Servo - August 2008

the calculations for the resistors were pretty simple:

R1

• Time On = R2 C1

• Time Off = R1 C1

1 !K

2! K

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.

1! 0K

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 potentiometer.

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 the perfboard:

+5V

+Vcc

18

27

555

36

45

R2

Output

Red White Black

4 .7K

4! .7K

+

1µF

0.01µF

0V

555 VEX PWM Controller

FIGURE 2

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 ₃the 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