In this article, we will focus on Ham Arduino External Control of electro-mechanical devices. These include relays, stepper and servo motors and high resolution analog outputs.
Let’s start with relays. Relays enable the low power Arduino to control higher power circuits. With a few external components, you can use the Arduino digital pins to control 5 or 3.3 volt relays. Also, you can buy boards or shields that control multiple relays directly off the Arduino.
Sometimes you might want to use an analog voltage for control. The easiest way to get analog voltage control from an Arduino is Pulse Width Modulation. PWM can provide a voltage from 0 to 5 volts. All you do is program the duty cycle, and add an RC filter on the output line. Unfortunately, this approach only provides 8 bits of resolution, or roughly 20 millivolts per step. Consequently, you might want to attach a real Digital to Analog converter. You can buy a small breakout board with the MCP4725 DAC for a few dollars. With 12 bit resolution, you get roughly 2 mV resolution. This is what I have used to control varicap circuits. And since the breakout board uses the I2C bus, you only need two digital pins for control.
Ham Arduino External Control for Motors
Starting out, you should learn how to use stepper and servo motors. Because they draw a lot of current, you need to power motors independently, and just use Arduino for control.
Servo motors use a type of PWM to rotate an actuator arm up to 180 degrees. Start with the popular SG90 9g Micro Servo. For $3, this little guy has enough torque for many projects. Servos are closed loop control devices, which means you can set and read its position accurately.
On the other hand, stepper motors are generally open loop devices. You just program the number of steps you want a stepper to “step” in either direction, and keep track of steps in your program. When you use a stepper, it is normal to “home to zero” at the beginning of control. The 28BYJ-48 stepper comes with its own driver board.
You can buy these motors on e-Bay and download control libraries easily. Once you master them, it’s easy to move on to more powerful servos and more complex stepper control. I have used the 9g servo to control pan tilt cameras and small antennas. Also, I have used this cheap stepper to remote control the tuning capacitor of a magnetic loop antenna.