remote mobile switch - RFEDA Forum

How does a mobile phone make different frequeny sounds. Does it depend on the current?If so, could you replace the speaker with a current activated switch so when you phone it and press a button dipending on which switch you want, switch a relay?I think it's called a touch tone switch thingimy or something, if anyone knows how they work then please reply ta. pete pvkev
pvkev

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1	mxg330	The frequency of the noise gives the pitch of it. And i believe that the keypad tone is just at different fequencys. So if you send a 1khz and a 5khz signal through a buzzer you will find a big different in the noise produced. There is a thing called a DTMF Decoder and it does exactly what your after. But i believe you will have to have a pic to decode the output or some logic or something. As i believe thereis only 1 output pin and the decoder sends the result in binary.I think the Volume depends on the voltage and the distance between signal source and speaker depends on the current. But i aint too sure.If you look at the datasheet for the component it will tell you how to work it. As it will have the best explanition.I have often thought about making a R/C car and being able to control it with my mobile. Just because it would be like a james bond gadget but with a toy car instead of the real thing. But ive never looked into the project properly let alone the DTMF decoder chips.	2008/9/3 21:02

2	bubulu	Nope, that's DTMF and is actually TWO sounds (two seperate frequencies) for each button pressed. The frequency is derived from dividing the output of a crystal oscillator or master clock. DTMF is absolutely dead easy to use. Encoders (at least the dual inline species) come in two main flavors, Keyboard or BCD input - simply put the former expects to see pushbuttons connected to it and the latter expects to see a 4 bit digital input. Most need nothing more than a colourburst crystal, a few buttons, and something to hear the sound to work. Forget ordering them. They can be found way too easily in neighborhood trash as every modern phone uses them. Even the "higher end" electronic office phones often use a stand alone DIP encoder. Decoders may come in more than one flavour but I've only seen 4 bit BCD outputs myself. Just as easy to use, but not as easy to find. At least from the trash perspective. Nope. 4 LEDs if you'd like. A nice TIL-311 HEX display will look uber cool and connect right up. Can use run of the mill 7447/7448 BCD to 7 segment decoders too but there are actually 16 valid "touch tones" and the standard 7447/8 show ugly sybols for hex A-F.If you actually want the outputs to do work, well the sky is the limit of course, but think of say a 74154 with 16 relays on it's outputs... Nice project. Fairly easy to do to. Need a fair soldering hand, and a mobile to hack. DTMF can also be sent over walkies - or for that matter just about any audio capable device.	2008/9/3 21:02

3	netlgc	Well i'm not exactly electronic fluent so am i looking for a DTMF schematic?pete	2008/9/3 21:02

4	mxg330	Do i have to have a programmable chip, because i havn't got the microchip boards or knowledge about it...	2008/9/3 21:02

5	rickli	Nothing programmable required. Is a good beginner project then, will chat with you later then - gotta get on with the long commute at the moment.	2008/9/3 21:02
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