Hardware Reference
In-Depth Information
Discussion
This wiring causes the PowerSwitch to operate in the opposite way you would likely think.
When you output a
HIGH
to
P9_14
, the PowerSwitch will turn off. When you output a
LOW
, it turns on. Read on to see why.
The GPIO pins on the Bone can
source
4 or 6 mA at 3.3 V, depending on the pin. This
means it can supply a positive current of 4 or 6 mA. All GPIO pins can
sink
8 mA — that
is, they can absorb up to 8 mA. The PowerSwitch Tail II requires 3 to 30 mA and 3 to 12 V
to turn on.
P9_14
cannot
source
enough current to cleanly turn on the PowerSwitch, but it
can
sink
enough; therefore, we've wired the
+in
input to the 3.3 V power supply on the
Bone (
P9_3
) and the
-in
input to
P9_14
.
When
P9_14
is at 3.3 V, the PowerSwitch doesn't see a voltage drop (both inputs are at
3.3 V), so it doesn't turn on.
When
P9_14
drops to 0 V, there is a 3.3 V drop across the PowerSwitch, and the internal
relay clicks on. Therefore, a
HIGH
output makes it turn off, and a
LOW
makes it turn on.
