■ FIGURE 8
reflector has been
attached in this photo.
Before you can
mount the enclosure to
the float, drill four holes
in the wood to clear the
heads of the 4-40
screws that hold the
board standoffs to the
box. That way, the
enclosure will fit flush to
the wood as the screw heads fit into the holes. The holes
don’t have to go all the way through; 1/4 inch deep
should do. Use a drill bit that is slightly wider than the
screw head. Before drilling, hold the box against the base
and push so that the four screw heads mark the wood.
That tells you where to drill.
To attach the box to the base, hold the box flush to
the wood and drill an 1/8 inch through the middle of the
aluminum and through the wood. Then, put a wood screw
through the hole and tighten it to hold the box to the wood.
base just above the handle as
■ FIGURE 9 shown in Figure 8. Then, draw two
diagonal lines as shown. Where the
diagonals cross is a point on the
center line of the base. Use the
carpenter’s square to draw the
center line through the point.
Position the reflector along the
center line and mark the spot
where you will drill the mounting
hole. Before mounting the dish to the base, we need to
drill a 1/4 inch hole through the side of the dish for the
microphone wires to pass through. Locate the hole close
to the bottom of the dish, but not so close that it’s blocked
by the wood base. Insert a rubber grommet in the hole.
Figure 9 is a detailed drawing of the mounting
hardware. (It is not to scale.) To keep it simple, lock
washers are not shown. Note the 1/4 inch flat washer
inside the mounting hole of the dish. Its outer diameter is
about 0.74 inches; its purpose is to help center the
mounting screw. The fender washers are 3/16 × 1 inch.
The one under the coupling nut is used to hold the dish to
the base. A 3/4 inch 10-32 socket head screw is used. You
need to get the screw very tight, and a hex key allows
much more torque than a screwdriver. The coupling nut
has a 3/4 inch height and a 5/16 hex size.
Figure 9 is just one way to do it. You may have a
better way to mount the reflector to the base. If you use a
different reflector than the one used here, you’ll have to
figure out a way to mount it.
Mounting the Microphone
Mounting the Reflector to the
The reflector used is an Edmund Scientific model
3053875. It has a 12 inch diameter, a three inch focal
length, and a 0.75 inch center hole for mounting. To
mount the dish to the base, drill a clearance hole for a
10-32 screw through the short end of the base. Use a
carpenter’s square and a pencil to draw a line across the
■ FIGURE 10
■ FIGURE 11
For the reflector to work properly, the microphone
must be mounted as close to the focal point as possible.
The Big Ear uses two small PCBs and a length of threaded
rod as shown in Figure 10 (again, not drawn to scale).
The bottom PCB is mounted on the coupling nut at the
bottom of the dish. The mic is soldered to the top board
and faces towards the bottom. The top PCB has a
clearance hole so it can be moved along the threaded rod
and be locked in place by two nuts and
lock washers. Insert the twisted-pair wire
through the grommet and wrap it loosely
around the threaded rod a few times. Cut
off any excess length and solder the wires
to the top board. The square pad should be
the mic ground, but verify that it is (see
Figure 11). Figure 11 shows the PCBs used
to mount the microphone. Figure 11(A) is
the bottom board while 11(B) and 11(C)
show both sides of the top board. The mic
is mounted on the side with the word MIC
on it. The mic is placed so that it lies over
the strip of copper near the mounting
holes. In Figure 11(A), note that the two
(C) holes are different sizes. The larger hole is