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Delay unit for
automatic antennas
You move the ignition key to the ON position
and your car's antenna starts up. Then when
you move to the START position, the antenna
retracts again. This simple delay circuit will
stop the antenna jitters.
By GREG SWAIN
Ever tried to buy a replacement
automatic antenna for a Holden
Commodore? I'll bet that you got the
same shock that I did. Oh yes,
they're readily available but the
$220 asking price is really "over
the fence ".
I didn't go ahead with the purchase. Instead, I took the alternative approach and bought an
aftermarket antenna from an auto
accessory shop for about 1/3rd the
price of the GM part.
"We sell lots of these for Commodores", the salesman assured
me. "It's easy mate. You just plug
straight into the existing wiring".
Oh yeah? To cut a long story
short, it's not quite as simple as
that. Mounting the antenna was no
problem but there was no way it
would work with the existing wiring. I eventually solved the problem
by unplugging the antenna control
box inside the car and installing a
couple of jumper leads across the
plug.
Antenna control
When the radio was turned on,
the original antenna would rise to
about half-height after a delay of
+12 V VIA
IGNITION
SWITCH
O
Ii
1 1 1 1 11
IO
SWITCHED +12 V
r-_CO_NT_RO_L_LI_NE--i
AUTOMATIC
ANTENNA
Jt·- ---- +12V
FROM
BATTERY
CAR RAOIO
Fig.1: how the antenna is normally wired up. The antenna extends
when the control line from the radio switches to + 12V.
+12V VIA
IGNITION
SWITCH
+12V FROM
BATTERY
t
AUTOMATIC
ANTENNA
CO NTROL LINE
CAR RADIO
Fig.2: the delay unit is installed in the control line between the
radio & the antenna. It holds the control input to the antenna low
for 4 seconds after the control line switches high & high for 11
seconds after it switches low.
30
SILICON CHIP
.-------i.,..__
100
10P.
____,.~,W,,.-------0+12V FROM
BATTERY
+
16VW+
D1
1N4002
SWITCHED +12V
CONTROL LINE C>--1-+l-+---"\M\---"f
FROM RADIO
01
BC328
IC1
555
s__.,____
TO CONTROL LINE
0 OF ANTENNA
,---eC
220k
.,.
~
l
.01
D2
1N4002
I
RmY
.,.
180!1
.,.
ANTENNA DELAY UNIT
.,.
B
eOc
VIEWED FROM
BELOW
Fig.3: 555 timer ICt is wired as a monostable. When the control line goes
high, pin 3 switches low after 4 seconds and turns on Qt and the relay.
When the control line goes low, pin 3 goes high 11 seconds later and Qt
and the relay switch off.
four seconds. It could then be raised or lowered to any height required using pushbutton up/down
switches mounted on the dashboard. Similarly, when the radio (or
the ignition) was turned off, the
antenna would fully retract after a
delay of about seven seconds.
Because the control box has now
been removed, the dashboard switches no longer work and the new
antenna is either fully up or fully
down (which is really more convenient). Fig.1 shows how the antenna
is wired up. It has three leads: one
goes to + 12V from the car's battery, one goes to ground and the
third goes to a switched + 12V output from the radio.
The way in which the antenna
works is quite straightforward.
When the radio is turned on the
control line goes high and the
antenna extends. Conversely, when
the radio (or the ignition) is switched off, the control line drops to 0V
and the antenna retracts.
The control line, by the way, controls a changeover relay inside the
antenna base. This in turn switches
the direction of the current through
the antenna motor to provide up or
down operation as required.
There's just one problem with the
circuit of Fig.1: the antenna starts
up as soon as the ignition is switched to the ON position when starting
the car [assuming the radio switch
is on), but then immediately
retracts when you go to the ST ART
position. If the engine doesn't fire
quickly, the antenna can have
several false starts as the ignition
switch is moved between the ST ART
and ON positions.
All this stopping and starting inevitably causes wear on the motordriven nylon rack inside the antenna and can lead to premature
failure of the drive system.
Antenna delay unit
Our Antenna Delay Unit solves
that problem. As shown in Fig.2, it
is installed in the control line between the radio and the antenna. It
works by holding the control input
to the antenna low (ie, at 0V) for a
further four seconds after the ignition has been turned on. At the end
of this 4-second delay period, the
output of the Antenna Delay Unit
goes high (ie, to + 12V) and the
antenna then extends in the conventional manner.
This 4-second delay period gives
you ample time to switch through
the ON position and get the car
started before the antenna is
activated.
Conversely, when the ignition (or
the radio) is switched off, the
Antenna Delay Unit holds the control input to the antenna high for a
further 11 seconds before dropping
to 0V. This means that the antenna
now stays extended for 11 seconds
after the ignition has been switched
off before retracting.
This feature is useful if the
engine stalls soon after starting or
if you switch off the ignition but immediately decide to restart the
engine to move the car. It also
allows you to switch off the engine
and switch through to the ACC (accessory) position without having the
antenna start retracting as you go
through OFF.
In practice, the Antenna Delay
Unit will greatly reduce the number
of antenna operations. This translates into less wear on the nylon
rack and greatly extended antenna
life. It also means more convenient
operation - it's quite annoying
having the antenna stop and
change direction after moving only
a few centimetres.
Circuit details
PARTS LIST
1 PCB, code SC05103901,
57 x 65mm
1 1 2V SPOT PC-mount relay
4 PC stakes
Semiconductors
1 555 timer {IC1)
1 BC328 PNP transistor {01)
1 15V 1W zener diode (ZD1)
2 1 N4002 diodes {D1 ,D2)
Capacitors
1 1 OOµF 25VW electrolytic
capacitor
1 4 7 µF 16VW electrolytic
capacitor
1 .01 µF metallised polyester
Resistors {0.25W, 5%)
1 220k0
1 4700
1 47k0
1 100
Fig.3 shows the circuit details of
the Antenna Delay Unit. It uses a
555 timer IC (what else?), plus a
transistor, a relay and a few other
minor components.
The 555 timer (ICl) is wired as
a monostable oscillator and is
powered directly by the + 12V rail
from the car battery. Let's assume
initially that the control line from
the car radio is low (ie, the ignition
is off). This means that the 4 7µF
timing capacitor on pins 2 & 6 will
be discharged and thus pin 3 of ICl
will be high.
Pin 3 of ICl controls Ql which is
a BC328 PNP transistor. When pin
3 is high, Ql and the relay are off,
and the control input to the antenna
is low.
Assume now that the ignition is
MARCH 1990
31
0
ASSIS
I
~
NTROL LINE
ANTENNA
1
HED + 12V CONTROL
E FROM RADIO
•+
,LO
V FROM
TTERY
100uF
0
u
Z01
en
1
.2.1
Fig.4: parts placement on the PCB. You can install the
parts in any order but leave the relay until last. Fig.4
(right) shows the PCB artwork.
suddenly switched on. When this
happens, the control input jumps to
+ 12V (assuming that the radio was
on in the first place) and the 47µF
capacitor charges via Ill and the
47k0 resistor. After about 4
seconds, the voltage across this
capacitor reaches 2/3Vcc (ie, 2/3 of
the supply rail) and pin 3 of !Cl
switches low and turns on Ql.
Ql, in turn, switches on the relay
which connects the + 12V rail to
the control input of the antenna.
The antenna now extends in the
conventional manner (ie, 4 seconds
after the ignition was switched on).
Switching off
If the ignition is now switched
off, the control line from the radio
goes low again and the 47 µF timing
capacitor on pins 2 & 6 discharges
via its parallel 220k0 resistor.
After about 11 seconds, the voltage
across the capacitor reaches
1/3Vcc and pin 3 switches high and
the relay turns off.
This switches off the + 12V supply to the antenna control line and so
the antenna now retracts. Diode Ill
prevents the timing capacitor from
discharging into the switched control line from the radio, while D2
protects Ql from any back EMF
generated by the relay.
Power for the circuit is derived
directly from the car battery. A 100
resistor and lOOµF capacitor provide decoupling and filtering of the
+ 12V supply rail, while ZDl protects !Cl and Ql from any high
voltage spikes that may be on the
line.
32
SILICON CHIP
Construction
The parts for the Antenna Delay
Unit are all mounted on a small PCB
coded SC05103901 and measuring
57 x 65mm. Fig.4 shows the parts
layout on the PCB.
Before starting construction,
take a good look at the copper side
of the PCB and compare it with the
published pattern. Any board
defects such as open circuit or
shorted tracks are best corrected
at this stage.
You can now install the parts on
the board. Install four PC stakes at
the external wiring points first and
leave the bulky relay until last.
Note that many of the parts are
polarised so be sure to install them
the right way around. These parts
include the IC, transistor, diodes
and electrolytic capacitors. It's
easy to identify pin 1 of the IC - it's
adjacent to a notch on one end of
the plastic body.
The PCB can be installed inside the
car behind the kick panel or housed
in a watertight plastic and mounted
with the antenna.
To test the unit, apply power and
connect a clip lead between the
control input and the + 12V rail.
The relay should pull in after about
4 seconds and drop out 11 seconds
after the clip lead is removed.
Installation
The Antenna Delay Unit can
either be mounted inside the car or
installed adjacent to the antenna
under the mudguard. If you choose
to mount the unit inside the car, we
suggest that you install it behind the
kick panel, adjacent to the access
hole for the antenna leads.
On most modern cars, the antenna will be hidden behind a plastic
panel secured to the inside of the
mudguard by self-tapping screws.
Remove this panel, then use your
multimeter to identify the leads to
the antenna. In most cases, you will
find that the connections to the
antenna are made via bullet
connectors.
A splashproof plastic case should
be used to house the PCB assembly
if you decide to mount the unit with
the antenna. This can be secured
either to the antenna mounting
bracket or directly to the antenna
body itself (possibly by using plastic
cable ties). Seal all holes in the case
with silicone sealant to prevent
water entry.
Finally, use an automotive crimp
connector kit to terminate the four
leads from the PCB. That way, it
will simply be a matter of unplugging the existing wiring and then
plugging the Antenna Delay Unit into circuit.
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