How to Add Automatic Power on to the Omnitech GPS

This mod will make your Omnitech GPS unit power on whenever external power is connected to it. It is intended for those that would like their units to turn on when they turn on their car. After performing this mod, connecting external power will cause the following to happen depending on the state of the unit:

1. Completely powered off. -> It will begin boot up process.
2. In standby mode. -> The screen will flash once and come out of standby.
3. The unit is turned on and running a program -> The screen will flash once but remain on.

Disconnecting the power will have no effect on the unit. An unintended side effect of this mod is that the power button becomes disabled when external power is connected. The hard reboot button is not effected. When the external power is not connected, it behaves as if the mod is not there (the power button works).

Although this may look like an easy mod, you can easily ruin your unit. You need to have some soldering experience and be comfortable with electronics.

What you will need:
2 x 680 Ohm resistors
1 x 51 Ohm resistor
2 x 1000 uF capacitors*1
2 x 220 uF capacitor*2
1 x 4.5 V mini relay
1 x 5V relay*3
thin gauge wire
soldering equipment

*1. Not exactly the same capacitors I used. Mine came from here.  The one in the link above is slightly longer. But shipping will kill you if you order from both sites and Jameco doesn't have the correct relays (coil resistance is very important).
*2. I used a single 475uF capacitor that I had laying around, but the substitution above should work.
*3. The relay I used came from this assortment, but there is no guarantee that you will get the exact same one. The important characteristics of the relay are: 5V, ~307 Ohm coil resistance, non latching. The mini relay has a coil resistance of 167.5 Ohms (measured) and is non latching as well. 

1. Remove the 4 Philips screws on the back cover.
2. Lift off the the back over, and disconnect the battery and speakers from the main board.
3. If you have added a microphone (previous instructable), you will need to de-solder the wires and tuck them out of the way.
4. Disconnect the GPS antenna by pealing up the copper sticker over the connector and gently popping the connector off.
5. Remove the two screws at the bottom corners of the main board.
6. Slide the main board down unit the clips at the very top no longer hold it down.
7. You can now swing the main board up. Be careful to not lose the power button as it will now be removable.
8. You have done all of this to access the leads to the power switch. It is a momentary switch. Solder on two wires to the leads of the power switch and run the wires to the other side of the main board. You can verify that you have done this correctly by briefly connecting the power to the unit and crossing those wires for a few seconds. The unit should power on. Now turn it off! We have work to do!
9. Place the main board back exactly as you found it. Connect the GPS antenna. You can solder the microphone back on as well.
10. Leave the two wires hanging there for now.

Because of the limited space inside this unit, you will have to build the circuit in an unsophisticated way. There will be no custom etched pcb boards here. Only wires and squished capacitors (yes, squished).

Here is the layout:

Relay 1 is the 4.5 V mini relay
Relay 2 is the 5V, 300 Ohm coil resistance relay.
We use both poles on Relay 1 and only one pole on Relay 2
The wires we attached to the main power switch in the previous step are obviously the wires in the diagram running from the "Main Power Switch". They will need to be branched so that they go to both relays.
Make sure to orient your capacitors the correct way. If you have no idea what I mean, google "capacitor polarity".
To figure out the pin out of your relays, search for their data sheet or use a 5V power source and an Ohmmeter.
If all of this looks alien to you, you should probably not do this mod.

Like I said in step 2, you will need to squish the capacitors to make them fit. Don't rupture them! Gently press them flat using whatever tools you like to flatten things with. The only reason I have listed two 1000 uF capacitors instead of one 2000 uF capacitor is because the 2000 uF capacitors are way too big. Below is my layout. You can see where I put the largest capacitors. You can place yours there, attempt to put the back cover on, and see how much squishing is needed. Squish them a bit, test with the back cover, and repeat until they fit.

Some things to note:
1. The gray sticker covering most of the board is actually a ground. It is conductive. Protect your contact points. Notice all the yellow tape I put down?
2. Relay one is small enough to fit on top of the enclosure/heat sick of the GPS receiver. The enclosure is grounded and conductive.
3. I placed the relays on their backs and trimmed their leads.
4. I put the battery in there to show where you can't put components. There is some space just to the left of it. This is my custom battery that I will post about in an upcoming instructable. It is the same size as the original.
5. My back cover fits on with all of this inside.
6. I don't have a close up image of the contact points for the USB port, but any freshman with a voltmeter can figure it out.

Because the mod needs to be robust enough to handle the initial power on and stand by mode, timing is important! Before you screw on the back cover and pat yourself on back, test it out.

It took me a lot of testing and failed designs before I came to this one. It is very hard to figure out the exact timing of the power button without the proper equipment. So calculating exact values for the capacitors and resistors was pointless for me. 

The large capacitors control the timing for the initial power on.
The smaller capacitors handle the standby mode. 
The "big" resistors handle the minimum time needed in between plugging and unplugging and plugging in the power again. They also handle the maximum voltage available to trigger the relays. Too small of resistors will result in the relays staying triggered. The smaller resistor helps in the that task.

If it does not work for you, you can try tweaking some of the components. If it does work, fantastic!

This is my unit. As you can see, the back cover fits on just fine. I use the built in function of Amigo to turn off the power when the car turns off. You could do the same with any program using the auto shutoff feature of the miopocket skin. If you want way to toggle standby mode automatically with the power instead, see step 5.

In the simulations for my circuit, it looked like I needed diodes on the wire connecting the two capacitor packs. In practice, I didn't see any ill effects from not using it. The danger is when the power is disconnected and the capacitors are discharging via the resistors, charge from the larger capacitor could circulate through the relay and cause it to trigger.


A different mod:
If you only use the Omnitech GPS in standby mode, and you never do a hardreset, and therefore you don't have a need to automatically have the unit power on, but you would still like the ability to come out of standby when the power is connected (gasp), then you can remove the large capacitors and leave the rest of the circuit the same. This will have two effects. First, if the unit is completely powered off, connecting the external power will have no effect on it. Second, if the unit is powered on, the power being connected will determine whether it is in standby mode or not. So, if the unit is on and you connect the power, it will remain on. If you then remove the power, it will go to standby automatically. If you connect the power again, it'll come out of standby, and so on. Although the act of removing the external power will put it into standby mode, when the external power is not connected it behaves as if the mod is not there (the power button works).