telephone ring detector

Thread Starter

kygh_qin

Joined Nov 29, 2009
6
I use this circuit as telephone ring detector, but I can’t get a pulse at pin 5 when the phone rings. When there is no incoming call, the voltage at pin 5 is 5V. When the phone rings, the voltage drops to 3V instead of 0V. Anyone can tell me how to solve this problem to get a ring pulse? What is voltage at pin 5 I should get when there is no phone ring? 5V or 0V?
 

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hgmjr

Joined Jan 28, 2005
9,027
I use this circuit as telephone ring detector, but I can’t get a pulse at pin 5 when the phone rings. When there is no incoming call, the voltage at pin 5 is 5V. When the phone rings, the voltage drops to 3V instead of 0V. Anyone can tell me how to solve this problem to get a ring pulse? What is voltage at pin 5 I should get when there is no phone ring? 5V or 0V?
In particular I would like to know what value you are using for R2 in your circuit.

hgmjr
 

Thread Starter

kygh_qin

Joined Nov 29, 2009
6
C1 330 nF 100V
R1 10 kohm 1W
D1,D2 12V zener diode
D3 1N4148 diode
U1 4N27 optoisolator


this is the values of the components that I used in ring detector circuit. I want to connect the output of the opto to PIC microcontroller for ring detect when there is a incoming call.
 

Thread Starter

kygh_qin

Joined Nov 29, 2009
6
Hi bertus & hgmjr,

Thanks for your reply.why I can't get a single pulse of ring when the phone rings? is the circuit correct? Can I connect the output of opto directly to microcontroller?
 
Are you using an oscilloscope to look at the output of your circuit?

A ring signal is typically around 20 Hz sine wave and will therefore give you a output signal from this circuit that is also 20 Hz. On a voltmeter, this could show up as a constant ~3V signal as it does some averaging of the signal. You will not get a single pulse but a train of pulses as long as the phone is ringing.
 

Batista230

Joined Feb 11, 2009
14
if the ring is the U1 in the schematic
then there is no way you are going to get 5V out of it if your putting 5V in with a resistor of any value in the middle
 

BMorse

Joined Sep 26, 2009
2,675
if the ring is the U1 in the schematic
then there is no way you are going to get 5V out of it if your putting 5V in with a resistor of any value in the middle

U1 is an optocoupler, the resistor is basically going to give an active low output, normally ouput will be high until ringing is detected and the output will pulse low at the rate of 20hz.
 

flat5

Joined Nov 13, 2008
403
Hi BMorse, I'm having trouble understanding why it's important which way the diodes are connected because they are back to back zeners fed an ac signal.
 

BMorse

Joined Sep 26, 2009
2,675
Hi BMorse, I'm having trouble understanding why it's important which way the diodes are connected because they are back to back zeners fed an ac signal.
I had built a ring detector circuit some time ago, and I had the diodes the same way as the first post and it did not work, so I inverted the diodes and then the circuit worked perfectly..... this is the actual circuit I used in my phone ring detection circuit >>>ring detector circuit.jpg
 

rjenkins

Joined Nov 6, 2005
1,013
As long as the two zeners are in series and opposite ways round, it makes no difference.
For either polarity, one conducts at about 0.6V and the other conducts at 12V.

If only one was reversed, it would mess things up.

Also double check that the diode across the LED in the opto isolator is the correct way round, so it protects the LED from being reverse biassed rather than clamping it off..


An alternate version of this circuit does away with the zeners and puts the opto LED in a 'bridge rectifier' configuration of diodes.

In any version, the sudden voltage changes on the phone line when the phone goes off and on hook or the line polarity reverses means you may get spurious single output pulses from the opto. You need to look for several consecutive pulses in software to be sure it's actually ring voltage on the line.
 
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