SC_Radar_Speed_Gun_Pt2.pdf

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Last month, we
unveiled our
new low-cost
.
mIcrowave
Doppler Radar
Speed Gun,
designed for
nleasuring the
speed of cars,
bikes, boats, horses
and even human
sprinters. This
month, we show you
how to build
it
and
describe how
it
is
used.
Pt.2: By JIM ROWE
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EXPLAINED last month, all
the components in our new Dop­
pler Radar Speed Gun are on two
PC boards. The smaller DOPPLER1
board contains the microwave head
circuitry and fits inside a small shield
box attached to the underside of the
coffee-can antenna barrel. By con trast,
the larger DOPPLER2 board carries
the counter/display unit circuitry and
fits inside a standard UB 1 plastic uti
1­
ity box.
The two units are linked by a single
cable that's fitted with a Type A USB
plug at each end.
Although the larger DOPPLER2
board has more components on it than
the smaller board, it's a little easier
44
SILICON CHIP
Fi~
as
sin
A
s
to assemble because it's only single­
sided and the components are all of
the familiar "leaded" type. This being
the case, we're going to assemble this
board first.
Counter/display board
Fig.5 shows the assembly details
for the counter/display board. Begin
by fitting the 12 wire links. These can
all be made using tinned copper wire
or resistor lead offcu ts, except for the
one located just below transistor Q3.
This link should be made from a short
length of insulated hookup wire, be­
cause it runs quite close to a lead from
the 1kQ resistor just below it.
Once all the links are in place, fit
the six 1mm PC board terminal pins
which are used for the three test points
and their accompanying ground con­
nections. Follow these with the four
3-pin headers which are provided for
reprogramming the timebase divider
links LK1-LK4, just below the right­
hand centre of the board. The three
connectors along the bottom edge of
the board can then go in - ie, CON1
(the USB Type A connector), CON2
(the mini stereo headphone jack) and
CON3 (the concentric DC input con­
nector).
Now fit the IC sockets, taking care
to orientate each one with its "notch"
end towards the left, as shown on the
overlay diagram. This will help ensure
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Fig.5 (right): install the parts on the counter/display board
as shown here. Above is the completed prototype, which has
since been modified to include the HoldlRun switch facility.
DOPPLER SIGNAL
FROM HEAD UNIT
PHONES
pins
oints
con­
four
d for
,ider
light­
:hree
ge
of
ONl
'ON2
rand
r.on­
care
Itch"
1
the
Isure
that you later fit the lCs the correct
way around.
The resistors can go in next, fol­
lowed by lrimpot VR1. Be sure to fit
the correct value resistor in each loca­
tion. Table 1 shows the resistor colour
codes but we also recommend that you
check them using a digital multimeter,
as some of the colours can be difficult
to decipher.
The small ceramic, monolithic and
metallised polyester capacitors can
now all be mounted. These capaci­
tors are all non-polarised, so they can
be fitted either way around. Follow
them with the electrolytics which are
of course polarised, so take care to fit
them with the correct orientation.
siliconchip. com.au
That done, fit the 38kHz crystal (Xl).
As shown on Fig.5, this mounts on its
side, with both leads bent downwards
about 2mm from the case so that they
pass through the holes in the PC board.
Solder its leads to the pads under­
neath, then fit a small U-shaped piece
of tinned copper over the crystal's case
to secure
it
in position (the ends ofthe
wire "U-loop" are soldered to match­
ing pads on the the board).
Now for the semiconductors. Begin
with the two diodes, taking care to in­
stall them with the correct orientation.
Also, be sure to use the lN4004 power
diode for Dl and the smaller lN4l48
diode for D2.
Follow these with three 7-segment
LED displays. These must all be ori­
entated with their decimal point LEDs
(which we don't use here) at lower
right. The four transistors can then
go in - the PN100 device goes in the
Q4 position (near the bottom of the
board), while the three PN200 devices
go in the Ql-Q3 positions below the
displays.
Finally, plug the eight lCs into their
sockets, taking care to ground both
yourself and the PC board earth be­
fore handling them. This is necessary
because most of the lCs are CMOS
devices and are vulnerable to damage
from electrostatic discharge.
Your counter/display board is now
finished and can be placed aside
DECEMBER 2006
m.au
45
SOLDERING IRON TIP
UPPER TIP OF
CROSSOVER TWEEZER
'HOLD DOWN')
I
COPPER TRACK
AND PAD
' - - - - PC BOARD
~LOWERTIPOF
~
CROSSOVER TWEEZER
SOLDERING IRON TIP
'-..
"'­
CD
HOLDING SMD CHIP IN PLACE
WHILE APPLYING A TINY SOLDER
DROP WITH SOLDERING IRON
TIP TO 'TACK' ONE END
HOLDING SOT, SOIC OR SIMILAR
SEMICONDUCTOR DEVICE IN
PLACE WHILE TACKING ONE PIN
BOTH LEADS CRANKED DOWN AND CUT
SHORT FOR 'SURFACE MOUNTING'
~
@
SOLDER TACK NOW
HOlDING CHIP
IN PLACE
.--/
~~
®
..,I,
SOLDER TACKED LEAD
NOW HOlDING
DEVICE IN PLACE
.--/
ONE LEAD CRANKED FOR SURFACE
MOUNTING, THE OTHER END BENT
DOWN FOR THROUGH HOLE MOUNTING
OTHER END OF SMD CHIP NOW
SOLDERED TO PAD IN
NORMAL WAY
PIN OR PINS ON OTHER SIDE
OF DEVICE NOW SOLDERED
TO PAD(S)IN NORMAL WAY
~
CD
FIRST END FINALLY
RE-SOLDERED IN
NORMAL WAY
~
Fig.7: the leaded resistors are
soldered to the PC board as
shown here.
@
PIN ON FIRST SIDE
RE-SOLDERED, OTHERS
SOLDERED IN NORMAL WAY
Fig.6: this diagram shows how the various SMD parts are soldered to the
PC board. Be sure to use a temperature-controlled iron fitted with a very
fine chisel-shaped bit and take care not to overheat the tiny components.
while you assemble the microwave
head board. Note that we haven't
discussed the Hold switch (Sl) at this
stage, because it mounts on the box lid
and is only connected to the display
board later.
Microwave head board
This second board is considerably
smaller than the first but is more chal­
lenging because about half ofthe com­
ponents on it are small surface-mount
devices. It's also double sided but this
shouldn't cause you any problems
because the board supplied in the kits
will have plated-through holes and
solder masking on both sides.
Only one component mounts on the
underside of the board - the Type A
USB socket. Everything else mounts on
the top of the board, because virtually
all of the underside copper is used as
an earthed ground plane and shield.
Before you begin fitting any com­
ponents to this board, examine the
46
SILICON CHIP
overlay diagram ofFig.8 to familiarise
yourself as to how it all goes together.
That done, begin the assembly by fit­
ting the surface-mount (SMD) parts.
To do this, you'll need a soldering
iron with a very fine chisel-shaped
tip, which you need to keep particu­
larly clean. Ideally, it should also be
a temperature-controlled iron, so it
doesn't get too hot and damage the tiny
components. In addition, you'll need
a small pair of crossover tweezers to
handle the SMD parts and a wooden
toothpick to hold each part down
while you solder it in position.
You'll also find an illuminated mag­
nifier a big help - especially if it's on
the end of a spring-loaded arm, so you
can place it in just the right position
above the PC board.
By the way, manually soldering
SMD parts in place isn't all that dif­
ficult
if
you tackle them carefully
and one at a time. Fig.6 shows how to
solder both passive and active SMD
parts to a PC board.
You should fit the SMD parts to
the head-end board in the following
order:
(1) the loon 0805 resistor at upper
right;
(2) the InF 1206 ceramic capacitor
near the top of the board, just to the
left of centre;
(3) the five 10nF 1206 ceramic capaci­
tors;
(4)
oscillator transistor Ql (this must
be orientated with its "fatter" collector
lead at upper right);
(5) mixer diode Dl, orientated with its
"two-lead" side towards the antenna
microstrip line on its right;
(6) the ERA2-SM microwave amplifier
chip (JCll, orientated with its locat­
ing dot and diagonal-cut end (pin 1)
towards the bottom of the board;
(7)
RFC1, the UHF choke, which is the
largest of all the SMD devices (orien­
tated with its pin 1 identification dot
at lower right).
That completes the trickiest part of
the board assembly and you should
now be ready to fit the rest of the
parts.
Begin this second phase by fitting
the USB connector, which mounts on
the underside of the board. It's fitted in
the normal way by carefully pushing
siliconchip. com. au
ANTENNA WIRE
(28mm FREE LENGTHI
BLOBS
Of
EPOXY
CEMENT USED TO
GLUE BODY
Of
lllf
TANTAlUM
CAPS TO TOP
OF
PC
BOARD
220pF ELECTRO
HELD DOWN
USING 'U' OF
HOOKUP OR
TINNED COPPER
WIRE
CONI
(UNDER PC BOARD)
Fig.8: the microwave head board carries quite a few surface mount devices (SMDs) and these should be
mounted first, as described in the text. The USB connector mounts on the underside of the board.
all its connection leads and mount­
ing clips tlU'ough the matching board
holes, then soldering them to the pads
on the top of the board.
Next, fit the leaded resistors but note
that most of these are mounted in a
slightly unorthodox way - either with
both end leads cranked down and cut
short for "surface mounting" or with
only one lead dressed this way and
the other bent down in the usual way
to pass through a board hole.
Fig.7 shows how the leads are pre­
pared and the resistor fitted to the PC
board in each case.
Start with the resistors that are fitted
""ith one end passing down through
the board hole. These are:
(1)
the lOOQ
resistor which connects the emitter of
Ql to ground; (2) the lkQ load resistor
for mixer diode Dl; and (3) the 470Q
DC return resistor between the antenna
microstrip line and ground.
In all three cases, it's the lead at
the "earthy" end of the resistor which
passes down through the board hole.
These leads are then soldered to the
copper pads on both sides ofthe board.
In contrast the "cranked down" leads
at the other ends of these resistors
are soldered only to the pad on the
top layer.
The remaining "leaded" resistors
can now be installed. Three of these
have both leads cranked down as at
the top of Fig.7 - ie, the lokQ collec­
tor load resistor for Q2, the 1.5MQ bias
resistor for Q2 and the lOOQ collector
resistor for Q1.
The last leaded resistor to fit is the
lokQ bias resistor for Ql, which is fit­
ted in a different way again. As shown
in Fig.8, this resistor is fitted alongside
the lOOQ collector resistor. One lead
is bent down and over before cutting
0
0
0
0
0
0
:J
Q
0
0
[J
0
siliconchip.com.au
No.
1
1
1
1
6
4
2
6
1
2
4
7
2
3
2
Value
2.2MQ
1.5MQ
1MQ
330kQ
100kQ
47kQ
22kQ
10kQ
6.8kQ
4.7kQ
1kQ
680Q
470Q
100Q
47Q
4·Band Code (1%)
red red green brown
brown green green brown
brown black green brown
orange orange yellow brown
brown black yellow brown
yellow violet orange brown
red red orange brown
brown black orange brown
blue grey red brown
yellow violet red brown
brown black red brown
blue grey brown brown
yellow violet brown brown
brown black brown brown
yellow violet black brown
5-Band Code (1%)
red red black yellow brown
brown green black yellow brown
brown black black yellow brown
orange orange black orange brown
brown black black orange brown
yellow violet black red brown
red red black red brown
brown black black red brown
blue grey black brown brown
yellow violet black brown brown
brown black black brown brown
blue grey black black brown
yellow violet black black brown
brown black black black brown
yellow violet black gold brown
DECEMBER 2006
47
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MATERIAL: O.3mm OR O.25mm BRASS SHEET
BEND DOWN
I
~
___
Fig.9: the shield box which encloses the microwave head board
is made from a rectangular piece of O.3mm or O.25mm-thick
brass sheet. Cut
it
out as shown in this diagram and fold down
the sides to form the box.
it short, so that it can be soldered to
the same pad on Ql'S collector line
as the loon resistor. The other end is
then bent around in a hairpin shape
and then down, so that
it
can be sol­
dered to the copper pad just below
the base lead for Ql, where the two
lOnF SMD bypass capacitors are also
connected.
Be sure to cut this lead to length
before you solder it, as it's not easy to
cut off the excess afterwards.
The four lOnF leaded monolithic
capacitors are next on the list. These
all use the same arrangement used
for some of the resistors - ie, one
cranked lead and one lead bent down
for through-hole mounting.
It's just a matter of carefully dress­
ing their leads and cutting them to
length before fitting them. The leads
that pass through the board holes are
again soldered on both top and bottom
sides of the board.
Diode 02 goes in next and this is
fitted in the same way as the resistor
shown at the bottom of Fig.? Make
sure that it's the anode lead that passes
down through the earthing hole.
Transistor Q2, the
PN100
leaded
transistor, can now be installed. This
is again fitted in an unusual way: its
---,.
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-t'rl'­
500g
INSTANT COFFEE CAN 'B'
(127
DIAMETER,
173
LONG)
WITH TOP INSIDE FLANGE AND
_ _ _ _ BOTTOM BOTH REMOVED­
-
500g INSTANT COFFEE CAN 'A'
(127
DIAMETER,
173
LONG)
WITH TOP INSIDE FLANGE REMOVED
_ _ _ _ BUT BOTTOM LEFT I N T A C T _ _ _ _ _ .
6mm DIAMETER
CLEARANCE HOLE IN
CAN~
(POSITION SHIELD BOX SO
ANTENNA CUTOUT IS CENTRED
OVER THIS HOLE BEFORE SOLDERING)
1 -
47 .5 ­
~'"
PLASTIC LID FROM COFFEE
CAN CLIPPED OVER OPEN
END TO KEEP OUT
DUST
&
MOISTURE
SOLDER FILlET ALL
AROUND BUmD RIMS
CANS JOiNED BYMNE SEAMS CANIS/V
UP
Of
AT THE BOTTOM
(FOR NEATNESS)
SOlDER FILLET ALL AROUND
SHIELD BOX TO FORM
UNBROKEN SEAL WITH
ANTENNA CAN
(HEAD UNIT
PC BOARD
IN
SHIElD
BOX)
AlL DIMENSIONS IN MlllIMETRES
NOTE THAT BOTH COFFEE CANS
ARE SHOWN CROSS SEcnONED
FOR ClARITY
Fig.10: the antenna barrel for the microwave head unit is made from two coffee cans. It's made by first drilling a
hole for the antenna in can "A", then soldering the two coffee cans together to form the barrel (see text).
48
SILICON CHIP
si/iconchip. com. au

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