Telephone DTMF

[Electroenthusiast]

I'm on a project where i need to obtain a DTMF signal from a call, DTMF signal will be pressed by the dialler(a person who calls the my telephone). What exactly i want to do is i need to obtain DTMF signal from the Telephone and input it to the Micro controller. I'm using HT9170B IC DTMF Decoder for the purpose, and it connects to the microcontroller. I'm okay with connecting 9170 circuit to the microcontroller and programming the Micro controller.

The problem is that the telephone would be dead when i do so, because i'm connecting the wires which (i would otherwise- before) be connected to phone. The telephone-connection would be always dead(cut). So i need to overcome this prob, i want the telephone connection to be on, and therefore receive the call only when a caller calls and switch off when the call is over(and the phone should not be dead at anytime).

Any idea about overcoming this?

 

[Electroenthusiast]

Any Answers?

 

[crutschow]

Use a telephone answering machine to answer the phone for you. The DTMF circuit can be connected to the phone lines all the time (as long as it doesn't draw any DC current and with proper protection from the high voltage ring signal). After the phone is answered, any DTMF signals generated by the sender can be detected by your circuit.

 

[Electroenthusiast]

Use a telephone answering machine to answer the phone for you. The DTMF circuit can be connected to the phone lines all the time (as long as it doesn't draw any DC current and with proper protection from the high voltage ring signal). After the phone is answered, any DTMF signals generated by the sender can be detected by your circuit.

Answering Machine: a good idea, but can i make some circuit on my own for the purpose?(that would be bit complicated)
@underlined: When will it draw high DC current? (I guess the voltage is -24V).
Is HT9170B suitable/ should i use any other IC?

 

[unclejed613]

i'm going by what we have here in the US... the open circuit voltage is 48V, the ringing voltage is about 90-120VAC @20hz. the off-hook DC voltage drops to 6V, and the audio is on a 20mA current loop (not voltage variations, but current variations). so while idle, you have an open set of contacts, and a capacitively coupled ring detector that rectifies the AC ringing voltage and triggers the relay circuit. the closed contacts place a 600 ohm load on the line, in parallel with an optocoupler which takes current variations through the LED side, and turns them into voltage variations on the phototransistor side. the resulting audio feeds your DTMF decoder. once the microcontroller has the control signals decoded, it trips the relay circuit off. the device can be powered in standby mode through a regulator that's only active when the line voltage is 48V, and directly powered by the 6V off-hook voltage when the line is connected.

 

[Gary B]

So why would the telephone be dead? Bridge your device across the line. Its idle DC resistance must be equal or greater than 15K and the 20Hz impedance should be on the order of 7 to 14K. If there is only one other ringer across the line, you won’t affect the network.

One of the niftiest ways to detect the presence of ringing is with a NE2 lamp, diode, capacitor and resister voltage divider. It forms ½ wave rectifier that can turn on a CMOS gate. Select a RC time constant long enough to ensure that voltage spikes won’t accidently trip it. The minimum must ignore time is 25ms. Once ringing is detected, close the loop with a relay and your DTMF detector. Note that the impedance of your circuit should be either 600 or 900 ohms depending on if you intend to use it on a trunk or line but the receiver is specified to work into a 600 ohm source. In your case, I would guess the correct answer would be 900 ohms to the network and 600 ohms to the receiver. Also, it should be designed to pull a minimum off hook current of between 20ma and 35ma. The CO must drop the call if the current drops below 18ma and will have a feed resistance of 200 ohms plus loop resistance. Loop current will not normally exceed 35ma but could go as high as 50ma if you live next door to the CO or if they are using loop extenders. If the latter is the case, the maximum DC voltage could be as high as 130 volts so caution is advised.

If you have never done it before, DTMF can be picky about what is a good set of tones and what should be rejected.
• Both frequencies must fall within 1.5% of their nominal values to be detected and must be rejected if either is more than 3.5% from nominal.
• Must reject tone pairs that have a duration of less than 23ms and record ones with a duration of 40ms or longer. It must also keep up with a cycle time of 93ms or longer.
It goes on from there to get picky about tone levels as far as absolute levels, twist (the difference between the levels of the two tones) and about a dozen other factors. All the details are spelled out in the EIA/ANSI/TIA-464-C section 7.1.5.

Last, if you intend to use this on the public network, you need to apply to the FCC for type acceptance of your device.

 

[mvs sarma]

perhaps we cant discuss without seeing the schematic used by the OP
he has to upload what he has actually wired and not a tentative for discussion sake. Hope the OP gets it.

 

[audioguru]

The OP doesn't understand that the DTMF dialed goes to the central office, not to his telephone line. The central office will send his line a digital "caller ID" (if he pays a fee for the service) during ringing.

My telephone line is cheap because I don't pay extra for DTMF (my phone line is a rotary dialed line). The telephone company wanted to force us to pay the fee and switch to DTMF so we sued them and we won. My phones dial "clickety-click". I switch my phones to DTMF (after the bank answers) when I do banking by phone. So I don't have caller ID.

 

[mvs sarma]

hi audioguru,
Perhaps the Op wants to sample the number he dialed through a circuit he designed, and display the dialed number. It is very much possible.

 

[audioguru]

hi audioguru,
Perhaps the Op wants to sample the number he dialed through a circuit he designed, and display the dialed number. It is very much possible.

Hi Sarma,
No.
The OP wants to display the number dialed to him by the distant caller. Before his telephone rings. Before the caller ID circuit knows anything.

 

[unclejed613]

actually the OP's description sounds more like some type of remote control app or data logging of some kind. he seemed to want something that would pick up the line when a call comes in, and wait for DTMF data, then hang up after the data comes in.

 

[Gary B]

The OP doesn't understand that the DTMF dialed goes to the central office, not to his telephone line. The central office will send his line a digital "caller ID" (if he pays a fee for the service) during ringing.

My telephone line is cheap because I don't pay extra for DTMF (my phone line is a rotary dialed line). The telephone company wanted to force us to pay the fee and switch to DTMF so we sued them and we won. My phones dial "clickety-click". I switch my phones to DTMF (after the bank answers) when I do banking by phone. So I don't have caller ID.

Actually, those are two different features. Caller ID is a 2400 baud modem signal that is sent between the first and second cycles of ringing. DTMF was originally invented to speed up CO call processing so more customers could be served by the existing equipment. That they can get most people to pay them for it is a bonus.

 

[Mr RB]

...

My telephone line is cheap because I don't pay extra for DTMF (my phone line is a rotary dialed line).
...

What the??? Wow AG I always suspected you were living in some type of time warp... Not you've proved it!

Come on admit it, you're that old professor guy in "Back to the Future" aren't you?

Do you get utility bills that read; "Electricity usage; 1.21 Gigawatts"

 

[Electroenthusiast]

here's the block diagram

 

[audioguru]

What the???Do you get utility bills that read; "Electricity usage; 1.21 Gigawatts"

All electricity meters in my city were changed to "Smart Meters" so soon we will be billed more during the day and billed less at night.
Mine and my neighbour's meters were changed within about 2 minutes. They started reset to zero and I can see my neighbour's big LCD display.
Her meter reads double the amount of electricity used than mine. She lives alone and I have 2.25 people in my home. The 0.25 person is my mother-in-law who stays with us for three months of each year.