not designed for continuous rotation, and using them
that way can shorten their lives. The modification involves
rewiring the potentiometer and snapping off the
mechanical stops that prevent 360° rotation. If you’re
interested, there are several sites on the Internet that will
show you how to do it.
Now, let’s build our buddy.
The Servo Buddy Circuit
Figure 6 shows the schematic. It uses two CMOS 555
timer ICs. The first is an oscillator running at 50 Hz to
generate the 20 ms spacing between pulses. The second
is a one-shot timer with its output pulse width set by a
potentiometer. I used CMOS because bipolar 555s often
have a transient internal short of power to ground when
they switch. Such transients generate noise in the form of
high current spikes. I used 555s because I had a lot of
them. You can also use a 556, which is a dual 555 in a
14-pin DIP. Figure 7 shows the parts placement on a
printed circuit board (PCB), and Figure 8 is a
34 SERVO 05.2008
photograph of the finished unit. Note the loop of wire
along the bottom edge. It’s soldered to ground as a place
to attach the ground lead of an oscilloscope or voltmeter.
The board is designed to be powered by an unregulated
DC wall-wart supply; a 9V 1A unit should do the job.
Power is connected to a two-position terminal block, and
D1 protects against reverse polarity. An LM7805 regulates
the supply down to five volts and is hefty enough to handle
one amp slugs of current. C2 is a 100 μF cap, also to
handle high current draw. The servo connector attaches to
a three-pin header on the board edge.
The oscillation frequency of IC2 is set by C3, R2, R3A,
and R3B. R3A and R3B are connected by a removable
jumper (JP). With the JP out, IC2 runs at 50 Hz for analog
servos. With JP in, IC2 runs at 250 Hz for digital servos.
Digital servos will work at 50 Hz, but having the higher
frequency allows them to be tested with a typical input
signal. Using 1% resistors for R2 and R3B, the exact
frequency depends on the tolerance of C3. Monolithic
ceramics can vary ±20% depending on temperature so