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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. ~