the calculations for the resistors were
• Time On = R2 C1
• Time Off = R1 C1
Opting for simplicity, we stuck with a 1 µF
capacitor and the closest we could get with
standard resistors. For R2, this came out to be
19.4K which can be made with three standard
resistors (10K + 4.7K + 4.7K). However, for
the R1 which controls the duty cycle we
needed a bit more accuracy than we could
get with the typical 5% tolerance resistors.
For this we knew we needed somewhere
around 1.8K. For this, we chose to combine
a standard 1K resistor with a 1K, 15-turn
The only other challenging part we
needed was a way to connect the Vex motor
to the circuit. If you are using a breadboard to
build the circuit, the Vex motor cable is on
standard .1” centers and just plugs into the
breadboard. If you put it on a perfboard, there are
several options short of actually soldering the motor pins to
555 VEX PWM Controller
affected the other circuits of the car. To clean this up, we
simply connected the + 5 to the battery through a five volt
regulator (such as a 7805 — RadioShack P/N 276-1770.
1) Use an IC socket and just push the motor into the socket.
This works to experiment with, but isn’t very durable.
Putting It in Motion
2) Get a Vex extender cable (or the equivalent from a local
R/C hobby store) and solder it to the board.
3) Use an old IDE, floppy, USB block, or even an internal
audio cable that the Vex motor plugs into — make sure you
find the right wires to connect into the board. Audio cables
work great as you can see in Figure 3.
Once you have everything assembled and have
powered up the circuit, you need to adjust the potentiometer
to get the optimum speed. You can do this by either
watching a scope connected to the outputs or tune it by
just watching the motor. You should be able to see the full
range of the motor from maximum reverse to idle to full
forward by adjusting the potentiometer.
Building on It
The circuit is easy to lay out. As you can see in Figure
4, the students built the circuit so that parts occupy just the
bottom half of the
perfboard (the top half has FIGURE 3
the original timer circuit
which controlled how long
the motor ran for). The
circuit isn’t too sensitive to
parts placement, so just lay
everything out wherever you
have the most room.
The only special thing to
note is that the circuit in
Figure 2 has both a + 5 and
+Vcc indicator. This is
because we found that in
testing the car, sometimes
the noise of the motor
One obvious improvement to this circuit would be to
SERVO 08.2008 47