
and off.
~
021"4_ _ _ _~2..,.2k~-------"if-1
15
RST
04
B0649
QJ;l'-7_ _ _ _ _ _..._2Y,.2k,.__ _ _ _ _---"t-t.
BCE
13
~
12V/24V LIGHT CHASER
switches high and so on until Q4
switches high and resets IC2. In other
words, the transistors act like switches
which are opened and closed one at a
time in sequence.
Each output ofIC2, at pins 3, 2, 4 &
7, is fed to the base of an NPN Darlington transistor (Ql-Q4) via a 2.2kQ
resistor which limits the base current
to about 3.5mA. Thus, as each output
of IC2 goes high in turn , the corresponding Darlington transistor turns
on and switches on the lamp(s) in its
collector circuit.
The Darlington transistors are
TO
TO
BD649s. These can dissipate up to
60W with suitable heatsinking and
also have a gain of at least 750 (but
typically above 1000). This means that
our 3.5mA of base current is turned
into about 3A of collector current
running through each transistor.
However, since only one transistor is
on at any one time, 3 amps is the
maximum current drain of the circuit, so the load on a car battery is
minimal.
To protect the ICs from the spikes
and surges that occur in most car electrical systems , we've included a 12V
LAMPS-Fi-.
q_
8
E C~~
100~1
N
~
ZD1
(t~
s1:c::;~SIS
TO
TO
E C~
~
N
'
k
IC2
Fig.2: follow this layout diagram
carefully when installing the parts on
the PC board. Fig.1 shows the pinouts
for the four power transistors (Q1-Q4).
The lOOQ resistor should be changed to
lkQ 0.5W if a 24V supply is used.
54
SILICON CHIP
zener diode across the supply line.
This also regulates the supply rail to
the ICs to +12V if a DC supply of
greater than 12V is used. Added protection is provided by the 5 amp fuse
which fits directly on the PC board.
The circuit also has protection
against reverse connection of the battery supply, although it may not be
apparent. Each of the Darlington transistors incorporates a reverse-connected diode from collector to emitter and these will conduct if the supply is reversed, turning on all the
lamps and (possibly) blowing the fuse.
The two ICs are also protected against
reversed supply by the 12V zener
diode which acts as a forward biased
diode if the supply polarity is wrong.
Construction
The 12V Light Chaser is built on a
small PC board coded SC08106911
and measuring 105 x 67mm. Whether
you buy or make the board, check
that there are no shorts or breaks in
the tracks. If you find any, use a small
knife to scrape away the excess copper or a dash of solder to bridge the
gap a~ required.
Fig.2 shows the parts layout on the
PC board. Begin by installing 12 PC
stakes at the external wiring points,
then solder in the wire links and the
two fuse clips. Make sure that the
fuse clips are M205 versions. These
are smaller than the standard 3AG
types.
Next, install the resistors. Table 1
shows the resistor colour codes or
better still, use your multimeter to
check the values. Note that the 100Q
resistor will have to be changed to
lkQ 0.5W for 24V operation. The three
capacitors can now be installed.
Check the polarity of the two electrolytics carefully (they both face in
the same direction).
Now for the semiconductors: install the two ICs and the zener diode
as shown in Fig.2, then mount the
four power transistors (Ql-Q4). Check
Fig.1 for the transistor pin connections. When they are installed on the
board, their metal tabs should face
the centre.
The 100kQ pot is connected externally as shown on the wiring diagram. It can be replaced by a fixed
value resistor if you don't need to
vary the flash rate. If you like, you
can mount the completed assembly
inside a folded aluminium case. Each
transistor should be fitted with a small
TO-220 clip-on heatsink for loads
greater than 12W. Make sure that the
heatsinks don't short together.
It's up to you how you wire up the
lamps. If you are using the MES
(miniature Edison screw) lamps, then
matching MES sockets are probably
the most convenient. However, these
sockets cost as much as the lamps
themselves and 48 at around 50 cents
each is getting a bit expensive.
As an alternative, you might also
consider using 12V mini lamps which
come with 10mm leads. These are
more expensive than the MES lamps
but may be more suitable for some
applications.
Make sure that the lamps are correctly rated for the voltage you are
using. If you are using a 12V supply,
then you can wire groups of 12V
lamps in parallel. If a 24 V supply is
to be used, you can still use 12V lamps
but these should be wired in series
pairs of two, with each pair then wired
in parallel.
PARTS LIST
1 PC board, code SC081 06911 ,
105 x 67mm
10 PC pins
2 M205 PCB fuse clips
4 TO-220 clip-on heatsinks
1 aluminium box to suit
1 100kQ potentiometer (VR1)
Semiconductors
1 NE555 timer (IC1)
1 4017 CMOS decade cou nter
(IC2)
4 B0649 NPN Darlington
transistors (01-04)
1 12V 1W zener diode (ZD1)
Capacitors
2 10µF 16VW electrolytics
1 0.1 µF metallised polyester or
ceramic (5mm lead pitch)
Resistors (0 .25W, 5%)
1 10kQ
4 2.2kQ
1 1.8kQ
1 100Q
Testing
Once you've finished the board, you
can install the fuse and power up the
circuit. To do an initial test , use a
LED and a lkQ resistor as the load for
each transistor. If it is working cor-
Miscellaneous
Heavy duty cable, hookup wire,
solder, etc
rectly, each LED should come on in
turn and the circuit should respond
by either increasing or decreasing the
chase rate as the 100kQ pot is rotated.
If it doesn't work, check the board
thoroughly for any solder splashes
which may be causing shorts between
the tracks. In particular, check between the IC pins. This done, check
that all components have been installed correctly and that the correct
value has been used at each location.
Once the circuit is working with
the initial LED load, you can then
replace it with any load you choose
up to 36W total for each transistor; ie,
you could use 12 3W bulbs or six 6W
bulbs, etc.
SC
Fig.3: here is the full size pattern for the PC board. Use it to check that your
board has been correctly etched before mounting any of the parts.
TABLE 1: RESISTOR COLOUR CODES
0
0
0
0
0
No.
Value
4-Band Code (5%)
5-Band Code (1%)
1
4
1
1
10kQ
2.2kQ
1.8kQ
100Q
brown black orange gold
red red red gold
brown grey red gold
brown black brown gold
brown black black red brown
red red black brown brown
brown grey black brown brown
brown black black black brown
APRIL 1991
55
