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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.
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.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.
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.
No.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.
Some of the tone goes to ground through R18 and most of the tone goes to the remote end of the telephone line.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.
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.
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.
No.In figure both StD and B signal energize IC7 to produce AF beep signals.
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.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.
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.
The datasheet for the LM555 recommends two supply bypass capacitors for each 555. The values are shown.I will add 555 supply bypass capacitor, pls inform me about its value (Is it OK with 0.1 uF).
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."
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.