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

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!!!

Comments

author
Mythbuster Kid (author)2012-01-02

Seems a little bit over complicated.

author
deeceb (author)Mythbuster Kid2014-08-14

how would u simplify it

author
darweesh8 (author)2014-03-24

Dear Boss, How about if i want to convert the existing OnStar to a Free Hand Calling through Bluetooth device? any idea

author
Chebog (author)2011-10-16

Sir, question? how many source is required in the circuit? is it 2 source? Source from the 7805 voltage regulator to supplied to the circuit and the 12volt for the relay? or is it the same source to use in the relay?

author
Chebog (author)2011-10-11

Hello Sir, I have a question?. The voltage regulator IC has a Input, common, and output, is the common pin there serve's as the Ground?.

author
Chebog (author)2011-10-09

sir good day again can something about relays.what relay could turn on and off a bulb with 220V and what DIODES and transistor can amplify or power up that relay?.tnx

author
unknownpocketnerd (author)Chebog2011-10-10

I've already explained that. You need to find yourself some basic electronics handbook. The automotive relays I suggest cannot handle 220V, they are low voltage high current relays designed for no more than 40 V. However there are PCB mounted relays that handle up to 400 V. The diode is to eliminate the back emf after the relay switches off so the transistor and IC do not get damaged. 1N4001 will do. For transistors: BC548, 2N2222 just about any small signal npn transistor will do.

author
Chebog (author)unknownpocketnerd2011-10-11

Thank you very much sir, I really appreciate it...

author
Chebog (author)2011-10-08

What i mean is where do the supply of the relay goes?in order for it to function?

author
unknownpocketnerd (author)Chebog2011-10-08

Just read along the rest of the comments.

author
Chebog (author)2011-10-08

Sir good day.can i ask you again about where do the goes in the relay is it connected on the coil to drive it? and do the ground goes is it on the common of the voltage regulator IC?please reply.

author
Chebog (author)2011-06-19

good day, i just would like to ask you again regarding with the transistor.If where is the collector and emitter connected to the circuit.

author
Chebog (author)Chebog2011-06-24

excuse me sir. is it OK to use any type of cellphone just like nokia phones?.or motorola phones only?.

author
unknownpocketnerd (author)Chebog2011-06-25

Motorola phones have the behaviour that this gadget demands: They can be set to auto answer even when the hands free and charger are plugtged. I tried a nokia one, but it didn't work the same way. Old motorolas worked for me.

author
unknownpocketnerd (author)Chebog2011-06-25

I have already answered that, just read the blog.

author
Chebog (author)Chebog2011-06-24

ahmm,good day sir.regarding on your circuit how about the relay how do you connect it?.because my relay comes with no diode inside.

author
unknownpocketnerd (author)Chebog2011-06-19

DTMF outputs small current 5/0 V states, thus to drive a relay, a transitor is used. The base is connected thru a resistor (4k7) to the DTMF. The emmiter to ground, the collector thru the relay's coil to 12 V. This is the most common way to drive a relay. Use any small signal generic npn such as 2N2222.

Then use the realy's contacts as you please.

author
Chebog (author)unknownpocketnerd2011-06-19

THANK YOU Sir for your reply i really appreciate it.tnx again.

author
giacomine (author)2011-06-15

Como eu faço a ligação do celular para o circuito? E como eu faço a programação do celular (teclas)? Obrigado!

author

Sorry I don't read/speak Portuguese. Can you write that in English?

author

How I make the linking of the cellular one for the circuit? E as I make the programming of the cellular one (keyboard keys)? Thank you!

author

How does the phone connect to the circuit? I just indicated it: Using the handsfree cable earpieze wire. It is and audio signal, and the DTMF decoder needs an audio signal coupled thru a cap. So just connect the audio signal from the phone's handfree to the DTMF input.

author
giacomine (author)giacomine2011-06-16

How I make the linking of the cellular one for the circuit? E as I make the programming of the cellular one (keyboard keys)?

author
Chebog (author)2011-03-25

Hello Mr. Author.can i if where do the 4.7 resistors go in the circuit?because I'm trying your project and I really can't figure it out where the resistors goes in the circuit.please tell me what to do?.thank you......

author
unknownpocketnerd (author)Chebog2011-03-26


the 4.7 k resistors go between the DTMF decoder outputs and the transistors' bases. 

author
insaneidiotic (author)2010-10-08

hey iam trying your project to my car actually . iam not that high knowledge to do it .
iam an automobile engineer actually which makes me geek to your project...

hey please assist me with your content that u made for this project .
ill be very happy if u help me out .

since iam weak with the circuit i gonna make it through my friend to fix it in my car as considered to my car how all should i change the settings and pls help me out through mail since iam not a premium user iam not given the contents so pls do try to provide me fully.

iam from india:

my mail ID gokulnathrao23@gmail.com
contact no:+91 9790712498

ill be very helpfull if u provide me your contents of the circuit where will the major fault occur while doing this project pls assit me.....!!!!!!!!!!!!!

author

Well, everything you need is right here, that's why I've wrote it as an 'instructable'.

A person who engages in building this gadget needs to be experienced in electronics, or have some one with that expertice help her/him out.

The hard part is to get the IC. Newark sells it here in Mexico, but I don't know about India.

Anyway thanks for your post.

author
Chebog (author)unknownpocketnerd2011-03-25

Mr. author good day, can i ask you something where do the 4.7K resistor go in the schematic?please reply.cause I'm working on that project of yours.

author

It was nice hearing from you ...iam trying out in my car..... here the IC is easily available...:-) thats great of it...:-)

i have done the circuiting with my friend as explained in the picture.

sorry man that guy just did the circuiting and gave it to me. he just did not fix it to my car ... so only asking these questions.... sorry again....

My doubts .......:-)

hey man .... there is no steps after connecting to the relays, should theses realys be the command giver to the mechanism used in the car. where should i give the relays connected to...

how do i find the input from the headset of the phone it has two ends of ear peice.... which one will do for the project ?

then pls tell me about how to signal the IC through my phone this is where i hung up totally... like first call the phone number of the mobile placed in the car then should i give a combination of the numbers....... i am totally confused here ... please elaborate... how to signal the IC.... ...

hey iam at my final stage to connect to the car and work it out correctly..... if i do this i might impress my girlfriend man....... please do help me man...... whatever happens i have to do it completely.... pls help me out man.....

my face book url:
https://www.facebook.com/reflectorr

do provide me email address of yours so that i can contact u further......

author

The DTMF decoder needs and audio input, so the handsfree's 'ear piece' will have to be desoldered and the pair of wires soldered thru a dc decoupling capacitor to the DTMF. Then make sure you setup the phone to auto answer incomming calls.

The relays can switch on/off anything you want in your car: horn, lights, engine, locks, etc.... Just ask your friend to wire you the stuff you need to control.

Good luck impressing your fiance!

author

thaks buddy....

will soon say u a result of what happened to my car.......:-).

author

It was nice hearing from you ...iam trying out in my car..... here the IC is easily available...:-) thats great of it...:-)

i have done the circuiting with my friend as explained in the picture.

sorry man that guy just did the circuiting and gave it to me. he just did not fix it to my car ... so only asking these questions.... sorry again....

My doubts .......:-)

hey man .... there is no steps after connecting to the relays, should theses realys be the command giver to the mechanism used in the car. where should i give the relays connected to...

how do i find the input from the headset of the phone it has two ends of ear peice.... which one will do for the project ?

then pls tell me about how to signal the IC through my phone this is where i hung up totally... like first call the phone number of the mobile placed in the car then should i give a combination of the numbers....... i am totally confused here ... please elaborate... how to signal the IC.... ...

hey iam at my final stage to connect to the car and work it out correctly..... if i do this i might impress my girlfriend man....... please do help me man...... whatever happens i have to do it completely.... pls help me out man.....

my face book url:
https://www.facebook.com/reflectorr

do provide me email address of yours so that i can contact u further......

author
gormly (author)insaneidiotic2010-10-17

I am still amazed everytime I see a comment like this... sad.

author
DiamondBack (author)2009-03-07

"Now call your cellphone. Once it answers ..." This is suggesting that we have a second phone line on our cell plan, am I understanding that right?

author
stapler117 (author)DiamondBack2010-03-05

 For this use, adding a second phone to your line would be necessary. it would be better to use a prepaid phone since you wouldn't use many minutes.

author
Kasm279 (author)stapler1172010-04-05

Yes but when the service expiration date nears you have to buy more minutes. Pretty soon the minutes pile up, even on my mom's phone that she occasionally uses...

author

I guess they add up.  I am not sure since the service provider some times changes its fees and policies,  but I am pretty sure the phone can take phone calls right.  Anyway, I once in a while call it to see if the phone is still alive and test the functions I have (siren, engine off, locks, etc.).  Only once I had to replace the battery of the cell phone as it was dead and wouldn't charge no more. This is something you will like to do to make sure your system is up and ready whenever you need it.

author

Umm, no? No one said you needed a second plan to call your cell phone. Just call with another person's phone or your home telephone.

author
AndrewNoNumbers (author)2009-08-28

Wow you must have a lot of patience to do something like this. I gave up just reading the parts list. =]

author

Thanks. It has proven to be very useful as one day my alarm's remote control battery died, I was unable to get into my car, so I dialed the system's phone from my cell and was able to unlock the doors then I got in and disable the alarm according to the alarm's emergency procedure (ignition on, valet parking switch pressed for 5 seconds, and so on). But I was able to get back home and then took the other remote control. It is like a "plan B" stuff also.

author
y2j514 (author)unknownpocketnerd2010-04-04

My plan B for when my alarm remote dies is called a key.  It works pretty well - you put the key in and the door unlocks =D

I mean sure the alarm goes off but you do the emergency override.

author
unknownpocketnerd (author)y2j5142010-04-06

I forgot to mention that I disengaged the keylock from the shaft that drives the mechanism, so the only way to get in the car is by means of the electric locks (I am afraid of lock picking).  So my key only works on the ignition switch (for now).

author
imakethings (author)2010-03-12

can u post a schematic of relay driver and further connections between CM8870 and the relay drivers??

author
solpredator (author)2010-02-02

I really like the 'structable but I have a couple of questions about it.
What was the voltage of the ceramic capacitors?
Since there were differences in your build, can you provide the actual schematic used for your board?
I was able to locate a good source for the CM8870 dtmf decoder IC at http://www.dpcav.com 
This project has inspired me to make a similar system that will (hopefully soon) become another 'structable here!  :D  Thank you!

author
WillTheRescue (author)2009-09-12

Could this -with a little modifying- control anything? For instance: could I have it control my home? So I could call my house and turn on lights before I got there?

author
letapjar (author)WillTheRescue2009-11-19

you could do what you're thinking about this with a web server and an ardunio.  I just did something similar - hooked up a lamp to the arduino and now I can send a text message to turn the lamp on and turn it off.  I will write an instructable for it when I have some time.

author
WillTheRescue (author)letapjar2009-11-21

 I have seen people do this before, and I have been very curious about it. I would love if you could post an instructable!

author

Hells yeah! Just about anything you can switch on. Just use the proper relays (the ones I used are rated for high amps, but low voltage). And a 5 V power supply (like a discarded mobile phone charger) can power it up. Halloween is up next and it might be a good change to test this thingy and freak out your neighbors as you are standing outside. There are instructables with spooky stuff you can try.

author

 Haha, I live in Maine. No neighbors. Except the crazy lady next door. It would just save energy if I could call and turn my lights on before I get home instead of having to leave them on all night so I can find my way up the drive way. Thanks for this 'ible!

author

Correction: When I said "control anything" I meant "control anything else'

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