How to electronically put a telephone line "off-hook"

In the USA putting 600 ohms across the line will produce an off-hook condition. The audio can be pulled off through a capacitor. Surge suppressors like MOVs should be used also. Tom

What are you going to use as a "telephone?" You indicate that you want your equipment to auto-answer an incoming call, but then what happens?

A good answer to your question can't be provided without more information.

By the way, your telephone line isn't 600 Ohms impedance, and tripping the ring with a 600 Ohm resistor is a marginal approach which, depending on how it is done, can degrade the transmission path.

Don

Hi,

I know how to detect the ring signal and create a logic signal for a microcontroller, but then how do I "pick-up" the phone line to stop the ringing and connect with the caller ?

Perhaps with an optocoupler as a relay but what does the relay connect up to the line to stop the ringing ?

Help appreciated, cheers !

Oh ok thanks, is that why I see 600 ohm 1:1 transformers in circuits ? So once the line has 600 ohms across it there is a circuit between me and the caller and I can then do DTMF or audio ?

To hang up I just remove the 600 ohm resistor ? Cheers!

your

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I want the ring detect circuit to inform a microcontroller and then have the micro 'answer' the phone i.e by triggering a relay. Then once a circuit is established, decode the DTMF pulses sent be the other end (using a DTMF decoder IC) and then have the micro put some audio on the line.

Miguel's circuit below seems its what I want I believe ? L1----------------8------- RD 200 L2----------------SW------

RD= ring detector circuit in parallel to line

Tom, your circuit here

Thanks ! Linden.

Hi. Here at the bottom of this page is the RD part of my circuit below, plus the whole enchilada if you want to do it in another way:

Miguel

Hi. To make an steady output DURING the duration of the 20 Hz AC ring cadence, you have to replace RD with a full wave rectifier, and its output filtered. The size of the filter capacitors will determine the duration of the steady output by integration. It may even last between rings. That will be about 200 Volts DC, that must be current limited to drive an optoisolator to feed the microcontroller at logic levels. Miguel

Thanks Miguel, I have something similar to your ringdetect but my ringdetect signal doesnt stay low for the duration of the ring it oscillates low/high in sync with the ring.

Does your circuit give a steady output ? Maybe I have to integrate the input or the output first ?

Cheers, Linden.

Hi Linden. The optoisolator output should have a NPN transistor collector and emitter pins, no biased base pin. The emitter must connect to microcontroller ground, the collector to the microcontroller {low when off hook} sense pin AND to a weak pullup resistor to +5V. (about 100K) If this resistor is much smaller, it may cause the behavior you describe.

->Do NOT have any direct metallic connection to the telephone line. Everything must be transformer-coupled, capacitor coupled or opto-coupled.

->There are logic output opsoisolators you may try if the above somehow does not work for you.

->The current to the optocoupler led should be sufficient to saturate.

->Or, canibalize a defunct modern telephone/anwering machine and look at its circuit for your country telco system. Miguel

Thanks, i've setup your RD circuit without the sidactor and 33V zener. D1=1N4148, OPTO=4N25 but my /RINGDET just never goes low enough.

i.e Vcc=5v, /ringdet goes from 4.95V down to only 3.5V on each ring so I think the opto LED doesnt have enough forward voltage across it ??

-Linden.

All the ring detect circuits I have seen around the place look similar, but for the life of me I cannot get a 5 - 0v swing on the ouput of the optoisolator. Usually only 5 - 3.5V during the ring. When putting a PNP transistor on the output to give proper logic levels it further reduces the swing from 5V - 4.5V during the ring and this is not enough to switch the transistor. Ive tried a couple of opto's now (H11G1 and 4N25) and get similar results.

It switches properly using DC input instead of the telephone line though! So obviously the line is not turning on the opto LED enough ?? Frustrating for an amateur !

- Linden.

I still dont think it is driving the LED hard enough to saturate... I had the exact same setup (optocoupler with NPN output, with 100K pullup to 5V) no mcu connected yet but it still didnt go low enough.

Changed optocoupler to a 6N138 with darlington transistor output, found a circuit online and it works perfectly... I should measure but upon looking at the specs the avg forward input current for the 4N25 is

Thanks again for your help, lets see how I go with the rest hehe.

-Linden.