DTMF dialer design problem

On a sunny day (Fri, 25 Jan 2008 10:57:19 -0800 (PST)) it happened Alexander Avtanski wrote in :

I am not 100% sure why, but you feed in series with the line. I would feed in parallel, with a capacitor to block DC ,and a resistor in series to avoid shorting the conversation.

I tried this too - an extreme case of it indeed - I just shorted the microphone totally - this way the transformer is like it's connected in parallel straight to the mic input of the phone. The signal is still not strong.

Curiously enough, if I use my home phone's headset on the same phone, the signal IS very strong. I'm puzzled...

- Alex

Actually - when shunting the mic I'm missing the capacitor... But I don't see how this would change the things here.

- Alex

On a sunny day (Fri, 25 Jan 2008 12:13:03 -0800 (PST)) it happened Alexander Avtanski wrote in :

Been a long time since I played with phones, but in the old days there used to be a transformer, and shorting the mike is _not_ the same as creating a parallel feed. There is a spec somewhere for how many volts should appear on the line during speech, maybe somebody here knows.

You have a point about shorting the mike. I'll think about this.

And just for whoever may reply on the question about the volts on the line - I'm plugging the transformer between the phone and the handset, not between the phone and the wall.

Thanks,

- Alex

On a sunny day (Fri, 25 Jan 2008 12:43:41 -0800 (PST)) it happened Alexander Avtanski wrote in :

Oops, my mistake, now it gets more complicated, as there are many phone types, and the modern ones here have an electret mike, and some electronics, while very much older ones used the old carbon mike and transformer? Feeding in series with electret electronics probably causes strange effects? Feeding in parallel with an electret, even with a DC blocking cap, would drive different circuits in different phones, this explains you different results.

Here is the answer to your problem: the connections between the telephone and the handset are not standard. You'll need to figure out which wires are used for the microphone on the telephone in the office.

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Hmmm, now you got me thinking... With an electret microphone the way the transformer is currently connected does not make sense at all. Looking at how an electret microphone is usually connected (and yes, I didn't know that and had to look it up), do you think that the following might work - forget about the transformer (apart from providing galvanic uncoupling) - use a FET transistor in parallel with the mike - between lines 1 and 4 of the RJ6 jack. This, the way I see it, would vary the resistance between lines 1 and 4 - maybe this is what's needed with the ShoreTel phone?

And generally, how would you (or anybody else reading this) connect it? I'm ready to try.

Thanks,

- Alex

Thanks. The thing is:

1) The office phone works OK with the home phone's handset - the DTMF signal is loud and clear, but the microphone level is too low when i speak.

2) When I use the office handset on the office phone, the microphone level is great, but the DTMF signal is too low.

3) When I try to "simulate" the home phone's handset on the office phone by using a relay that puts a resistor in parallel with the office handset when the dial tone is submitted, still doesn't bring the DTMF signal to the same level as with the home handset.

4) Everything works just beautifully on the home phone.

Anyway, Jan's reply gave me some ideas to try of alternate ways of connecting. If you have any ideas what can I try, let me know. Assume that the wiring is as shown on the schematics - if it is not the same way, I'll figure it out.

Thanks,

- Alex

What's wrong with the tried ant proven method of acoustic coupling? I've seen several brands of self contained DTMF generators with a speaker that is held over the mouthpiece

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Central Florida

On Fri, 25 Jan 2008 10:57:19 -0800 (PST), Alexander Avtanski wrote:

:Hello, : :I'm working on a DTMF dialer box - something that you plug in between :the phone and the handset that sends DTMF tones over the handset's :microphone line. Here is the current design: : :

: :The way it is connected to the phone is shown here: : : : :The box works great on my home phone, but on the office phone the DTMF :tone level is too low. I guess it comes from the difference in the :microphone used in my home phone's handset and the ShoreTel system at :work. When I put my home's handset on the office phone, the dialer :works fine - the tone level is high, - but this time the microphone :level is too low and the person I'm talking to can barely hear me. : :So, what I tried to do was to put a relay that shunts the ShoreTel :microphone while the box is dialing - when the relay is switched on it :adds a resistor parallel to the ShoreTel's mic, lowering the :resistance to match what I measured on my home phone's handset. This :makes the signal a bit louder, but still not to the level when I use :my home phone's handset on the office phone. Even when I shunt the :microphone totally, the tone is not loud enough. Again, on the same :phone if I use my home handset, the signal is strong. : :My questions are: : :1) Am I missing something? Are there any connections between the :microphone and the handset speaker that I should be concerned about? :Is this why just shunting the microphone doesn't work? : :2) I would like to avoid adding an amplifier if possible - currently I :rely on whatever signal comes from the NTE1690 chip - it is enough on :my home phone. If I have to add amplifier, that's OK, but I'm not :sure it will solve my problems - maybe the real issue is impedance :matching? Any ideas how to test/solve the problem? : :3) Any other ideas? : :The thing is I'm not very good with analog electronics - I suspect :impedance mismatch, but I find it hard to do anything about it. Any :help will be greatly appreciated. : :Thanks, : :- Alex

I am not familiar with the ShoreTel VOIP phones but has somebody actually checked with ShoreTel to determine if the particular system you have is capable of sending and receiving dtmf signals during a call? Perhaps there is some system setting which is not enabled. It could also be a problem with the VOIP ISP provider depending upon whether it will allow transfer of dtmf signalling during a call.

It works fine if I put my home phone's handset on the ShoreTel phone. DTMF is loud and clear and is detected fine - I can dial using only the box if I want - it works. But the microphone level is too low with my home handset. With the original handset the DTMF signal gets too low.

I tried different things (described above) without luck. I'm woring on something now - trying to vary the resistance between the lines - let see if this will work.

Thanks,

- Alex

That works. I was using one "ear" from a bulky headphone for this though - maybe I should look for something smaller, that I can strap on the so I don't have to take it on and off all the time.

I was hoping for a more elegant solution, rather than having things attached to the mic.

Thanks,

- Alex

When I'm searching through archived news, I don't like it when people ask questions and then never come back to tell if the answers they got helped or not. I'm trying not to do this and every time I ask something at the end I post a message what happened at what the solution of a problem was.

The problem with the DTMF dialer was really the electret microphone, as Jan Panteltje suspected. The fix was to use a transistor in parallel to the mike as a voltage controlled resistance - simple and works fine on both phones. The updated schematics, if somebody is interested or have forgotten what is all this about, are on the same place (

).

Thanks to Jan, Michael, Nico, Ross and Christopher for the help.

Regards,

- Alex