When you pull into your home driveway after a night on the town, it's normal to feel a sense of security. A very false sense of security! You see, a lot of cars are broken into or stolen at the owner's home address - usually while they're sound asleep in bed. Dreaming of, well, whatever takes their fancy... For some of us, a conventional alarm is simply not enough - I mean, you hear those same alarm tunes playing so many times you kinda ignore them. And, in the middle of night, you're also prone to want to believe that it's somebody else's alarm going off! Another concern is that remote car alarm scanners are also now within the realms of the average car thief.
Here's the cheap solution. A stand-alone pager alarm - that's built up from nothing more exotic than a Dick Smith wireless doorbell and a few other components! Think about it. This package comes complete with a separate transmitter and a receiver unit capable of covering up to 30 metres - making it ideal for home pager alarm purposes. Once we've finished screwing around with it, though, it won't bear much resemblance to its former pathetic ding-donging self. No way. It'll let rip with a high pitch scream to grab you by the nads whenever some bugger's breaking into your car! Forewarned, you can then select your weapon to confront the thief. And wouldn't that be f-u-n?!
Anyhow, here's how the conversion kicks off...
After spending $25.39 (plus batteries) for the Dick Smith Wireless Door Chime (cat number Y8405), you then take it home and proceed to thoroughly rip it apart. The first - and easiest - part of our process is to give the receiver (the bell) a bit more attitude. You want something that's capable of scaring the you-know-what outa you when you're asleep! Just take a small Phillips-head screwdriver, pull the case apart and de-solder the two wire connections on the terminals of the existing ringer.
This should be replaced with a much louder piezo buzzer - like this $3.35 item purchased from Jaycar Electronics (cat number AB-3462). Solder the positive (the red) lead of the buzzer to the connection as shown in the photograph. With some slight trimming and filing of the inside of the plastic casing, you should be able to j-u-s-t squeeze the new piezo buzzer into the confines of the receiver unit. Test out the ringer tone now. Bit louder, eh?!
The transmitter unit also needs to have a little preparation before you permanently sling it into your car. Grab that Phillips head screwdriver again, pull the backing plate from the transmitter and solder two lengths of shielded wire to the two switch conductors at the end nearest the batteries. These wires - when connected together - will activate the transmitter. If you want to run them out of the transmitter case nice an' neatly, simply put a small hole into the side of the case and squeeze them out through that. Done.
Now we come to a more difficult part - how to trigger the transmitter from inside your car. The most effective way of monitoring the car's doors is to tap into the interior lighting circuit. Most cars - such as our demo Holden Commodore - have a switched earth to trigger the courtesy lights (which means the door plunger earths a permanent 12V supply whenever you open the door). That sounds simple to tap into but the thing is, you also need to have an exit delay so that you don't trigger the pager alarm while you're stepping out.
Answer: Purchase a Flexi Timer construction kit from Jaycar Electronics - this will serve to form the basis of our exit delay. Notice that the Flexi Timer kit can be soldered together in only around half an hour and is classed "simple" - so don't be scared off by the thought of daunting electronics! Following the instructions for assembly, you'll notice that the timer can be set to various durations before triggering the (supplied) output relay. We elected to make our exit delay around 7 seconds - and achieved this by placing our time duration link at what's called "Q4" on the circuit. This duration can be further adjusted with the PCB-mounted trim-pot.
Now we come to the component needed to link the exit delay with the door trigger. For this, you'll now need to pick up a 12V latching relay from Jaycar (part number SY-4060, RRP $9.53). The "latching" part is very important because this will keep the transmitter (and, therefore, the receiver) operating continuously once the alarm has been tripped. In other words, the pager won't stop buzzing if the thief happens to close the car door.
Piecing the Components Together
By now we've loaded ourselves up with a receiver unit (equipped with a loud piezo buzzer), a transmitter with external activation wires, a completed Flexi Timer kit and a latching relay. How the hell do we put it all together?!
Probably the best place to start is with the Flexi Timer. Note that the Flexi Timer's only function in life is to allow you to flick the system on, and give you time to get out of the car without triggering the pager transmitter. Put simply - once activated - the timer is the device that sets the pager alarm into its steady-state "armed" mode. Now, to power the timer, we bought a simple two-position toggle switch (which was installed well out of view for security reasons). This switch controls a 12-volt power supply to the timer, while the module's earth is made to a convenient nearby lug.
Meanwhile, the output (relay) side of the Flexi Timer needs to have its own permanent 12V feed connected. One of its output pins should then run to both positive sides of the coils on our separate latching relay. That means - once our timed 7 seconds is up - the Flexi Timer relay switches power through to the latching relay.
As mentioned, the Jaycar latching relay - the one that connects to the door switch - contains two separate coils. That means you need an energy pulse to switch the relay to one side, and another pulse across its other coil to bring it back to its original position. These two relay positions - in our case - represent "normal" and "triggered". Now, since we we're already feeding 12V to the positive of both coils (straight out of the relay on the Flexi Timer), the only way that we can switch them over is with switched earths - which is, not coincidentally, just like the car's door switch.
We switch the latching relay by making a closed-circuit across each individual coil - either via the door switch (to trip the alarm), or a fixed earth isolated by a small (normally open) push-button switch. This push-button switch (also bought from Jaycar) serves to switch the relay back to its normal position once the alarm has been triggered. Think of it as the reset switch.
So - with the timer controlling our fully functioning latching relay - all that's left to do is connect the two open end wires of the transmitter to the outputs of the latching relay. These two wires should be soldered to pins 3 and 4 on the same side of the relay. Ha-ha - we're finished!
Show Time!
With the timer, the latching relay and the transmitter mounted in position - not to mention those concealed toggle and push-button switches - our built-up pager alarm is very easy to operate. It helps to label the toggle switch (the one that fires up the timer module) as the Main switch. If this is set to off (and the timer therefore not operating), then no power will reach the latching relay. No power to the latching relay equals no paging - even if the doors are opened. Once switched on, it activates our countdown 7-second exit timer and then puts power through to the latching relay - ready to nab a thief.
Our push-button reset switch needs to be pressed whenever you return to the car and the system has previously been armed. Upon opening the car door, the alarm will be tripped and you must reach for the reset switch (to bring the latched relay back to its normal position) and the Main switch (to switch the whole paging system off). Note that you first have to close the door to rest the relay to its normal position. If the car has been broken into, then pushing the reset button will shut the buzzing pager up.
Notice that the existing switch on the side of the Dick Smith receiver is also quite useful. In cases when you want to take the pager out with you, you can simply switch the receiver off so that it wont scare the bejeezus out of you when you get back into the car and haven't yet pushed the reset button. Conversely, if you just want to leave the receiver at home at the start of the day, just switch it off so that no one else in the house gets scared when there's a loud noise coming outa your bedroom!
So there you have it, a relatively cheap DIY pager alarm that's perfect for home (or other short distance) car security monitoring. Just make sure you keep an eye on the charge off the AA batteries!
Contacts:
www.jaycar.com.au
http://www.dse.com.au/cgi-bin/dse.storefront
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