Van Alarm

[frusciante89]

Hi guys, I'm building a little alarm for my dad's van... I live in italy and recently a lot of vans have been stolen near my house, so we want to find a cheap and reliable way to protect it... These are my requirements:
- I want a simple system, which doesn't need drilling the van structure or similar invasive operations. Something that won't affect the value of the vehicle (which is brand new)
- the system needs to be a normally closed type, so that cutting the wires won't result in the deactivation of the system
- I want to use an ultrasonic sensor coupled with a siren for notification of intrusions
- I want very low power consumption

From here, I started designing the circuit of the system, with these things in mind:
- the ultrasonic sensor (HCSR04) will be placed inside the van on the roof, pointing towards the driver-seat. When the sensor detects a variation in this distance (e.g. someone is sitting there), the alarm (unless when it's not engaged) will turn on.
- I want the owner to have a 10-15s time span for the deactivation of the system from when he enters the vehicle. My idea of the deactivation system would be a spring loaded toggle switch (or a pushbutton) that needs to be continously pushed for, say, 5s, otherwise no deactivation occurs (this is a way to prevent the burglar from deactivating the system even if he's able to find the switch)
- The system, when deactivated, goes to sleep and it engages again if the ultrasonic sensor detects the roof-seat distance (that means that no-one is inside) for more than, say, 5-10 mins.
- the ultrasonic sensor is wired so that if one the 4 wires is cut (5V, echo, trigger, GND), the Arduino starts the siren. Since I'm new to burglar wiring, I designed a system using 6 wires and 2 resistors (that create a voltage divider). I'm open to suggestions, don't know if this is a good wiring scheme
- the siren will be placed in the back of the van (which is not communicating with the cabin) so that it'll be difficult for a burglar to cut the power. This, however, poses a problem: I need to find a way to wire the siren to the arduino without drilling holes in the cabin panel... I was thinking of using rf or bluetooth communication, but this would mean using two arduinos, two power supplies and more hassles... So this weekend I'm gonna carefully inspect the van to check if there are already pre-drilled holes...
- as far as the siren wiring is concerned, I want the siren to start screaming not only if the arduino tells it to, but also if the wire connecting the arduino to the siren is cut. How can I do this? Maybe using a transistor as a switch? The power supply for the siren will be placed in the back of the van so that it's hopefully untouchable...
- finally I want to install a small buzzer that turns on during the 10-15s deactivation time... This will notify the user that he needs to turn the system off with the toggle switch before the siren starts screaming

With these things in mind, I designed a mock-up circuit in Fritzing...

https://postimage.org/index.php?lang=italian

However, the siren is still missing the wiring... What do you suggest? Do you think this will be a good alarm system?
Thanks for your help

 

[alec_t]

A FET could be used to supply power to the siren. Its gate would be resistor-biased to turn the FET on; the Arduino would be programmed to keep it turned off.

 

[Wp100]

Hi,

If you search on arduino Car alarms or Ultrasonics you will find a loads of good info.

Also consider adding a arduino controlled tracking device so if it is stolen you know where it is.

What about fitting a good quality wheel clamp ?

 

[frusciante89]

Thanks for the answer guys! I think I'm close to the final design... I put a FET for the siren... What do you think?

 

[heydonms]

That's not a FET, it is a transistor and relay and it will draw quite a bit more power than a FET to remain in the off position. If you are driving the car everyday and use a small relay it probably won't be an issue but if you are leaving it for extended periods then you might end up running the battery down. Of course, the arduino might do that on its own.

 

[Wp100]

Hi,

You would be better using a relay with a 12v coil.
That has two advantages, one, the power/sudden drain and any noise is on the 12v rail not the arduinos /sensor 5v rail; two with all the heavy power being on the 12v rail you could use a much smaller and more efficient 5v regulator like a 2950 so saving on power, though doubt either would make much drain on a van battery .
Don't forget the caps either side of the regulator used, see their datasheets for the correct values

You could always use power saving modes in your program code, eg sleep

Think R1 might be a bit high, perhaps try around 2k, check the current either way.

 

[heydonms]

Or just use a FET...

 

[frusciante89]

Sorry, that's obviously not a FET... However: there's a problem with this circuit... if someone cuts the relay power, then I can't activate the relay from the arduino (pin 10)... I assume that in order to avoid this, I should keep the coil of the relay always excited so that any cut in the circuit will trigger the siren: but this will mean spending a lot more energy, right? Moreover, such a system feels to me like a bomb ready to explode, and that the relay/siren might be triggered for other reasons (like vibrations, false signals from the sensor, power spikes...).. Is this feeling reasonable or is this just paranoia?

As far as the power is concerned, I'm thinking of using an old car battery stored in the back of the van, I will prefer not to hook up to the van's electric system.. But I don't know if this will be enough to guarantee a sufficient life-span of the system (I don't want to change battery every week)

 

[heydonms]

However: there's a problem with this circuit... if someone cuts the relay power, then I can't activate the relay from the arduino (pin 10)... I assume that in order to avoid this, I should keep the coil of the relay always excited so that any cut in the circuit will trigger the siren: but this will mean spending a lot more energy, right?

Right, so use a FET. Pretty much zero current draw in the off state.

Moreover, such a system feels to me like a bomb ready to explode, and that the relay/siren might be triggered for other reasons (like vibrations, false signals from the sensor, power spikes...).. Is this feeling reasonable or is this just paranoia?

Mostly paranoia, but in any case the solution is the same as the the power problem. Use a FET, they aren't affected by vibration and as long as the circuit design is sound then you won't have issues with the sensor or power spikes.

As far as the power is concerned, I'm thinking of using an old car battery stored in the back of the van, I will prefer not to hook up to the van's electric system.. But I don't know if this will be enough to guarantee a sufficient life-span of the system (I don't want to change battery every week)

It would be a far better idea to just tap into a power wire somewhere, since the alarm battery isn't doing something noticeable recharging it is likely to be put off after the first few months and then why bother even having the alarm? If you can find an existing hole through the body near where you want to mount the siren you can run a new +ve line from the battery without making any permanent modifications to the vehicle. Put in a little SLA if you want to give the siren an independent power supply.

 

[frusciante89]

Thanks heydoms, you're giving me some precious hints!!
So, I still have some questions...
1) how can you wire the fet so that it's in a normally closed state? I searched around but I didn't find much... do you have a schematic handy? Or, alternatively, I could use a normally open configuration and run a parallel wire that will somehow trigger the alarm if someone tries to cut the wire bundle...
2) I was thinking of hooking up the system to the van's battery as you said, but I have a doubt: when you start the car, don't you have a lot of spikes and pulses? Won't this kill my little arduino?

 

[heydonms]

1) just pull the gate high with a resistor (assuming you just replace the relay with an N channel FET) and then pull it low with the micro. You may want to use a transistor since "high" will probably be >12V, or if your arduino has open collector outputs that will tolerate 12V (with some safety margin) you can just use that.

2) Yes cars are fairly hostile to electronics and it is possible that you might damage the arduino if you aren't careful. I'm not terribly familiar with arduinos but I believe they take a 5V input, in which case you will already have a Vreg between the 12V line and the micro, that will provide some protection. If you put in a SLA battery, that will also provide some protection (and then you can hook into the accessories wire rather than constant 12V and you avoid the worst of the transients produced by cranking). You can also add filter caps, zener diodes, etc. to add further protection.

 

[4pyros]

Charge the SLA battery in the back when the van is running.
Your ultrasonic sensor (HCSR04) is not meant for out door use and may false trigger in the heat and/or cold.