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How to make an "auto-on" ?

Matt_C

Feb 9, 2015
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Sorry for the crap title, but I couldn't think how to word it? Hi guys, I'm new, and I don't know an awful lot about electronics (in fact very little) so I was wondering if I could pick your brains.

I've got a little project on the go, using a pretty cheap Chinese GPS sat nav tablet, which I want to install into my car (my car has a housing for a 7" screen, but the factory nav doesn't work, and is well out of date anyway. The idea here is to take a more modern touch screen one - http://tinyurl.com/msfsggp - and install it in place of the screen)

Hard wiring in the power adapter is easy - all I'm going to do there is wire a 12v socket directly into the switched live and ground for the existing cigar lighter socket behind the dash. Then I can easily plug the power adapter into it behind the dash, and tie it up out the way. That way I don't have to mess about with stepping down the voltage etc etc, as the power adapter takes care of all that.

My problem is, the unit doesn't have an auto power on and off feature - seems the only way to power on and off is using the press button on the top > press and hold for 2 or 3 seconds and it powers on, press and hold for 2-3 seconds and it goes to a shut down / sleep option screen, and turns off if no choice made in 5 seconds. However, once this screen is inside the housing, there will be no way to access this button! I had to rip apart the casing on this GPS to relocate the PCB inside, in order to be able to plug into the USB cable for power (and later a video cable for rear view camera, but that's further down the line), and this also "releases" the power button from the external button on the casing. It's a simple, two-wire affair - one wire from PCB to a press button, and then second wire back to PCB.

What I'm hoping is someone might know a way to trigger the device to turn on using voltage, rather than a physical press - i.e., vehicle ACC is turned on, and the power from the switched live turns the device on. Powering off isn't really an issue - I'll just cut the battery off so when ACC is switched off, the device will simply "die" (I wouldn't want to leave the battery inside the car anyway, as in summer it will get stupidly hot in the sun, and there's the fear of the battery going pop!)

Some not very good pics;

http://i60.tinypic.com/fmhts9.jpg

http://i59.tinypic.com/jj8e1k.jpg

Now, this device is only my "tester" device. It owes me nothing, so doesn't *really* matter if I kill it - and so far with all the physical messing around with it, making it fit in the housing, taking it out again, etc, the white wire (which is for charging the battery) and both the black wires from the switch have come off the PCB. Yep, it's cheap, and soldered poorly. So if there's a way it could be done so that upon receiving power (to the USB socket, from either in-car adapter or home plug adapter, or even computer USB - I'm thinking all are rigged up to provide 5v to the device) it could power on, that'd be great.

Any ideas?
 

mohammadrahimpoor

Feb 9, 2015
19
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Feb 9, 2015
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hi matt as u know ur tablet is controlled by a microcontroller or something like that and the only way to add auto on feature to ur device is to reprogram it whitch i think is not doable there isnt any kind of circuits outside all controlling protochols r given by ur device microcontroller and thats why u can not change it
 

Matt_C

Feb 9, 2015
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Feb 9, 2015
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I was thinking it'd be something like that. However, I was hoping that since the power button simply makes a temporary electrical connection, then this could be simulated in someway physically, with voltage from the switched live
 

mohammadrahimpoor

Feb 9, 2015
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Feb 9, 2015
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when u push the on botton a counter starts to count when it counts to a specific number it turns ur devise on thats how it works there isnt any kind of relays on swithes for that
 

Matt_C

Feb 9, 2015
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Thanks Kris. That might be a bit over my head - I really know nothing about making circuits etc! When you say contacts, do you mean the the points the wires of this button connect to the PCB, or do you mean actual contacts under the physical button itself? You can see (well, kind of) in the poor pictures I posted, the button itself sits on a little PCB and two wires run from it to the main PCB that runs the device
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Thanks Kris. That might be a bit over my head - I really know nothing about making circuits etc!
Well, this is a relatively simple project to start learning with then!
When you say contacts, do you mean the the points the wires of this button connect to the PCB, or do you mean actual contacts under the physical button itself?
Either. If you can make electrical connections to both sides of the button, you can connect a relay contact in parallel with it. It doesn't matter whether you connect your wires to the actual leads of the button, or to the pads on the button circuit board, or to the wires, or to the pads on the main board where the wires connect.
You can see (well, kind of) in the poor pictures I posted, the button itself sits on a little PCB and two wires run from it to the main PCB that runs the device
Yes. If that's all there is on that little board, and you can run two thin wires from an external circuit into the unit and connect them to those pads, you can do it.

For an introduction to circuit assembly, Google how to build a circuit on stripboard. Ask here if you have any problems.
 

Matt_C

Feb 9, 2015
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Would I need to keep that little button board or could I do away with it, and put this circuit in it's place? IE, the two wires that run from the main board - instead of them going to the button board, they just go to this pulse circuit instead?
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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You could remove the button if you don't want to use it. I would keep it in case you want to replace it some time in future though.
 

Matt_C

Feb 9, 2015
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Feb 9, 2015
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Gotcha. Reason I ask is because the button board has detached itself from the main board anyway (it's a cheap GPS and he soldering is very flimsy. Where I've been constantly moving it around the wires have become detached). So really I need to figure out this pulse circuit and solder it to the points on the main PCB that originally had the button on. Thanks again for your advice - no doubt I'll be back!
 
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