Pls find the attachment of a self-explanatory telephone ring detection circuit.
The problem is that my 4N25 is not secure.
'After' four or five times of operation, it destroyed itself.
Then I replaced it with another, but same fate.
I think the protection circuit of 4N25 has no better immunity against high ring volt.
The limiting devices should keep the current through the opto LED down to 3 ma and the LED will only support 2 or 3 volts, so voltage should not be a problem. I suggest that a part of the limiting circuit has failed. For instance, if voltage is eating the opto, then diode D2 must have failed.
Atually, there were 3 mistakes. The post about ring voltages was by indulis, and thanks for the spell check. I haven't found another spell checker on this site.
This guy posted same question and schematic on another site and never came back.
I would suggest checking the wiring very carefully, then a base resistor of maybe 1 megohm from base to emitter.
I set-up the same circuit again on a separate bread board with all new components except 4N25.
The circuit was fine for the first few checks, again the problem arises. That is I cannot get +5V
at pin4. This seems to be the problem is not with opto.
CI tantalum, electrolytic is not avail.
I cannot measure AC voltage across R1 with ordinary multimeter as intermittently occurring ring volts. I don't have any oscilloscopes.
R1 10K 0.5W
Deliberately I avoided isolation transformer as it only for electrical protection.
When I set-up 0-DETECT-SCH, I got constant VCC continuously at pin 4 when Ringing occurs. But I would like it to be of on-off pulses. ON pulse indicates the presence of Ring volt.
Hey bychon,
[The limiting devices should keep the current through the opto LED down to 3 ma and the LED will only support 2 or 3 volts, so voltage should not be a problem. I suggest that a part of the limiting circuit has failed. For instance, if voltage is eating the opto, then diode D2 must have failed. ]
Judging from post #10, "I get constant Vcc at pin 4 continuously when ringing occurs" and post #14, volts supported at 1 ma (missing all the decimal points) the circuit is working.
Judging from post #10, "I get constant Vcc at pin 4 continuously when ringing occurs" and post #14, volts supported at 1 ma (missing all the decimal points) the circuit is working.
The way to test the circuit is this:
Connect everything up and leave the input to the optocoupler open.
With a 12v supply and 1k resistor plus 10 pot, reduce the resistance to the LED and see when the voltage on the emitter starts to rise. I would put a 100u in place of 1u to see this effect.
The problem is this: The optocoupler needs more current to illuminate the LED to turn the transistor on sufficiently to charge the 1u.
I always put the ring voltage through a bridge as you don't know the polarity of the phone line. Some phone lines actually change polarity!
I have done the same thing with an 8 pin PIC and a few components. It is much better to time the rings rather than count them. I would also join 5 6 7 outputs to get a more-reliable result.
When I set-up 0-DETECT-SCH, I got constant VCC continuously at pin 4 when Ringing occurs. But I would like it to be of on-off pulses. ON pulse indicates the presence of Ring volt.
That is your problem. Assuming that you mean C1 instead of CI, it should not be tantalum or electrolytic. It should be a film capacitor rated at 150V or more.