Device Control using telephone(DTMF)

vinodquilon · Dec 24, 2009

Please find the attached schematic of DTMF Telephone Controller.
But, I have no idea about the use of ic7 555. Kindly help me to get out this problem as we are planning to do a mini project based on this. Hoping for a good reply.....

audioguru · Dec 24, 2009

The very old circuit uses very old 74xx TTL logic ICs.
IC7 gives a tone to the telephone line when pin 15 of the touch-tone decoder goes high.

vinodquilon · Dec 24, 2009

audioguru said:
The very old circuit uses very old 74xx TTL logic ICs.
IC7 gives a tone to the telephone line when pin 15 of the touch-tone decoder goes high.

Sir, kindly explain in detail.I cannot get the idea 'tone to the line'.

audioguru · Dec 24, 2009

vinodquilon said:
Sir, kindly explain in detail.I cannot get the idea 'tone to the line'.

When the touch-tone decoder receives a valid touch-tone pair of frequencies then it turns on the transistor that turns on the IC7 555 oscillator. The output of the oscillator is coupled to the telephone line through a coupling capacitor and the tone from the oscillator is fed to the telephone line which makes a beep sound.

vinodquilon · Dec 24, 2009

audioguru said:
When the touch-tone decoder receives a valid touch-tone pair of frequencies then it turns on the transistor that turns on the IC7 555 oscillator. The output of the oscillator is coupled to the telephone line through a coupling capacitor and the tone from the oscillator is fed to the telephone line which makes a beep sound.

Thanks for valuable information.
In the circuit there is other two beep generating sections(B) at the output of AND gates.Does it possible to couple all three beep generators into same one ic7 555.

audioguru · Dec 24, 2009

The circuit does not show what B connects to.

The circuit is a nightmare of errors:
1) None of the relay coils have a spike prevention diode so the driver transistors will probably be destroyed.
2) Transistor T1 short-circuits the power supply when it turns on.
3) The 5V regulator does not have an input capacitor and does not have an output capacitor.

vinodquilon · Dec 24, 2009

dtmf

audioguru said:
The circuit does not show what B connects to.

The circuit is a nightmare of errors:
1) None of the relay coils have a spike prevention diode so the driver transistors will probably be destroyed.
2) Transistor T1 short-circuits the power supply when it turns on.
3) The 5V regulator does not have an input capacitor and does not have an output capacitor.

I have made the following changes-
1. connect IN4148 across relay coil in backwards
2. no change for T1 as it works in switching region. When it turns on, it will permit current to flow from collector to emitter only, thus producing a negative trigger pulse at pin 2 of 555. No issue of shorting both Vcc's.
3. 0.33uF at input and 0.1uF at output of regulator IC

Kindly inform me about the validity of above changes.
The link of project is Device Control Using Telephone: ElectroSofts.com
I am very glad to you to give all these suggestions and expect more to fulfill our mini project work.

gary350 · Dec 24, 2009

If you want to produce the tones try Motorla 4410 IC. It only needs 5 resistors, 1 crystal, 1 diode, 2 transistors and a key pad to produce all 16 tones. It includes the extra tones your home telephone does not have.

Also try LM2430 or LMX2433

The 4 extra tones are special tones used by the operator.

Lets assume you live in New York and you want to call California. You dial the long distance operator on your home phone. This connects you to a direct line to California. As soon as the long distance operator answers you push the KP button and this disconnect the operator and leaves you connect directly to California. Now you can dial any telephone number in that California area code as if it is a local phone call. The telephone will not ring until you push the ST button. If you do not push the ST button you continue to be connected to that phone like a wire tap. If that phone makes or receives a phone call you can hear both sides of the conversation. It works but be warned the phone company computer will record the whole thing so don't do it unless you would like to receive a visit from the FBI. It also works on a pay phone if you can find a pay phone these days.

audioguru · Dec 24, 2009

vinodquilon said:
no change for T1 as it works in switching region. When it turns on, it will permit current to flow from collector to emitter only, thus producing a negative trigger pulse at pin 2 of 555. No issue of shorting both Vcc's.

No.
T1 short circuits the VCC to ground when it turns on.

vinodquilon · Jan 8, 2010

audioguru said:
No.
T1 short circuits the VCC to ground when it turns on.

Sir,

I will replace IC5 & IC6 with a perfect 555 mono without shorting VCC. Now I think the circuit is free from all bugs.
Finally I have one more doubt about the working of the circuit, when someone calls to the specified number RLY 1 automatically places R18 across line thus reducing line voltage from 50 to 12 V, this is equivalent to accepting the call. RLY 1 is controlled by IC5 for 60 sec.
When MT 8870 detect one digit, it would produce one StD high pulse and it is used for producing audible beeps to the remote caller. For this StD would activate IC7 for producing audible frequencies and it is fed back to the line as shown by the T point on figure. Here is the doubt, why the AF signals doesn't goes to ground through R18 instead of going to the remote end......

audioguru · Jan 8, 2010

vinodquilon said:
For this StD would activate IC7 for producing audible frequencies and it is fed back to the line as shown by the T point on figure. Here is the doubt, why the AF signals doesn't goes to ground through R18 instead of going to the remote end?

Some of the tone goes to ground through R18 and most of the tone goes to the remote end of the telephone line.

vinodquilon · Jan 8, 2010

Dtmf

audioguru said:
Some of the tone goes to ground through R18 and most of the tone goes to the remote end of the telephone line.

Sir,

Between two telephone lines how we can identify which line to be grounded and which one goes to DTMF IC.

audioguru · Jan 8, 2010

I am not a SIR, I am just an ordinary guy.
There is only one telephone line that has two wires that are perfectly balanced.
The "grounds" in the circuit must never be connected to an earth ground, they are just a common point of the circuit.
It doesn't matter which wire is which.

The TIP wire of a telephone line is usually at 0VDC and the RING wire is usually at -50v when the line is not in use. 20Hz 90VAC ringing is applied to the -50VDC on the RING wire during ringing.

vinodquilon · Jan 8, 2010

dtmf

audioguru said:
I am not a SIR, I am just an ordinary guy.
There is only one telephone line that has two wires that are perfectly balanced.
The "grounds" in the circuit must never be connected to an earth ground, they are just a common point of the circuit.
It doesn't matter which wire is which.

The TIP wire of a telephone line is usually at 0VDC and the RING wire is usually at -50v when the line is not in use. 20Hz 90VAC ringing is applied to the -50VDC on the RING wire during ringing.

So I will connect Red Ring wire to DTMF IC and Green Tip wire to lower end of R18, no other connection points for Tip wire. Is it OK now........

vinodquilon · Jan 8, 2010

dtmf

audioguru said:
I am not a SIR, I am just an ordinary guy.
There is only one telephone line that has two wires that are perfectly balanced.
The "grounds" in the circuit must never be connected to an earth ground, they are just a common point of the circuit.
It doesn't matter which wire is which.

The TIP wire of a telephone line is usually at 0VDC and the RING wire is usually at -50v when the line is not in use. 20Hz 90VAC ringing is applied to the -50VDC on the RING wire during ringing.

In figure both StD and B signal energize IC7 to produce AF beep signals. Does it is possible to combine these 3 control signals (one StD, two B) towards IC7 as each control signal occur at different times. If yes, how we can combine three inputs to a single transistor T4 base point.

One more doubt, if we fed back beep signal 'T' towards phone line directly without any couplers does it cause any loading problems when high volt ringing signal comes.

vinodquilon · Jan 9, 2010

dtmf

vinodquilon said:
In figure both StD and B signal energize IC7 to produce AF beep signals. Does it is possible to combine these 3 control signals (one StD, two B) towards IC7 as each control signal occur at different times. If yes, how we can combine three inputs to a single transistor T4 base point.

One more doubt, if we fed back beep signal 'T' towards phone line directly without any couplers does it cause any loading problems when high volt ringing signal comes.

In figure both StD and B signal energize IC7 to produce AF beep signals. Does it is possible to combine these 3 control signals (one StD, two B) towards IC7 as each control signal occur at different times. If yes, how we can combine three inputs to a single transistor T4 base point.

One more doubt, if we fed back beep signal 'T' towards phone line directly without any couplers does it cause any loading problems when high volt ringing signal comes.

audioguru · Jan 9, 2010

vinodquilon said:
In figure both StD and B signal energize IC7 to produce AF beep signals.

No.
B does not do anything to iC7.

One more doubt, if we fed back beep signal 'T' towards phone line directly without any couplers does it cause any loading problems when high volt ringing signal comes.

IC7 and C9 will be destroyed by the high voltage ringing signal. They should be disconnected by the relay when the circuit is on-hook and connected when off-hook.

The circuit is a mess:
1) Every relay coil needs a reverse-connected diode across it to stop the flyback high voltage from destroying the driver transistors.
2) The 7805 regulator needs an input capacitor and an output capacitor.
3) The 555s need their own supply bypass capacitors because they cause a high current spike when their outputs switch.

vinodquilon · Jan 9, 2010

audioguru said:
No.
B does not do anything to iC7.

IC7 and C9 will be destroyed by the high voltage ringing signal. They should be disconnected by the relay when the circuit is on-hook and connected when off-hook.

The circuit is a mess:
1) Every relay coil needs a reverse-connected diode across it to stop the flyback high voltage from destroying the driver transistors.
2) The 7805 regulator needs an input capacitor and an output capacitor.
3) The 555s need their own supply bypass capacitors because they cause a high current spike when their outputs switch.

You have informed me about the needs of relay diodes and 7805 regulator capacitors.
In addition to all these I will add 555 supply bypass capacitor, pls inform me about its value (Is it OK with 0.1 uF).
In the circuit schematic the function of 'B' is to produce beeps to inform remote end about the status of control device. One B for Device Status Check, caller can press the specified device control key if the device is already ON caller hears beeps. Second B for Device Status Feedback, after turning on a device by caller it produce beeps to inform about the success of his action. StD informs the remote end through beeps when a key press is decoded and registered by MT8870. So I asked the question---
"In figure both StD and B signal energize IC7 to produce AF beep signals. Does it is possible to combine these 3 control signals (one StD, two B) towards IC7 as each control signal occur at different times. If yes, how we can combine three inputs to a single transistor T4 base point." Otherwise I think three separate IC7 needed and thus three separate fed back lines to telephone line through separate newly added relay coils.

audioguru · Jan 9, 2010

vinodquilon said:
I will add 555 supply bypass capacitor, pls inform me about its value (Is it OK with 0.1 uF).

The datasheet for the LM555 recommends two supply bypass capacitors for each 555. The values are shown.

it is possible to combine these 3 control signals (one StD, two B) towards IC7 as each control signal occur at different times. If yes, how we can combine three inputs to a single transistor T4 base point."

Make an OR gate with one resistor to ground and a series diode from each of the 3 signals.

vinodquilon · Jan 9, 2010

audioguru said:
The datasheet for the LM555 recommends two supply bypass capacitors for each 555. The values are shown.

Make an OR gate with one resistor to ground and a series diode from each of the 3 signals.

That is a good idea.

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Device Control using telephone(DTMF)

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