Introduction: Get "On Star" -like System for Your Car, Free.

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With a cheap mobile phone and a DTMF (dual tone multifrequency) decoder, you can get a system to unlock your car's doors, to turn off the engine, to trigger the siren or horn, or just about anyting you can think of with remote control (from any place you can dial up to your car's phone number).

Step 1: Get Your Stuff.

You'll need:

-A mobile phone that has a hand free port. And which you can configure to self answer any incoming call. (Motorola C133, which I used has this feature).
-A hand free for your cell phone. (got it for 1 US at Waldo's 1 dollar mart)
-A car charger/adaptor for your cell phone (Waldo's to the rescue)
-The most critical of all: a CM8870 dtmf decoder IC. In Mexico, AG electronica has them. It is considered obsolete in USA, but If you can get it in Mexico, you can certainly get it in the US.
-a 3.579545MHz quartz crystal
-2 x 100 KOhm x 1/4 W resistors
-1 X 330 KOhm x 1/4 W resistor
-3 X 100nF ceramic capacitors
-7805 voltage regulator IC
-1000 uF X 25 V Filtering capacitor
-4 x 12 V Relays (I used automobile type)
-4 x BC548 or any small power PNP transistor.
-4 x 4.7 KOhm x 1/4 W resistors
-4 x 1N4001 diodes (to dissipate back emf from the relays)
-Printed circuit board (prototype board)
-A nice plastic housing (I used a discarded wireless phone base station)

Step 2: Build the Circuit.

Some ciruit assembly skills are needed in this step, so I'll assume that and show you the schematic. Use your 7805 IC and 1000 uF capacitor to provide 5 V steady supply to it. (Ignore the zener and 100 Ohm resistor, the 7805 takes it place and is a lot better). Since we'll be supplying only 5 Volts to a single IC, the heatsink can be obmitted. From your hand free, take the cord to the earpice, cut it of (earpice). and solder the mesh or shield to ground, then the inner cable to the 100nF audio input capacitor.

This way your cellphone will pass the tone presses audio signals to the DTMF and it will decode them to its binary output.

On every binary output you will connect a buffering transitor to drive a 12 V relay. Now the relays are fed with 12 Volts, the transistor only grounds them so they operate, but the relays drive no current from the 5V supply (thus, we keep it simple).

BC548 is common, but you can try any low or mid power npn, like BC238, 2N2222, etc.

Reverse emf diodes should be mounted in parallel to the relays' coils but in reverse mode (so they catch back emf). I was lucky my relays came with diodes, so my boards doesn't show them. (snap!)

Step 3: Supply Voltage to Your Cellphone

Now assemble everything in your plastic box. Take cables from your relays to what ever load you would like to drive (door locks, sirens, lights, horn, engine, stereo, neon lights, etc). Just thing that this basic circuit can drive 4 outputs simultaneously ( to expand it to 7, you may need a 3-8 lines decoder 74LS138 or a 4-16 lines decoder 74LS154) But for me having and alterantive mean to unlock my doors if I accidentaly leave the keys in, or if my car is stolen, I can turn the engine off, even if I don't now where it is, IS GREAT!!!

Step 4: Look for a Secret Place.

Under my dash board, there is some place, and cabling is kept hidden. So I just bolted it and wrapped the cables.

Now call your cellphone. Once it answers, press a key to set the input you desire on. The table below shows the functions (K, is for keypressed)

K Q1 Q2 Q3 Q4
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
0 1 0 1 0
  • 1 0 1 1
# 1 1 0 0
A 1 1 0 1
B 1 1 1 0
C 1 1 1 1
D 0 0 0 0

So there you have it. Enjoy it and have fun!!!