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GTM-21 & cradle receiving voltages

 
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Cypermethrin
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Joined: Sep 03, 2009
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PostPosted: Fri Sep 04, 2009 5:56 pm    Post subject: GTM-21 & cradle receiving voltages Reply with quote

Can anyone advise me if the GTM-21 flat plug, which feeds the Garmin cradle, is 12v ? And is then taken directly by the Garmin nuvi 760 and used ?

I know that if you use the USB connector the 12v is stepped down to 5v before entry into the unit.

I have taken some readings of the voltage inside the Garmin cradle at 12v which implies the step down is not required if you use the Cradle ?

Thanks
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PhilHornby
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PostPosted: Mon Sep 07, 2009 9:24 am    Post subject: Reply with quote

Yes, it is.

If you go into the 760's diagnostic screen (by holding the battery icon for 7 secs), you can see the external voltage detected by the unit.

I sometimes run my 760 on the bench, powered from a 12V unregulated adaptor, via a StreetPilot's straight-through connector. The Nüvi hasn't objected (yet!) to the 13.75V it is being fed with.
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Cypermethrin
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PostPosted: Mon Sep 07, 2009 9:41 am    Post subject: Reply with quote

Thanks Phil

Just to clarify you can feed the 760 with 12v through the cradle via the GTM-21 gizmo. But can you also feed it 12v directly through the mini-USB 5 pin plug too ?



PhilHornby wrote:
Yes, it is.

If you go into the 760's diagnostic screen (by holding the battery icon for 7 secs), you can see the external voltage detected by the unit.

I sometimes run my 760 on the bench, powered from a 12V unregulated adaptor, via a StreetPilot's straight-through connector. The Nüvi hasn't objected (yet!) to the 13.75V it is being fed with.
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PhilHornby
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PostPosted: Mon Sep 07, 2009 10:18 am    Post subject: Reply with quote

Cypermethrin wrote:
...But can you also feed it 12v directly through the mini-USB 5 pin plug too ?


Not something I've tried ... I think smoke might result Exclamation

In any case, I know some USB only units (Nüvi 2xx series?) won't charge from the USB port, if the voltage is even slightly out of USB spec.

Why would you want to do such a thing Question Confused
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Cypermethrin
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PostPosted: Mon Sep 07, 2009 12:07 pm    Post subject: Reply with quote

PhilHornby wrote:
[quote="Cypermethrin
Why would you want to do such a thing Question Confused


I don't want to - but was trying to understand !

What I do want to do is ....... currently trying to install the 760 into my dash as a permanent fitting. I want to be able to have the functionality of using the mini-USB 5 pin port keep the GPS updated with current maps and speed camera data etc while also using the same port to power the unit.

My plan is to take the 12v feed from the cigarette lighter into a convertor as a USB port female connector – and house this in the centre console cubby box. Then run the male USB to miniUSB 5 –pin lead to the 760 in the dash. In this way I would retain the ability of uploading new data from my laptop and powering the unit too – just by swapping the USB lead over in the centre cubby.

Make sense ?

Thanks for your help
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PhilHornby
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PostPosted: Mon Sep 07, 2009 2:01 pm    Post subject: Reply with quote

Are trying to avoid using the cradle?

Otherwise, wouldn't the easiest way, be to plug the GTM-21 into a cigarette lighter extension cable, which has the plug chopped off and connected to the fuse-box instead. You could put your USB extension lead in place for data transfers.

Otherwise, the problem you have got, is accessing Pin 4 (ID) of the mini-usb socket (there being 5 pins in total). If you were to plug a Garmin 5V adaptor in, at the end of a USB 'extension', the Nüvi would go into data-mode. It needs to sense the resistance between Pins 4+5, to know it's a different kind of peripheral attached. I don't think you can buy a mini-USB extension lead with all 5 pins connected. (Strictly speaking, the USB spec. doesn't allow for extension leads at all Surprised )
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Cypermethrin
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PostPosted: Mon Sep 07, 2009 2:43 pm    Post subject: Reply with quote

Hi Phil

Thanks for your help here- much appreciated. The pictures below should help explain the dilema. The plastic adaptor which has to be glued to the rear of the fascia holds the GPS. You then make holes in it to gain access to the ports. So if I used the GTM-21 as you rightly suggest as being the easiest way it will most likelt foul the fit of the unit in the mould.

The alternative way is to remove the unit from the housing everytime you need to update the data/maps etc.

What do you think ? I agree using the cradle is the easiest away to achieve this - but I would have to hack a considerable amount of the rear of the mount for it to fit ?

Thanks - again....









And the finished item looks like



Last edited by Cypermethrin on Mon Sep 07, 2009 3:25 pm; edited 1 time in total
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M8TJT
The Other Tired Old Man
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PostPosted: Mon Sep 07, 2009 3:11 pm    Post subject: Reply with quote

How about reducing the size of the piccies so that they fit the page? Shocked
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Cypermethrin
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PostPosted: Mon Sep 07, 2009 3:26 pm    Post subject: Reply with quote

M8TJT wrote:
How about reducing the size of the piccies so that they fit the page? Shocked


Fair cop ! - should be better now
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PhilHornby
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PostPosted: Mon Sep 07, 2009 11:19 pm    Post subject: Reply with quote

Cypermethrin wrote:
What do you think ? I agree using the cradle is the easiest away to achieve this - but I would have to hack a considerable amount of the rear of the mount for it to fit ?

You could dismantle the cradle and just use its PCB; this would make it much thinner - but you'd lose the spring attachment mechanism, so you'd need to think of a way around that. It would still require a sizeable cut-out though.

I'm pretty certain you won't find a ready-made mini-usb extension with 5 pins connected end-to-end. I think you'll really struggle to even find the plugs and sockets to make your own (The ones in the once-great Maplins, only have 4 pins (1,2,3 & 5)).
Even if you find them, you're going to be soldering by microscope Rolling Eyes

Maybe you could remove the Nüvi's back and trace the power connectors on to the board - and then make your own 12V supply connection. This would free up the USB for updates, so it would only need to be a 4 pin connection.

(See this thread on GPSPASSION.COM for some background discussion on cradles, connectors, charging etc.)
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