Get "On Star" -like system for your car, free.

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!!!