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Widget Of The Week
Linwood Simple Siren Car Alarm, 1984
 Judging by recent media reports car theft has
never been easier, thanks to many modern (and usually expensive) cars being
fitted with super-secure ‘keyless’ ignition systems… A cynic might mutter something
about Murphy’s Law and chickens roosting, though to be fair nicking cars, or
‘Twocking’, (taking without consent – I know all the modern words…), has been
with us, probably since before the first Model Ts rolled off Henry Ford’s
production line. But for some reason, until comparatively recently, car makers
have given it a relatively low priority. I clearly remember the well-worn
ignition key for my first car, a scary late 60s Austin 1100, could open and
start a surprisingly wide range of vehicles, made by the likes of Ford, British
Leyland (as was) and Vauxhall. My guess is the only thing that prevented
more widespread theft was that most British cars from that era were unreliable
rust-buckets hardly worth the effort of pinching. As recently as the 1980s if you
wanted to ensure your car’s security you’d have to have an alarm fitted -- it
tended to be an optional extra on new cars – or immobilise it with a Heath
Robinson device that fitted over the steering wheel. The other option was to
fit an alarm yourself, and the Linwood Simple Siren Car Alarm
featured here was one of dozens of DIY systems available at the time.
Back then car self-fit alarms were fairly
simple affairs and most, including this one, were triggered by a small drop in
the car’s battery voltage. This could be caused by the courtesy light coming
on,when a door is opened, or switching on the headlights or sidelights. Other
types of intruder sensors were also available, like ‘tremblers’, which
responded to movement, ultrasonic motion detectors and concealed pressure
switches under the carpets and inside the driver’s seat. A keyswitch located on
the outside of the car or, in this case, the driver turning on the ignition
disables the alarm. A variable entry delay stops the alarm going off straight
away. Typically the alarm box or siren module would be mounted under the
bonnet, so an additional switch might be fitted to trigger the alarm if it was
opened. Old-school alarms like this one tended to be really loud, and would
keep going until the owner turned them off, or the battery ran down. There was
a time when they went off all the time, usually around 2am, and the ones that
used overly sensitive tremblers could be set off by a light breeze, passing
lorries, drunks and scallywags. It has got better, though and nowadays car
alarms are supposed to cut-off automatically 20 minutes, in theory at least.
 The Linwood Simple Siren, as the name implies,
was some distance from the cutting edge of alarm technology, but that also
meant that just about anyone handy with a screwdriver could fit it, and at
around £20, it was a fraction of the price of a professionally installed
system. There seems to be two schools of though about the visibility of car
alarms; most are hidden away inside discrete black boxes, indistinguishable
from the rest of a car’s electrical gubbins. This one takes the opposite
approach, and there would be absolutely no doubt what it was for. This might be
a good thing on a car accessory dealer’s shelves, but fairly pointless when
it’s under the bonnet, and quite possibly a disadvantage if a car thief is
hoping to find the source of the noise and shut it up (a bonnet switch seems to
be an optional extra on this model).
 The most noticeable feature is the built in
siren/sounder, clearly visible through the thin plastic grille. This could be a
design flaw as it looks like it could be swiftly silenced by a quick jab with a
screwdriver. Incidentally, the sounder is actually a small loudspeaker but with
a rigid plastic cone, instead of the more usual paper material. It’s
effectively a giant ‘tweeter’, designed to operate most efficiently at higher
frequencies. The siren sound is generated by a 741 op-amp chip, configured as
an oscillator, and the alarm’s voltage sensing and logic circuitry is based
around a pair of 4001 quad NOR gate CMOS chips. These and the handful of
ancillary components live on a PCB mounted next to the sounder. Connections to
the car’s electrical system consist of 12-volt battery positive and negative
ground cables, a connection to the ignition switch and a pair of ‘sensor’
wires, to connect to the driver’s door courtesy light switch. There’s also
provision for another external sensor (probably the bonnet switch). Apart from the questionable housing it’s all
quite well made and there’s even a waterproof ‘conformal’ coating on the
printed circuit board, to protect it from moisture.
I found it in a big box of bits at a car boot
sale; I was actually hunting for a 12-volt mains adaptor at the time, which I
found, tangled up with the Linwood alarm’s cables. Since it looked like they
would take a while to separate I asked how much for both? Fifty pence sounded
like a reasonable price and the deal was done. The mains adaptor tuned out to
be a dud, so it went into the bin and the alarm went into the
might-be-useful-for-bits-one-day box. This was a couple of years ago, and it
came to light recently when I was looking for a small speaker. I vaguely
remember having this one and it was only when I pulled it apart that I realised
it was totally useless for my purpose. It was about to go back in the box when
I noticed the date on the PCB (‘pat pending 1984’) and that it was in such good
condition. This bought back memories of a review of a kit car alarm and couple
of articles that I wrote on car security for a motoring magazine that I was
involved with in the early 80s.
It clearly had been installed at some time and
fitted with industrial strength metal brackets but it was far too clean to have
been in the engine compartment and I suspect it was mounted in the boot. Thanks
to the well-labelled PCB I was about to figure out what the five wires emerging
from the case were for. It powered up and made a suitably deafening noise, when
hooked up to my bench power supply. I was also able to test the voltage sensing
function, which operated very effectively when the supply dropped from 13.5 to
13.4 volts.
What Happened To It?
Linwood in Birmingham was the home of Linwood
Electronic and for a short while, from the mid 70s to the mid 90s, or
thereabouts, they produced a range of car alarms and accessories, like battery
chargers, wheel clamps and so on. According to records at companies house they
were trading for over 20 years but when the company was formed and dissolved
wasn’t shown. They seem to have disappeared without trace, or at least left any
sort of footprint on the web, aside from an occasional product turning up on
ebay.
Whilst most cars are now sold with some sort of
factory-fitted security system as standard, the emphasis has shifted from noisy
alarms to ways of preventing would-be thieves gaining access in the first
place, and immobilising the vehicle if it is compromised. This has involved all
sorts of digital trickery, from basic stuff like remote control and
microchipped key fobs to the currently trendy (but seemingly hackable) keyless
systems, fingerprint recognition, remote monitoring, GPS tracking and probably
coming soon to a car near you (or maybe here already…) voice recognition,
retinal scanning and microchip implants for the owner’s body. It’s ironic that
the current spate of thefts of cars with keyless systems has prompted experts
to advise owners of vulnerable cars to buy a steering wheel lock…
I am not aware of car alarms having any sort of
following in the vintage tech collector community, and why would they?
Nevertheless it is a mildly interesting subject but I’ll not waste time putting
a value on this and other alarms from the same era. The 50 pence I paid for
this one (and that faulty mains adaptor) was probably too much, and it
certainly has no practical use. I won’t even suggest that you seek out bargains
as a potential investment -- that’s not
going to happen. It is possible that alarms from a much earlier time – and I’m
only guessing that they exist – might be of interest to vintage and veteran car
enthusiasts. But dull as they are I would hate to think that the brief history
of the aftermarket car alarm, and the Linwood Simple Siren goes completely
unrecorded.
First seen: 1984
Original Price: £20?
Value Today: 50 pence (0518)
Features: voltage sensing
trigger, internal siren, variable entry delay, auxillary trigger/sensor
Power req.
12 volts DC (car
battery)
Dimensions:
162 x 113 x 63mm
Weight:
450g
Made (assembled) in: England
Hen's Teeth (10 rarest) 7
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