[aprssig] Kenwood TH-D7 GPS - grounding

[Steve Noskowicz]

--- On Fri, 9/12/08, Greg D. <ko6th_greg at hotmail.com> wrote:
> Subject: [aprssig] Kenwood TH-D7 GPS connector grounding
> To: "'TAPR APRS Mailing List'" <aprssig at lists.tapr.org>
> 
> So, I'm still working on mounting a GPS unit on top of
> my TH-D7.  ...  EM-406A from Sparkfun.com; runs on 4.5 to
> 6.5v.  [50ma ] The TTL output is inverted with an NPN transistor,
> and then direct into the TH-D7.
> 
> ...So, I want to tap into the ... Batteries America BT-11h
>... I took an ohmmeter ...ground side of the GPS port ...
> tied to the negative side of the battery.  
> Instead I get no connection at all.  ...

Greg,
  You, unfortunately, have a double, or is it triple, whammy here.  *AND* a good piece of information for me (The D7 accepts unipolar RS-232] cool!

  First, there is a diode in the negative side of the BT-11h.  If you're meter has a diode test setting, it will show .5-.6 volt in one direction, open the other.  It's hard to see, inside, down by the neg contact, but you can see the black wire running up along that side to that end of the cells. It prevents charging non-rechargables through the radio charger.  
   With this diode in place, your transistor inverter will only be going down to about + 0.7 volts for the GPS input of the radio.  This may, or may not be enough to work - it depends on the input threshold of the D7 GPS input.  If it is TTL, this should still work as TTL threshold is more like 1.5 V -- if my memory hasn't rotted yet.  Try this first.
  IF you connect the emitter of the inverter to the GPS connector ground, you'll just have to allow for this difference in your base drive arrangement.  A slightly smaller base-emitter resistor.  I'd probably do it this way.

   I guess with NiMh cells you could short the diode and just use the radio charger for the required time (57 hour full charge - due to 70 ma charge current).  However, as previously posted here, this requires ship-in-bottle tools.

  Second, thanks to Dave telling that the current regulator is on the ground side.  This means that when charging the radio, the GPS will be several volts more above ground (charger voltage minus battery voltage), relative to the radio's GPS input.    I'd have to review the circuit and think more, but off hand, I don't think that this is a problem, in itself.

  Then, I see there is already another post from Patric, but I don't think his comment about other radios applies to the D7 given the above.

  Lastly, I don't believe the drain imbalance is something to sneeze at.  The numbers were posted here a while back and IIR the radio+APRS TNC drain is somewhat more than the 70 ma charge. I think it is around 120-140ma.
   50 ma is a significant part of that  30% - meaning some cells will either be 30% over or under charged.  A switching regulator, off the whole battery, would be nice if you can and keep it quiet.
Good luck

--
73, Steve, K9DCI