DTMF IC Test_failure

[vinodquilon]

Pls find the attachment.
After setting the circuit (using mt8870) , when I switch on the circuit LEDs 1,2,3, & 4 are all suddenly ON without detecting ring volt and key press. Does this mean the IC is out-of-work. I have only one IC, otherwise I would have gone for IC replacement.

 

[jimlovell777]

Make sure you didn't install the LEDs backwards.

 

[vinodquilon]

Make sure you didn't install the LEDs backwards.

ya, it is in correct polarity.

 

[jimlovell777]

What happens when you press a DTMF button on the Telephone the circuit is connected to? Anything? Does the proper sequence of LEDs illuminate for that moment?

 

[vinodquilon]

What happens when you press a DTMF button on the Telephone the circuit is connected to? Anything? Does the proper sequence of LEDs illuminate for that moment?

LEDs ON when the circuit is turned ON and it remains in the ON state after pressing keys. No change for output LEDs.

 

[audioguru]

The circuit is not supposed to detect the ringing voltage. The zener diodes stop the ringing high voltage from destroying the IC.

 

[vinodquilon]

The circuit is not supposed to detect the ringing voltage. The zener diodes stop the ringing high voltage from destroying the IC.

But the ringing voltage of around 50v is accepted by local telephone.When it is in off-hook 50v reduces to 12v. Then the dtmf tones are applied to IC. But in my check LEDs 1 to 4 except 5 are ON all time. When key press occurs at remote end no change in state of any LEDs 1 to 5.