Telephone Pad(URGENT)

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elvinme

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what happens when we press a button on telephone number pad. i know that it generates 2 frequencies . Why 2 frequency are needed to be generated. Why the tone should have low and high frequency component? How the tone is detected. my question is how can you detect frequency of signal by using Band pass or any kind of filter. If this is possible to detect frequencies by filters then what should be the input voltage of that filter. Does sinusoidal of square wave rally matter?

I have uploaded project
 
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elvinme said:
what happens when we press a button on telephone number pad. i know that it generates 2 frequencies .
Click to expand...

yep, Dual Tone, Multi-frequency

Why 2 frequency are needed to be generated.
Click to expand...

You don't,. But would have to consider frequency + duration. Say it was 1 sec at 1000 Hz. This would impact dialing speed. You get MUCH better rejection of spurious signals by using two. When routing was in-band 2600 Hz was used as the tone to disconnect billing. It also was the tone of a whistle enclosed in a cereal box. So, you blow the whistle into the phone and talk forever for nothing.

Why the tone should have low and high frequency component?
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To make them easier to detect and to make sure a harmonic isn't mistaken. Thus having 1 kHz and 2 Khz is a bad idea.

How the tone is detected. my question is how can you detect frequency of signal by using Band pass or any kind of filter. If this is possible to detect frequencies by filters then what should be the input voltage of that filter.
Click to expand...

Yep, bandpass filter and even zero crossings would yield a signature.

Levels are the choice of the designer and available voltages. it's really restricted to the power allowed for audio on a phone line. 100 V p-p, I don't think so. It has to fall within about 5 V p-p is I remember correctly. That's the off-hook voltage. Suppose we were doing this with tubes?

Does sinusoidal of square wave rally matter?
Click to expand...

Yes, because or Fourier theory. You can sum a series of the harmonics of a sine waves to "make" a square wave.
 
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Telephone tones are known as 'DTMF' or 'Touch tone'. Just Google for lots of information on those.

You can decode them using analogue band pass filters, but it's a bit messy.

Have a look at the application on page 13 of this data sheet;

https://www.electro-tech-online.com/custompdfs/2012/05/NE567_SE567_2.pdf

It's much more common these days to use an 8870 - see here for data sheet:

https://www.electro-tech-online.com/custompdfs/2012/05/mt8870dtmf20decoder.pdf

That uses internal digital counting techniques to 'decode' the tone pairs.

And yes, you do need sine waves as inputs. Although the 8870 'clips' the waves internally, and uses square waves to 'count' , it does need the input waves to be sine waves.
If you try and input the tone pairs as square waves, the 8870 decoder can't read them, to separate into high and low tone groups.
 
We are expected to design a circuitry related to this issue. i have uploaded this PDF file of project. can you give your suggestion for design?
in pdf file there is block diagram it says that use summer and amplifier can you explain what it means.

And how can we get sinusoid by opamp to control frequencies. we will use band pass filters to detect.

And should we design band pass filters with or without Opamps ?
thanks.
 
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If you're interested in decoding in further processing I might provide a DTMF code lock circuit.

If you use the voice recorder of your cell phone you can record any combination of four dual tones generated by "Audacity 1.3 Beta Version (UniCode)" at any sequential speed to make it impossible to copy the code.

It serves well for those who regularly forget their house key at home.

Boncuk
 
This school homework is silly. The teacher taught the students NOTHING!
The students know nothing about DTMF, opamps, bandpass filters or any electronic circuits.
 
rogs said:
Not so easy to get a DTMF coder to create the frequencies required in this case. (See the OPs project pdf in the 1st post)
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We are required to do this project as simple as possible to observe characteristics of using filters,adders and op-amps.
please make your suggestions on this direction.
 
Friends,
we have already found how to design transmitter part. But as you know receiver part is more involved. We have a signal having 2 frequencies at transmitter stage and we want determine this by receiver. Should we use two band pass filers for each button in series?
 
elvinme said:
i did not mean how i can design it. I mean which one is better ? Does op-amp has an advantage in Q or in any thing comparing to RLC filters?
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The 'Q' will depend on how you configure the circuit. The design tool I linked to will allow you to select different 'Q' values for op amp filters. There are lots of notes on LC filter designs online -just google lc filters. For audio frequency band pass filters, it's probably easier to use opamps. At these frequencies high 'Q' inductors tend to be a bit big and expensive!

For analogue 'adding' just look for opamp 'summing amplifier' configurations. They will normally be found in the 'inverting' amplifier configurations for opamps.
 
rogs said:
No, use them in parallel.
Click to expand...

By the way it is also required to get the sound of the output signal. It is also said that the current will not be capable of feeding current to speaker to work. By using BJT the current can be controlled but will that damage our output signal ? In what configuration should the speaker be connected?
 
I would suggest the use of a small audio amplifier. Look up the LM386. Messing around with discrete transistors is not a very good way of dealing with this aspect of the project.
Unless of course this is not project, but a homework assignment?... in which case you're likely to have to follow whatever your teacher is asking for, however silly that is, compared to the best 'real world' solutions.....
 
It's very easy to do a design for a low current boost stage. If you add current limiting, that makes it harder.

You can take a resistor + 2 diodes and a resistor from your +- Supply and then also have a positive supply transistor, e-resistor, output, e-resistor negative supply

The output of the OP-amp feeds the center of the two diodes and the feedback is taken from the output. You should be able to figure out how the bases connect.

The amplifier part was extra credit.
 
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