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Cell phone dtmf generator/decoder

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atollp

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Hi forum,

I am building this dtmf decoder circuit. Although Instead of using a landline I wanted to use a cell phone as the receiver--(you can't run a wireless robot with a landline phone attached to it)

all i need is for someone to verify that this circuit will function just the same with a cell phone, because I don't want to start building and then have it not work.

I figure that it will, because it will produce the same dtmf tones as a landline. I have all the connections from the phone and auto-answer, etc. figured out. I just need to be sure of this last thing!

Pleasse help!!
 

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betwixt

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The design has parts to withstand the high voltages present on land lines, you can safely remove most of them if you are connecting to a mobile phone loudspeaker or earphone socket. This makes the circuit much simpler without losing any functionality. Take a look at the design at ATV-Projects DTMF Control Unit and you can see that only three components are needed, C1, R1 and R2. You should use a capacitor at pin 17 (as in my design) or you will find the circuit is prone to interference, it sets how long the tones have to be present before the circuit responds, without it, there is a chance of it mis-triggering if it hears noise or other sounds.

Brian.
 
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atollp

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Hey so I have this pretty much figured out, thank you. Although I do have two last questions:

1. For C1 and C2, does the voltage rating matter? The caps I have are 100nF at 250v.

2. And where does the + side from the cell phone enter the circuit? I notice that in your schematic the - side goes into the proper pin on the mt8870, but the positive just connects to pin 4.

3. And finally, I'm just assuming the rest of the circuit remains the same. Am I right?
 

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1. The capacitors only have to be rated to withstand the highest voltage in the circuit and I'm guessing in your case that's only 5V so using 250V parts is fine. 100nF is also the correct value to use.

2. The design you showed uses the input in differential mode, mine uses a single input. If you connect it like I did, the audio (from the phone) goes to C1 and the phones ground connects to the 0V line.

3. Yes, you are just decoding singe digits. My design used a microprocessor to look for specific combinations of digits so some extra 'thinking' had to be done in software.

Brian.
 

atollp

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Hey thanks so much, you've been of great help. I will test out the circuit as soon as possible and post results on here
 

atollp

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Hey so I was wondering...on the schematic I posted--there is a part of it on the right that uses a 330k resistor, and a 100k, and a 1N4148 diode....my question is if I can scrap most of that and use what the schematic you linked me uses....which is a 330k resistor across pin 16/17 and the 100nF cap. I am almost done building...just waiting on my IC to come in the mail....
 

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The purpose of the diode network is to set how long a tone must be present before the IC responds. If you imagine a situation where the IC responded instantly, it would be likely to give false outputs every time it saw anything closely resembling the dial tones. The extra components are there to ensure it rejects very short bursts of tone but is still able to accept ones of reasonable duration. In a real situation, dial tones are normally present for the whole time the key is held down and simple human reaction time means that is usually 100mS (one tenth of a second) or more, even if you press and release the key as quickly as possible. The component values I used will make the IC respond to tones of about 40mS or more which is fast enough to react to the fastest key press but slow enough to reject noise and other random tone occurrences.

My design has been in use for nearly ten years now and responds to dial tones sent over a microwave radio link, it has never failed once so I think it passes the reliability test !

Brian.
 

atollp

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You have been very helpful...my MT8870DE IC should be here today or by early next week. Thank you. I'm assuming that my version of the 8870 and the one you show MV8870 will function the same, they seem to have the same pin diagram, although pin 5 on the mv8870 is labeled as SEL, and on my version its INH. However they both connect to ground. ?
 

betwixt

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The parts should be the same. I guess SEL = SELect and INH = INHibit which amount to the same thing.
Brian.
 

atollp

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So I've tried the circuit, but it's not working correctly. When I supply power, one of the outputs will turn an LED on sometimes but not others. The STD output on the chip will always flash however when power is supplied to the circuit, however. When calling the phone attached to the circuit, and entering the dtmf, nothing happens to either LED. Do you have any idea why this might be?

Thanks in advance
 

khaldun904

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can i send 10 tone per sec using t68i Sony Ericsson and what cell phone do you recommended.
i need it in my project i must send at least 10 t/s if not i will fail please help.

i am using at command to send dtmf during acall.
 

betwixt

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This is someone else's thread but I'll answer you, the normal maximum speed for DTMF is about 40mS per digit or 25 digits per seconds so you should be OK. The actual speed you can send from a mobile phone depends on the phone manufacturer and I doubt they release the figure in the specification as it isn't something a normal user would worry about. If you can't send AT commands to produce tones fast enough, try loading the digits into one of the phones memories (phonebook) then tell it to dial the number from there.

Brian.
 

khaldun904

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This is someone else's thread but I'll answer you

Brian.

Thanks betwixt sorry about that:oops:. after i put this i made a new thread:wink:

the normal maximum speed for DTMF is about 40mS per digit or 25 digits per seconds so you should be OK. The actual speed you can send from a mobile phone depends on the phone manufacturer and I doubt they release the figure in the specification as it isn't something a normal user would worry about. If you can't send AT commands to produce tones fast enough, try loading the digits into one of the phones memories (phonebook) then tell it to dial the number from there.

Brian.

i want to send at command using pic microcontroller what the procedure you suggest to achieve that (what at commmand can send the number to phone mem and then make it dial the num) this dtmf
(numbers)must sent when a voice call is started and then close.
i made a test for sending dtmf using AT+VTS (at command) but it just sent a (10 tone in 5 sec ) in other word( 2 tone in 1 sec) which is too long. i need to send 10 tone per sec at least. if i achieved 25 digit per sec as you said this will be awesome.
in my project i am using T68i Sony Ericsson and pic16f877a to send at command.
the pic will initiate a call and then send the digits.

Thanks for the reply.
 

khaldun904

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Hi,
i need to know how i can send fast dtmf using cellphone while in voice call, i am using T68i sony Ericsson to send the dtmfs and this will be automatic using pic microcontroller using at command.
and how i can cancel the back acknowledgment from the cellphone "OK" when i send to it the at command??for fast respond.
i will be very great full if you helped me this will be for my project........p.l.e.a.s.e.h.e.l.p..............
thanks
 

betwixt

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How fast an individual phone model works is decided by the phone manufacturer, there isn't a standard as far as I know.
Do a search for a document called "t68i_at_commands_dev_guidelines.pdf" which lists all the AT commands supported by the phone. On page 151 it tells you how to suppress response messages and on page 145 it tells you how to use ATDxxxxxx to dial number xxxxxx and also how to dial from the memory and last callers lists.

Brian.
 

khaldun904

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How fast an individual phone model works is decided by the phone manufacturer, there isn't a standard as far as I know.
Do a search for a document called "t68i_at_commands_dev_guidelines.pdf" which lists all the AT commands supported by the phone. On page 151 it tells you how to suppress response messages and on page 145 it tells you how to use ATDxxxxxx to dial number xxxxxx and also how to dial from the memory and last callers lists.

Brian.

thanks i saw this guide and went to page 151 and 145. But i tried the page 151 commands (before and after too) they just cancel the showing of the echo.but the time to send the command and being able to send the other is the same as before so the suppression of echo dose not effect on time needed to transmit the command.(return to zero point:|:-:)?)
and when sending the ATD command during the call it just initiate an another call.....so not useful for sending DTMF in call.

any other idea's ???????????? :cry::?:
 

betwixt

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Maybe there is no method of doing what you want.

Try this though, pages 71 and 87, the AT+CKPD command, it looks like the number comes next, then the duration of the key in 0.1s units, then the pause between digits in 0.1s units.

Brian.
 

khaldun904

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Stile in the same sending speed at max 10 tone per 5 sec OR 2 tone/sec
i did not get any higher speed.
but telephone system,the dtmf can be sent 10 tone/sec.:?:?:?:?::?::-(
 

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