■ FIGURE 8
As I mentioned earlier, I am
going to show you two different
types of interfaces.
SSC- 32 Interface
I got my hands on a SSC- 32 servo
controller (Figure 10) for another project I was working on. At $39.95, this is
the best darn servo controller I have
ever used. It has a built-in RS-232 driver
so it can be connected directly to the
PC or controlled via a microcontroller.
What makes this servo controller
so great is that it will control up to 32
servos via a very simple interface. You
can set the time the servo takes to
create its programmed move or even
send commands so that groups of servos can be controlled, all creating their
movement over a predefined period of
time. While we won’t be using any of
these advanced features in our mouse
trap, we will be accessing one of
the other cool features. The SSC- 32
has four inputs that can be read
independently of any of the servos.
I used the default power configuration on the SSC- 32 which allows a
single battery to supply power to the
servos and the logic. Lynxmotion also
sells a small, pre-wired power harness
■ FIGURE 10
■ FIGURE 9
that has a power switch and R/C
stick battery connector as shown in
Figure 11. I’m not going to show a
schematic for the SSC- 32 interface as
it is very simple:
• Apply power source to VL leads.
• Plug servo into Servo 0 position.
• Connect ramp leads to the B and
• Connect PC serial to nine-pin
Note that I used a 2,000 mAh
stick battery, but you can use the same
four cell battery pack that I used on
the Athena interface and it will work
just as well. You will have to charge the
batteries more often, though.
To control the SSC- 32 we will use
Zeus. Zeus is a simple programming
language that specializes in interface
design. I have included a special, free
version of Zeus called ZeusNV along
with the source code and compiled
applications available on the Nuts
& Volts website ( www.nutsvolts.com).
So even if you are not the best
programmer in the world, you have
lots of choices here.
To quickly test your mouse trap,
place the MouseTrap_DT.exe file
along with the mouse.jpg and
scream.wav and run the file.
■ FIGURE 11
If your PC is already connected
to the SSC- 32 and it is powered up,
the servo should arm. This means that
it should be slightly forward. When
the trap is tripped, the servo arm will
pull back then move back into the
Have fun and play around with
the program. The source is called
Athena Stand-Alone Interface
While the PC controlled trap is
cool to play with and fun to test, it is
not very practical in the long term for
a few reasons:
• A 2,000 mAh battery powering the
SSC- 32 will only last a couple days.
• You’ll need to tie up a PC or laptop.
• You need to tether the mouse trap
to the PC.
With this interface, we will use a
very inexpensive Athena microcontroller.
The Athena has a sleep state that
uses very little power when configured
properly. I built a light monitoring circuit that uses an Athena that has been
running on a single set of alkaline batteries for over two years. The problem
is the servo. Even with no signal being
sent to the servo, it still uses quite a bit
of power. This drastically shortens the
battery life, so I had to come up with
some way to remove the power from
the servo when it was not in use.
Kronos Robotics sells some relays
that can be connected directly to a
microcontroller. They have a built-in
diode that protects the microcontroller
from any EMF generated by the coil.
They are in a 14 pin DIP packages so
they work with a standard 14 pin DIP
socket. I used one of these relays to
remove the power from the servo
when not actually moving the servo.
Schematic 1 shows the complete
schematic that I used. If you want to
add an alarm to the circuit, simply
place it across the power leads
connected to the servo. That way,
when the relay turns on power to the
servo, it will also sound the alarm.
There are several sirens you can
use for the trap. The one I used has a