The P-MOSFET Drain or the PNP-Collector will drive the Vin of your boost/stepup regulator.
Drive the circuit from whatever pin you want from the Mega - when it goes high, current will supplied to your reguator.
I don't know what you're talking about with Tx and an LED.
The current has to go thru something if you want to control it.
Maybe switching relay on/off is what you need. Are you going to be turning this on/off/on/off quickly under software control? Or just once in a while?
Anyway, that's tomorrows question, I'm off to bed. 1:30AM my time.
There is a connection from the mega TX to the RX on the motor controller and it is causing problems. If the turn off the motor controller the power led on the motor controller is dimly lit and the step-up is outputting 3v vs the 7v it doesn't normally.
It seems that when I cut the power to the motor controller it seems to get power from any connection.
The only reason to turn off/on the motor controller is
1) when the robot is off to keep it in the off state, like for storage/transport, but batteries are connected still.
2) if the batteries are low, turn it off to charge faster or to be able to charge from solar (future option) (idea is if batteries are really low, turn off as much as possible. I pretty much can turn every thing off via software. I have a regulator for the sensors (instead of the onboard one) that can be shut off)
3) if I need to reset the motor controller / battery if it is tripped from overload.
I noticed that with the current setup the TIP120(NPN) gets much hotter than the TIP42(PNP). Wouldn't the PNP be carrying the load? The NPN is just turning the PNP on right?
I just noticed that the NPN get hot just being idle.
So maybe a relay would be better?
In testing I noticed that if I use just the TIP120 then the step-up outputs 4v. So that isn't right. So I guess this leaves using a relay.
Which one though? I have only worked with automotive relays.
Thanks so much. This robot has been a great learning experience and its growing. I can't wait till I can play with all the sensors and such.
The orginal problem was that 1 battery wasn't enough and the battery would go into protection and shut off.