Short Version: I verified that my scheme for getting the mute signal will work on my StarTac 7760 (CDMA/AMPS 800mhz). I will verify on my wifes 800mhz AMPS StarTac (7600 I think) when she gets home.

The tests are pretty easy to do yourself. A scope helps a bit, but a volt meter should work just fine. You'll need that mini-stereo plug. I got the connector bits at Radio Shack, about $6. If you had an extra headset you could cut that apart.

Drop me an email if intrested.

Oh, and be warned. I'll probably breadboard this setup. If I do a small run of PCBs I would sell my leftovers, but I don't really plan on making and selling the whole kit at this point, sorry.

The more I think about it, this will be pretty simmilar to Ed's circut (I plan to check with him before copying any of the details). I may use different parts, depends on what I can find laying around.

Long version:

For those that didn't know (or care) I'm working on a handsfree kit that is a bit more phone independent than some. The basic idea is to interface through the headset jack that seems to be on most phones (my last three at least!).

Pros: Phone independent! Less stuff to buy, hence less expensive. Can connect/disconnect without dropping the call.

Cons: No battery charge/power (use car cord, perhaps extra wires to trunk?). No external antenna. Mute signal scheme may work differently on different phones.

The analog signal interface should be straight forward. It was the mute signal that was a concern.

The mic requires a DC voltage to work properly. Usually this voltage is output as a bias on the mic input. BUT the phone designers I know are very **** about power consumption, I would think they would turn this off when the phone was not in use.

I did some testing. The mic input has a DC bias of about 1.8V whenever the phone is "active". This includes all phone produced noises such as keypresses when keypad tones are on. Incomming, outgoing, and active calls as well. There is about a 2-3sec delay after the event ends before the signal drops.

This behavior did not change when the phone was powered externally. It was intresting that turning keypad beeping off prevented the signal from asserting when buttons were pressed. I'm guessing that the whole audio interface circut (amp for the speaker and mic) is turned on and off as needed.

From what I can tell, on my phone at least, it's pretty close to the perfect mute signal...well that 2-3sec delay is a wee-bit long...

 

Well, almost exact. I'd been thinking about using the Ed P / James R setup, but using timer triggered by audio *output* to activate the muting. This would have the same delay problem that you're talking about (in fact, it'd have the potential for being worse). Cool that the phone provides a DC bias on the input side when the phone's active.

-dan

 

Steven,

As far as your handsfree kit, you are exactly correct, at least as far as the StarTac goes. A former reader of this forum built a kit like the one you describe, several years back... and that is EXACTLY how he did it, (ie: The mini phono hands free accessory jeck, and mic-to-mute transistor ground to pin 10 of the db25). I actually bought this home-brew kit from another AWer who no longer owns a StarTac. While it works perfectly, it is a little "messy" for my tastes, so I got in touch with the original builder to see how he did this. He told me essentially what you did. However, I prefer a less "universal" kit, and thus wish to do everything, (power, mute, mic, audio) through the butt connector. Anyway, a couple of other people had done their own research, and at this point we're collaborating to get things fully, and neatly integrated with the Motorola StarTac "Professional Install" kit so that everything functions through the butt connector and looks OEM. It should be universal to all StarTacs. So far, it seems we have all the theory and proper pin-out information. All that remains is implementation, which is should be forthcoming shortly.

In any case, what you describe will definately work, as that's exactly how it has been done in the past. Keep it up. Let me know how you fair.

 

Cool! What phone do you have? My goal is for something that works well and is simple.

This should work, and I'm thinking about it as well.. The only problem there I saw was the phone unmuting when you were..say on hold, or speaking more than listening? Noise concerns?

I noticed one bizarre thing about my StarTac. When it's charging the battery the bias voltage becomes a noisy. The voltage spikes are around 1.8V..the time the voltage was over my threshold of 1.4V was about 10ms. A couple times it went longer (20ms).

Also bizarre, the bias voltage was always present when the phone was OFF on external power.

I'm guessing that the power supply is doing something to make noise on this signal? Or..?? I'll have some sort of filter..I'm not sure how long though.

 

I know someone who modded the professional install kit for his S4. He had problems getting the proper mute signal. It works 99%, but he mentions quirks with the signal when the battery is charging and such.

Also his phone remains in the trunk (unacceptable to me) unless he pulls the signal cable through as the Audi wiring didn't have enough pins.

Can you send me info about your attempts with the pro kit?

Thanks!
Steve