the PropBot’s servo motors to the servo header. I’m using
the header for pins P16 and P17. There are three sets of
servo headers for pins P12/P13, P14/P15, and P16/P17.
Important! Note the position of the little jumper block
beside the group for pins P16/P17. As the board is shipped
from Parallax, the jumpers are in the 5V position meaning
the center pins are powered by the onboard 5V regulator.
We want more juice to the servos, so be sure to change the
position of the header for P16/P17 to Vin.
Here, the center pins are powered directly from the
main batteries without regulation. This is fine — as long as
you use a 6V battery source.
The servo headers are
connected to the input/output
(I/O) pins of the Propeller. The
numbers on the headers
correspond to the Propeller’s I/O
pins (the Propeller has 32 I/O
pins, though only about two
dozen are directly accessible on
the PAB via header connectors —
the rest are used internally for
other board functions).
Note the long row of pins —
labeled P0 to P15 — to the right of
the breadboard. These pins
provide an easy way to connect
stuff to the solderless breadboard.
Just stretch a jumper wire from a
socket in the breadboard to a pin.
There is some duplication
between the servo headers and
the row of breadboard pins, so be mindful of how you
connect things up. If you use the servo headers marked P12
and P13, you should not connect something into P12 or
P13 on the pins beside the breadboard. Installing the Optional Switches
The PropBot is equipped with two touch sensors on the
front for detecting objects. While the switches are optional,
I recommend you add them so that your PropBot has a
basic sense of feel. Several of the demo
programs in upcoming installments of this
series will depend on these switches. As
they’re fairly inexpensive — about $2-$3
each from sources such as All Electronics (a
SERVO advertiser) — they’re worth taking
the time to wire and install.
For the PropBot, you want a pair of
miniature (not micro) SPDT leaf switches.
The bulk of these have a consistent size and
shape, and already have two mounting
holes on opposite corners of the switch
body. Take a look at the switches in
Figure 1 and Figure 5 for a pictorial
example. SPDT leaf switches will have three
connection lugs: common, normally open,
and normally closed.
You want a switch with an attached
metal leaf — the length isn’t critical, but it
should be at least 3/4”. In a bit, I’ll show
how to add ordinary aquarium or metal
tubing over this leaf to extend the “touch
zone” of the switch.
If you happen to find a pair of leaf
switches with longer leaves, that’s fine, but
32 SERVO 10.2014
Figure 3. Here's how to wire the (optional
but recommended) leaf switches. Use a cut
length of servo extension wire, as it comes
with a nice connector on the end.
Figure 4. Connect the
switch wiring as shown.
Be sure the black wire
(ground) is connected to
pin 4 of the PAB. Use a
male header to mate the
breadboard with the wire