[TriEmbed] Is there a FET trick for this?

[Brian]

Let me see if I understand correctly:

When the FTDI is connected, you want power to flow FROM the FTDI adapter 
TO your board.

When the Adafruit doohickey is connected, you want power to flow FROM 
your board TO the adapter (the opposite direction) by the same pin.

If that's it, a simple diode between the pin and your board's power rail 
is all you need.  Oriented to allow current to flow from the connector 
to the power rail and not the other way.  Connect both the connector pin 
and the Arduino pin to the anode of the diode, and connect the power 
rail to the diode's cathode.  If the connected board is supplying power, 
the diode will be forward-biased and will conduct.  If the board is 
receiving power, the diode will be reverse-biased and will not conduct.

Make sure the diode is rated for the amount of current that will flow in 
from the FTDI adapter.

Crude attempt at ascii art:

[FTDI?] ----+---[>|---- +V rail
             |
             -
             ^ [optional]
             |
          arduino pin

[FTDI?] represents the connector pin.

The optional diode is if you want to prevent current flowing into the 
arduino pin from the FTDI adapter, but is unnecessary unless the FTDI 
adapter supply voltage is too high for the pin.

Ordinary diodes do impose a nominally 0.7V voltage drop when in 
conduction, but you can shop for a Schottky diode with a lower voltage 
drop if that will be a problem.

Hope this helps!
-B



On 12/2/2015 8:01 AM, Chip McClelland via TriEmbed wrote:
> All,
> Down to the final stretch on my new board - just need one more piece of
> advice.
>
> I am trying to see if there is a way to accomplish something without
> resorting to a physical jumper. I would like to use the FTDI header on
> my new board in two ways with respect to the power pin.
>
> 1) when I connect a FTDI connector, it could power the board. The power
> pin is connected directly to the board's power rail. This is how it is
> today.
>
> 2) when I connect the Adafruit UART friend, I can turn it on and off
> using one of the Arduino's digital pins.  In this scenario, I only want
> the power pin connected to the digital pin on the Arduino.
>
> Is there some way to automatically switch between these two modes based
> on whether there is power supplied by the FTDI adapter?
>
> Thanks,
>
> Chip
>
>
>
>
> --
> Sent from my iPhone - please excuse the typos
>
>
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