In the last two blogs, I described the issue I had.

Electronic Diet

Electronic Diet - part 2


I have a spare car that has an older model alarm system fitted just to provide remote lock/unlock.

The car has central locking and uses a transponder key, so it's not that easy to steal, but the flashing LED right next to the ignition barrel does suggest it might make noises if you break in.


The unit was fitted when we first got it, and it allowed my wife to open the doors from the footpath and get the kids into the rear in safety.

I've seen too many children out on the road getting into and out of cars .... not sure what the parents are thinking (or more likely not thinking).


Anyway now that it is a spare car used in winter and for towing, it sits around and the battery gets drained by the alarm system.



In the subsequent discussion I made this statement :-


Engineering a solution is a 4 step process

1.     It is important to fully understand the problem, and ensure the customer understands it OR the impact

2.     Find/Engineer the solutions that will cure the problem.

3.     Decide on the most appropriate solution, and apply it.

4.     Go back and check that it has solved the original problem, and almost as important, ensure it hasn't caused any other problems.



Step 1

The problem is the battery goes flat.

As I also elaborated the battery is new, and I hadn't mentioned that this has been an ongoing issue for several years, ever since it sits around as a spare car.


I've measured the current the alarm systems draws (65mA) and shown that this gives about 57 days until it will be totally flat.

Since I'm also the customer (and the one who gets complained at), I understand the impact.



Step 2

This step is about identifying the solutions that would cure the issue in Step1.

In some cases it may be impossible to fix the problem ... world peace has escaped everyone despite trying several solutions ... BUT you may be able to reduce the impact to an acceptable limit.


So far the choices are grouped to :-

Remove the unit.

Modify the unit to draw less power.

Provide external power.

Monitor the voltage and warn when it reaches xx volts.

Swap the unit.


Remove the unit

The unit was originally installed when we had small children, so removing it now, is not as much impact as it would be years ago.

The security of having an alarm system would disappear, so there could be some insurance concerns to address.

Having remote locking and unlocking is very handy at the skifield, as we leave a key inside just in case. At worst we can replace a window.

The drain on the battery would go away.


Modify the unit

I have the circuit diagram and there is room to reduce the drain BUT it does appear to be a lot of work, and if I'm honest a 50% reduction might be the absolute best.

Looking at the design and the schematic, I estimate it would be 8-16 hours work ... you do the math if this was charged out.


One suggestion was to only power the receiver (jw0752) and then have it power up the unlocking part.

This could work if the receiver only draws a small amount of current, but certainly wouldn't flash the LED or act as an alarm unit (insurance issues?)


HOWEVER the changes may result in other issues that negate the advantage. Having it oscillate between lock/unlock will result in burnt out lockers, lots of current draw and a flat battery.

The spare unit could be left unmodified UNLESS it was required to be permanently fitted.




Provide external power

There are three options here.

Cable supplying 12v (safer than mains).

This would require the vehicle to be parked within the range of the cable, or have several outlets.

The cable would be exposed to damage from the mower if it's near the garage. (the grass grows well in spring, and I'm not the only one on the mower)

You'd also need to ensure you undo it before driving off (I understand our fireman also have this issue, which prompted a modification to the start circuit on the Fire Appliance)


Solar charger

The alarms uses 65mA x 12v = 0.78w. every hour.

4 watt panels are approx $35, and would need to be wired to a permanent supply point.

Solar provides around 4 - 6hrs charge time, so 6hrs at 4W = 24Wh

The alarm is drawing 0.78Wh x 24 = 18.72Wh.

This would keep the battery topped up BUT it is likely it could end up overcharging the battery, as there is no charge controller.

Charge controllers are approx $45, but we could easily build something for less money (but more time).


Second battery

The option of a lead acid battery to run the alarm needs to be added to this list.

In other situations the addition of a storage battery is essential, however in this case there is no spare room to fit it.

Our existing battery is 90-125AH, so we'd need to have something about the same if we want to retain all the features of having this unit.

There would be some modifications to the wiring,but it is achievable.

The cost of a storage battery is around $3-400 plus a box.


Monitor the voltage

This option deserves investigating.

The requirement to monitor voltages and raise an alert to someone has many uses.

Power consumption of the monitor is negligble as there are many processors that run forever, or it could be powered using solar.

The monitoring and warning could be linked to the solar, so it doesn't starting beeping you at 3am (like so many smoke alarms do).

It won't stop the battery going flat (ulitmate goal), but it provides a warning if someone leaves the door open and the interior light is on (had that one).

The exact warning method would need to be established, (in this situation we have a close proximity to the house, so there are simple options).


Swap the unit

This one has a few questions.

Will the replacement draw more current, how reliable is it, can it be fitted easily.

Luckily I have one to check the current drain (9mA).

I had one fitted to another vehicle and it never false triggered.

I indicated we mainly use this for the remote central locking, so if it flashes the LED then it will provide the deterrant required.

The cost of the relacement was $30 (from memory).



Step 3

This is where you evaluate the possible solutions noted in Step 2.

Each solution will have advantages and disadvantages, and there  may be more than one that will resolve the problem.

Often there is a clear solution to resolve the problem, but it has a cost factor, and this may be much higher than another solution.


It's very easy to say " ... Take the cheapest option ..", but is that going to make it go away permanently, or will it come back again later.

e.g The battery goes flat after about a month, so just fit a bigger battery. This just means it goes flat in 6-7 weeks, and hasn't fixed the problem.


If the solution is being done for a customer, then you would need to be sure they understand the advantages, disadvantages and costs associated with each of the acceptable solutions.

After all if they were the expert, then they wouldn't be asking you ...




As you can see from the above solutions, removing it OR swapping it seem to stand out.

Since I already have the new unit, there is no cost (the original reason for buying it is not really necessary, and is a long way off), and it gives me what I already have.


So that will be my plan going forward.



Step 4

This one is important.

You do need to check the solution implemented has solved the problem, otherwise all you've done is spent money and wasted the customers time and confidence in you.

You also need to verify it hasn't caused other issues. In my case there are new remotes to used, and unlike the old unit, I'm unsure if I can find more spares.


If the work is for a customer, checking up will show you care about your work, and they may have some other problems they need you to solve.



So hopefully this has provided some insight into proper Engineering and solving problems.

Society have become lazy IMO and either fail to properly understand the problem or are more interested in making a quick buck.

This may work when there are thousands of customers, and there is a new one coming through the door each day, but if you are in a specialised field, the numbers are much lower, and your reputation is the one item you can control.