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sending a beep into the telephone line???


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Hi guys!

well! im making a circuit in which im using a 555 IC in astable operation. The frequency is about 152Hz which is an audible frequency. The output pin that is pin3 of 555 IC is connected to TIP of the telephone line. The idea is this that when the supply would be connected, the IC would produce a beep sound that can be heard on the telephone.

The problem that im facing is this, that the TIP terminal of my telephone line is providing current and voltage to my circuit in turns. as u know that when a bell rings, the exchange sends us a signal at 75V rms. SO my IC is behaving quite unexpectedly here, the telephone is engaged.


What i want is to stop the current and flow from TIP to 555. There should be only one flow and that is from 555 to TIP. How can i make it?

in a certain circuit diagram, i found a coupling capacitor connected between pin3 and TIP. I used a 1uf as a coupling capacitor, but even then im getting a very high voltage i.e. about 40V DC at my pin 3 of IC.

please help me as soon as possible! or give me some other idea of making this beep heard in the telephone!

thanks in advance!

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Hi Shaiq,
A telephone line is a balanced pair of wires. The balance must be maintained by connecting your circuit to both wires through an audio transformer or operating your circuit from a battery.
The DC voltage on the telephone line is maintained by capacitor-coupling your oscillator/transformer to it.
The high voltage ringing voltage is kept away from your circuit by using a relay to connect it when the line is not ringing.

The DC voltage on the telephone line will increase to about 50V when the telephone hangs up so you need to protect the output of the 555 from its damage with diodes.

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(I am assuming US style phone system here so forgive me if I am off target)

The phone line expects to see about 600Ohms per unit.

The main issue I have always dealt with is the ON/OFF hook changes in the system.
Idle the system sits around 60v
Ringing is around 90VAC  @ 20hz
Offhook and in use the DC component is around 40v

Here is a link to a wealth of Phone interfaces. 

http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html#audioint

-Mike

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HI guys!

thanks for ur tremendous help. that was very useful to me.

PLease tell me any solution  that doesnot require an audio transformer because that is not readily available in the market here.

If i go for diodes, do u think that this will be sufficient for blocking DC from coming to 555 timer??

if not then what should i do??

how these coupling capacitor would be rated, i mean im using a 1uf, is it enough???

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If you don't use a transformer then the 555 circuit must be battery operated to avoid unbalancing the phone line causing mains hum.
Diodes will protect the 555 in the moments when the relay contacts are flying.
Without the relay contacts then the protection diodes short the ringing voltage.

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