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Reliable remote trigger

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archont

New Member
Hello there,

First, let me apologize for not being an electronics expert. The most advanced thing I ever did was a circuit board with a few capacitors and mosfets, mostly because I never had the need for it. But hey, I learn fast.

I was thinking of a reliable way to make a remote trigger. For example, I want to be able to turn on the lights in my house, turn on the microwave while I'm on the bus or ignite a thermite charge on my HDD as the police is seizing my PC.

Since cell-phones are so cheap and offer unlimited range, they're a good way to reliably trigger something from afar, especially since you ALWAYS carry one. But while it would be OK to turn on the lights, it wouldn't be OK to use it to open the garage door. Suppose customer-support calls or I get an SMS. Or someone else by mistake dials that number. Well, you get the idea.

The point is to have a cell-phone that can be programmed to use the motor/speaker ONLY when a special SMS with a special code is received. Not when it's low on battery, not when there's an incoming call, not when there's new year and the phone wants to beep out a tune and display "HAPPY NEW YEAR". I don't want it to use the motor/speaker on any occasion except when the proper code is used. After all, I wouldn't want my garage door to open for no good reason.

Then I'd like a classic timer - albeit with a button that can cancel the countdown. Oh and a signal light would be nice, but all that should be as simple as programming a single chip and making a really simple circuit board. So I can handle that part myself.

What I won't be able to handle is the phone thing. I want to be positive it will work ONLY when I use the code - so the timer can't be triggered by electrostatic discharge or an accidental call. I walso want it to work when it has to - so I'll permanently plug the phone into the charger. In case the system goes offline the phone battery has full juice and will run for a week or so.

It would be best if the phone were a simple one - an old, monochromatic Nokia. Then again, easy programming means java, and java-enabled phones are more complicated, ergo less reliable.

Post ideas and advice here.

Cheers,
archont
 

crutschow

Well-Known Member
Most Helpful Member
You could have an answering machine pick up the land-line phone at home and have a circuit connected to the line that decodes DTMF signals. You would then program it (probably using a microprocessor) to accept a sequence of DTMF numbers to perform a particular task. You just punch in the numbers at the remote phone (cell or land-line) to perform a task.
 

archont

New Member
Problem is, I don't have a landline over here. Long story, but I probably won't get one. I'm stuck with cell phones only.

Although the approach is quite interesting. Assuming I could rip the code for tone recognition, this shouldn't be that difficult.

Certainly easier than trying to reprogram a cellphone, assuming it's not just hard-coded ROM.
 

crutschow

Well-Known Member
Most Helpful Member
I believe the DTMF sound comes through the speaker of the receive phone after it's connected so you could likely tap into the speaker wires and connect those to a DTMF decoder. Of course you'd also need a circuit to answer the phone when it rang.
 

archont

New Member
Well rigging the phone to auto-answer would be easier than reprogramming it.

Since that can be done using a button, soldiering both ends of the accept call button to a MOSFET along with a timer that enables the button each 2 seconds or so should do it.

That's assuming the MOSFET's internal resistance isn't too big. I'm not quite sure what kind of voltage runs through the keyboard, but I guess it's pretty low.

Rig that up to a 555 and it should be working nicely. I might even add a blinking red LED to that.

So in all the mechanism would consist of four separate circuits
1) The cellphone
2) Auto-accept button timer
3) Tone recognition
4) Countdown timer and some shutdown button.
 
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