Servo - September 2016

pressed, we will jump to a subroutine or function to handle the opening or closing of the door. In Figure 4, you can see a close-up of the control board and how the LED, servo, switches, and sensors are connected to it.

The servo and phototransistors are connected to the servo headers. The pushbuttons and LEDs are connected to the breadboard via solid wire. In Figure 4, the 10K resistors at the top left are the pull-ups for the phototransistors, while the 10K resistors just to the right are the pull-ups for the active-low pushbuttons. The 220 ohm resistor is used to limit current to the bi-color LED and the two 1K resistors on the bottom right are used to limit current to the IR LEDs used in the limit switches. See the schematic for more details.

The pushbuttons are pretty straightforward. They’re configured as active-low, which means that normally the I/O pin reads high, and when you push the button it goes low. This is the more common configuration. The optical limit switches are essentially two beam-break detectors consisting of an IR LED and a phototransistor. A 10K resistor normally pulls the collector of the phototransistor high. However, the IR LED saturates the phototransistor