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Build a Homemade GSM Car Security System

Swagatam

This super simple circuit design of a gsm car security system really works.Don't believe it? Find out and learn the simple method of constructing it. How would you feel if you had the power of controlling your vehicle through your own cell phone, from any part of the world? Learn more how to build a homemade gsm car security system.

Contents hide
1 Introduction
2 The Salient Features of the Circuit
3 The Trigger Circuit
3.1 PARTS LIST
3.2 The Main Control Circuit
3.3 PARTS LIST
3.4 The Auto Lock Facility
4 The Ignition Control Circuit and the Central Locking Circuit
4.1 PARTS LIST
5 The Call Back Feature
5.1 PARTS LIST
6 The Call Rejection Feature
6.1 How to do the Internal Wiring to the Modem Cell Phone
6.2 The Charger Section
7 Setting up the Attached Cell Phone Modem
7.1 How to Test the Unit?
7.1.1 Video clip showing how the control circuit is operated using a cellphone with a unique number

Introduction

I always wondered how to become rich by inventing or developing an idea that may be simple in design yet breakthrough in its features.

Cell phones have always intrigued me for the simple reason that, these gadgets are so compact and sleek in appearance and yet are able to do the monstrous task of sending signals wirelessly across the globe within seconds.

One day it struck me - can there be a simple way of exploiting this outstanding feature so that it may be used as a “cell phone remote control” to operate any equipment by just a flick of a button from any part of the planet?

Since all the commercial gsm based remote controls use a modem for the purpose so why can’t a mobile phone be used as a modem?

Such a “modem” will be easy to acquire, user friendly, low cost and easy to install. Besides, the conventional types of gsm modems are costly, difficult to wire and require special SIM cards and prior intimation to the network providers for it to become functional.

Inspired by the brilliant idea and after a relentless hard work it was a true sitting moment of success when I finally was able to complete a reasonably good, very economical gsm car security system that could not only immobilize a vehicle and do central locking but also send a revert back call to the owner’s mobile phone in case of an intrusion.

The system is armed and disarmed through subsequent miss calls from the owner’s cell phone and no call costs are incurred during the process.

Exuberant with the successful venture, I decided to contact relevant companies for the promotion of the unit. But the news was really shocking.

These companies rejected my product saying that the design wasn’t “industrial” and the use of an attached cell phone as a modem was not acceptable.

Very dejected I tried to promote and sell the unit through my friends and relatives, but, the task was uphill since the operation demanded lot of initial investment and after-sales service, so finally, I had to drop the decision.

But folks, there’s a good news, I am ready to share the success story with you all and unleash complete details of this truly amazing circuit.

The circuit is very simple, foolproof, fail proof and mind you it’s already working perfectly in four of the vehicles and in one of the jewelers shops (as a security door lock) since last six months or so.

The Salient Features of the Circuit

Locking and unlocking of the vehicle is done through subsequent miss calls, thus no call costs incurred. The above operation may be done from any part of the world.

The engine of the vehicle may be stopped even when it’s in motion, from any part of the world. Vehicle Locked confirmation is done by rejecting the miss call while a continuous ring in the owner’s cell phone indicates that the vehicle has been unlocked.

Any quantity of phone numbers may be assigned into the system’s cell phone so that it responds and can be operated only through these numbers.

A break-in or an intrusion is effectively converted into a revert back call to the owner’s cell phone as a warning. The device includes a built-in automatic locking system, in case the owner forgets to lock the vehicle.

Prerequisite: A prepaid SIM card to be fitted inside the modem (attached cell phone module) An entire unit will cost you not more than $80 USD.

Warning: The unit will not work if installed in conjunction with some other form of security system already present in the vehicle. Let's move on to know exactly how to build this gsm car security system.

The Trigger Circuit

As shown in the circuit diagram, the small two transistor circuit forms the basic “beep once” tone amplifier coming out of the headphone socket of the cell phone.

As this ring tone is accompanied with a variety of unwanted glitches or strong RF disturbances, it became very important to stop these from entering the main control circuit.

These disturbances could easily rattle the entire circuit and produce bizarre results. Resistor R18, capacitor C16 and inductor L1 are all included just for the above rectification.

The amplified signal moves into a relay to energize it momentarily as long the tone persists.

The unit’s supply voltage is wired across the relay’s N/O contacts so that when it operates, a logic high signal is transmitted to the main flip/flop control circuit.

The employment of a relay ensures that it is operated only by a genuine ringtone pulse and not by any other obscure stray disturbance emanating from the cell phone (if at all they are able to break into the defenses of R18, C16 and L1).

PARTS LIST

All Resistors are ¼ watt 5% CFR, unless otherwise stated.

Alternatively, L1 may be retrieved from any standard piezo buzzer.....the example image below provides a clear view of the buzzer inductor:

The Main Control Circuit

This circuit is basically a Flip/Flop circuit that toggles to alternately produce a stable logic Hi or a logic Lo in response to the input signal received from the above circuit.

I won’t go into a detailed circuit description as the particular circuit has already been discussed in one of my previous articles.

The out put of this circuit is used to activate/deactivate the ignition system and the central locking of the vehicle.

PARTS LIST

The Auto Lock Facility

The auto lock facility of this gsm car security system may be understood as follows:

Referring to the figure, as long as the output of N2 is high (system in locked position), the auto lock comprising of N4 and N5 is disabled and cannot operate.

The moment the output of N2 toggles low, N4 starts counting and after the set period of time (depending on the values of R1 and C1), the output of N5 goes high producing a trigger pulse at the input of N1, toggling it back to the locked position and once again disabling the auto lock.

Thus the system can never be kept in an unlocked position for more than the set period. Within this set period of time the owner of the vehicle needs to insert the vehicles ignition key into its slot to stop the auto lock from initiating.

Once the vehicle is locked, the ignition key will not work unless and until the system is unlocked through the owner’s cell phone.

The Ignition Control Circuit and the Central Locking Circuit

As shown in the figure, it is basically a simple relay driver circuit comprising of R24, T7 and T8. T7 and T8 alternately makes and breaks the ignition relay contacts in response to the pulse received from the above circuit.

The central locking relay group is also rigged to T1 for a simultaneous operation with the ignition system. Central locking operation is responsible for locking and unlocking the doors of the vehicle (the car).

The central locking machine generally incorporated in car doors is actually made up of a DC motor operated through subsequent positive and negative momentary voltage pulses to alternately lock (push) and unlock (pull) the lever of the door respectively. Keeping this particular feature in mind the present groups of relays and the capacitors have been arranged to result in the above operation.

PARTS LIST

The Call Back Feature

Looking at the circuit diagram we see that the IC 4060 is wired in its basic oscillator mode. The coil of relay (extreme left) is connected externally to the door switch of the vehicle. In the locked position if the door is opened, the system takes it as an “intrusion”, momentarily activating the relay and triggering a monostable comprising of N6 and N7.

As a result the output of N7 instantly goes low resetting pin # 12 IC 4060 and it starts counting. Pin # 2 of the IC goes high after a few seconds and latches itself, but during this particular period of time its pin # 15 produces exactly 4 pulses and is used to operate a relay whose contacts are wired with the call button of the attached cell phone (modem) internally.

Therefore the cell phone starts making a call and the owner is immediately informed of a possible theft or a break in.

Now monostable N6 and N7 will release itself exactly after one and a half minutes, until this period of time the entire circuit will be “sealed” and will not respond even to the miss calls from the owner’s cell phone.

This feature has been purposely incorporated to ensure that the circuit is not disturbed when the attached cell phone (Modem) is trying to make a call to the owner.

PARTS LIST

The Call Rejection Feature

Another relay driver circuit as shown in the above figure takes care of the “call rejection” facility.

When the system is locked through the owner’s miss call, a pulse is sent to the base of the driver transistor which operates a relay momentarily.

Now since the contacts of the relay are wired across the “cancel” button of the attached cell phone (Modem), the received call from the owner’s cell phone is immediately rejected and a “network busy” is indicated in the owner’s cell phone confirming that the vehicle is securely locked.

How to do the Internal Wiring to the Modem Cell Phone

The internal wiring of the modem of this gsm car security system may be understood as follows: You will have to do this part of the wiring very carefully and if you are not crafty enough, things can get pretty messy and spoil the whole “game”.

It would be better if you took the help of some mobile phone technician to complete the following operations:

It goes without saying that the outer cover of the cell phone is to be removed and also the inner metal mesh which is used to shield the keypads.

Carefully and very slowly peel off the keypad sticker and keep it in some secured place for later use.

You will find the embedded keypads exposed. But the sad thing is that the pads aren’t solderable, therefore the external wires from the relevant relay contacts cannot be attached to them through soldering.

So the only possible way is to fix the relevant wires by sticking there stripped ends to these keypads and let the keypad sticker and the metal mesh frame do the rest by firmly pressing these wires against the keypads to make a reliable contact.

The wires used may be the thin insulated ones, typically used to wire piezo transducers or simply a 36 SWG super enameled wire will do the job quite nicely.

Do not forget to “tin” the stripped ends of the wires so that they make good contact with the keypads.

Also remember to remove the small disc shaped metallic contacts in the sticker from the points which correspond to the cancel button and the call button. After this you may replace the sticker carefully back in its position so that it secures the connected wires further in there places.

Go on to fix the metallic wire mesh and screw it firmly to complete the “modem” assembly. The outer most plastic cover is not required, so do not try to replace it. This modem assembly is now ready and can be connected to the main circuit after the whole circuit assembly is over.

The Charger Section

The charger section of this gsm car security system utilizes a regulator IC 7805 and a current limiting resistor. It is connected to the cell phone modem permanently and is used to trickle charge its battery.

There’s no danger of overcharging the battery as every Nokia phone has a built-in self regulating automatic cut-off circuit.

Setting up the Attached Cell Phone Modem

Once the entire circuit assembly and all the connections are completed, the above cell phone modem may be set up through the following simple steps: Insert a pre-paid SIM card into the cell phone (Modem), Switch it ON the by short circuiting its “cancel button” wires externally using a piece of conductor. Since the keypads no longer exist, this is the only way to switch it ON.

Save the mobile phone numbers through which the unit would be operated. Go to the name section of all the above saved numbers – press options – Assign Tone – Select No Tone. Next, go to settings – Tone settings – Select empty (default ringtone switched OFF).

Similarly set the tone for the messages in the OFF position. Also switch OFF the keypad tones, warning tones and the start-up tone by selecting the relevant commands.

Finally, make a call through this modem cell phone into your cell phone by shorting its call button thrice externally using a piece of conductor, so that now the Modem "knows" exactly where to call back once a theft is sensed and will always call you back on this particular number every time the door of the car is opened (only when the system is in locked position). The Modem is now set perfectly and ready to use.

How to Test the Unit?

After concluding the whole assembly procedure as explained in the whole article, you may test this outstanding gsm car security system through the following simple methods:

Connect a good quality 12 volt regulated power supply to the circuit, The cell phone modem should immediately read "Charging", indicating that the charging process has initiated and is working perfectly.

Connect small 12 volt motor to the "central lock" outputs of the circuit. Start dialing the SIM card number, the moment your call hits the cell phone Modem, you will find the relays operating accordingly and the connected motor reverses its direction of rotation alternately on every subsequent calls made.

This confirms that the central lock section is operating perfectly and also the entire circuit.

Check the continuity of the ignition section using a DMM. It should make and break on every subsequent calls made. Similarly you may connect a horn (siren type) at the alarm output to check the relevant section.

The alarm should sound for a moment and stop at each and every received call by the modem. In the locked position (ignition relays in deactivated position), ground C10/R9's common point as might happen if the door of the car is opened (intrusion). The modem should immediately start calling the stored number and you will receive a call in your cell phone - the call back facility is working.

The above steps should be enough to confirm the correct operation of the system and you are all set to fix it in your car and actually witness the amazing feats.

Video clip showing how the control circuit is operated using a cellphone with a unique number

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