SCREENSHOT 10. This switch
emulates an eight-bit transparent
octal latch. When put into
a high impedance state, the
I/O pin relinquishes
all control of the logic signal
attached to it.
SCREENSHOT 11. Although the
P-channel MOSFET logic differs from
the perceived operation performed
by the open source switch, the
outcome is identical. The open
source's active output is logically
high, which is the 1 switch position.
SCREENSHOT 12. This is
about as dead as a circuit
can be. D4 is attached
to a PIC18F46J13
high impedance input
pin and the N-channel
MOSFET gate is floating.
Turning on the
gate brings the MOSFET
drain to ground level.
Right now, we don't have a way of
getting the MOSFET drain to present a
with the addition of a real resistor between D4 and Vcc.
Bang! The D4 status LED illuminates in Screenshot 14.
With the open drain switch in the Z position, the N-channel MOSFET is turned off and D4 assumes the + 3. 3 volt
logic level through the pull-up resistor. Moving the open drain
switch to the zero position turns the MOSFET on and draws
D4 to the ground level, producing a logical low output at D4.
56 SERVO 01.2012
SCREENSHOT 14. It's pretty
obvious how the open drain
circuit works. The open
drain's active output is
logically low, which is the
zero switch position.
SCREENSHOT 15. This circuit is
commonly used to drive LEDs
and high voltage/high current
relays using a logic level. The
circuit also can be used as a
simple logic inverter.
The on state of the MOSFET is supported by the extinguished
status LED in Screenshot 15. The switch logic seems flipped.
However, it makes perfect sense. The open source switch
output is active when placed in position 1. That’s because the
active state of the open source circuit is logically high. The
open drain switch is active when placed in the zero position.
Thus, the open drain switch active output is logically low.
Think of it this way. The open source and open drain active
outputs follow the logical output conditions of the P-channel
and N-channel MOSFETs when the MOSFETs are turned on.
The open source circuit in Screenshot 11 outputs a logical
high on D3 when the P-channel MOSFET is turned on. The
open drain circuit in Screenshot 15 outputs a logical low
when the N-channel MOSFET is turned on. Piece of cake.
Did you know that all of the illusions at Disneyland are
done above ground while all of the reality that supports the
illusions lies underground? The Electronics Explorer uses a
similar concept except that what happens on the Explorer’s
solderless breadboard is no illusion. The reality on the
solderless breadboard is produced by the multiple power
supplies and logic devices on the opposite side of it.
In actuality, the world of robotics is a complementary mix
of analog and digital techniques. I’ve presented the Digilent
Electronics Explorer in the role of a microcontroller stimulus
generator and virtual logic monitor.
The Explorer is just as flexible when used as an analog
workbench. In that the Explorer easily mixes digital and
analog experimentation and project development, it is a
perfect tool for any robot head. SV