ringing detection circuit

[summersnow]

Hi all..

I have a problem here.
i tried to build a ringing detection circuit.
But when i connect the ring and tip of the telephone cable to this circuit,i found out that it automatically being off hook.
As a result,it cant be used to detect the ringing signal and it get engaged when i call to the telephone.
Is there anyone who can explain to me why it get off hook even there are no ringing signal?
Thanks in advance

[audioguru]

The telephone ringing signal in North America is 90VAC. Your zener diodes are only 3.3V and the 330 ohm resistor creates a huge current with a peak of 370mA. I am amazed that your opto-isolator didn't burn out.

Look at all the ringing detector circuits in Google that use 15V zener diodes and a 10k resistor like this one from a modem:

[summersnow]

hi audioguru..

thanks for reminding..
actually,i write wrongly in the circuit..i'm using 16V zener diode..
anyway,after i change the resistor to 10kohm,the ringing detector circuit works..
Thanks for your help

[walid]

Hi guru
Now I'm  have a question:
in your circuit shown below:

why using two back to back 15v zeners. What they exactly do?
thank you

[audioguru]

why using two back to back 15v zeners. What they exactly do?

They block signals that have a peak-to-peak voltage of less than 15.6V such as tones and connection clicks. Then the ringer doesn't falsely ring.
The ringing signal in Canada and in  the US is 90V RMS which is 255V peak-to-peak.

[walid]

Hi guru
ok thank you i understand it.

another question: why D3?

[summersnow]

hi..

actually the D3 is to limits the voltage drop across the LED of the optoisolator to protect it from negative voltages,therefore avoid any possible damage..

[walid]

Hi summersnow
thank u very much for your reply
can u please explain it more with numerical example.

[audioguru]

Nearly every LED has a max reverse voltage rating of only 5V. The ringing signal has a peak voltage of 127V so the diode conducts and protects the LED.

[mvs sarma]

Hi guru
Now I'm  have a question:
in your circuit shown below:

why using two back to back 15v zeners. What they exactly do?
thank you

Hi

we can also use a DIAC (conducts  at  32 volts in each diection) and this will reduce comonent count. component is ST micro DB xx

[walid]

Hi guru

Nearly every LED has a max reverse voltage rating of only 5V.

It is a good point for me to put in my memory. But there remains a question: whether such a diode (4148) can bears this 127v ?

Hi  mvs sarma

we can also use a DIAC (conducts  at  32 volts in each diection)

This point is excellent and wonderful, thank you

[audioguru]

The protection diode conducts when its voltage becomes higher than only 0.6V. Then when the AC ringing voltage is reversed across the protection diode then the LED conducts and limits the voltage to only about 1.5V. So the 1N4148 has a max voltage during ringing of only 1.5V.