Dialer schematic

[lerameur]

Hello,

I made this simple dialer circuit. I want to use the transformer as shown but it just do not seem to work. It only works when I add the 9 OHM resistor across the transformer as shown below (not recommended to put there), anybody knows why it works and to make it work without that resistance at point A & D ??

http://www3.sympatico.ca/captoro/dialer.jpg

K

[mackrackit]

Maybe MAX7480 com to transformer B ?

[lerameur]

to the ground? I guess A connected to the ground after that?

[Archangel]

The little I know about telephones, and it's little, is they require a low impedance to signal the switching system to recognize off hook condition. In your circuit, you have a resistor, a capacitor and a transformer all in series, I do not think it's ever going to see that low impedance situation, and may in fact be able to ring when your circuit is powered. I think the resistor is allowing your circuit to pull the line to the required, low impedance (improperly).

[mackrackit]

Ground is pin#3 com is pin#1
I think COM is the other half of the signal to drive the transformer.

From the data sheet


Common Input Pin. Biased internally at mid-supply.


Just a guess.

[lerameur]

oo well I did not include the other pins on the max7480, because they are not related to the problem ( I guess). The ground is common to the left of the transformer, the pic and the max7480.
I used the following circuit : http://www.datasheetcatalog.org/datasheet/maxim/MAX7480.pdf
page one.

K

[Nicmus]

From the little I know about telephones I think your “On Hook” switch should be across the input capacitor. When the switch is OFF (On Hook) the AC (Ring signal gets through and shows high impedance to the telephone exchange system). When the switch is ON low impedance is shown and the DC circuit is flowing.
Hope this will help.

Nick

[lerameur]

what I found out is the following:
' On Hook: 40 to 50 VDC
' Off Hook: 4 to 6 VDC
' Ringing: 100 VAC
I would also like to have the voltage when the other party has pick-up their phone, so I will know when to stop the pic from sending DTMFout.

I did remove the resistor from the telephone line, and just keeping the capacitor, but no changes. Still cannot dial. I am going to try with an opto coupler instead of the transformer.

k

[Jerson]

Joe has identified the problem precisely. You should not have the capacitor on transformer pin C. That will allow you to 'pick up' the line via the relay and then dial out. No need for the 9 Ohm resistor.

[lerameur]

Actually in my second test I installed the relay parallel to the capacitor and remove the series resistor. So when the relay is activate it should bypass the capacitor.
It still did not work, I will remove the capacitor totally then...

k

[Nicmus]

Here is a tested (not by me) telephone line interface to a microprocessor with part numbers and sources:

http://www.eettaiwan.com/ARTICLES/2002MAR/PDF/2002MAR01_ICD_PL_ID_AN42.PDF?SOURCES=DOWNLOAD

Nick