AprilTags

The base station hosts 4 IR backlit AprilTags. The tags were made using 3D-printed frames, an IR LED strip, acrylic sheets, and non-reflective black adhesive material (for the individual AprilTag designs).

Wiring

The two outer tags are wired through drag chains that guide the wires down through the base station, connect to the central tags' wiring, and plug into the PWM Control board. There is an XT60 connector on either end of the base station that can be unplugged to disconnect power to the outer AprilTags. If the doors need to be manually opened all the way, these XT60 connectors MUST be unplugged. Additionally, there are two JST 2-Pin connectors under the base station lid that can be disconnected if the plexiglass sheet hosting the two inner AprilTags needs to be moved.

Note: For more information about the specific pinouts on the LattePanda, see the LattePanda Delta3 page.

Drag Chains

Because the doors move, and we did not want the wires connected to the outer AprilTags to hang and potentially drag around when the doors were open, we installed drag chains on either side. The drag chains carry the long wiring down and through the base station lid. They follow the movement of the doors as they open and close and do not interfere with anything.

PCBs

Two custom PCBs were printed for the project this year. Here is a zipped file containing the KiCAD files for both of them.

Ambient Light Processing

Purpose

This PCB reads ambient light using a phototransistor seated at the top of the base station lid. It then sends that analog data through an ADC to give us digital data that can be processed over I2C on the LattePanda.

Breakdown of circuits

The phototransistor is rated to measure wavelengths ranging from 400-700nm (visible light spectrum), so it is not influenced by the IR light coming from the AprilTag LEDs. There is a 1kHz low-pass filter that the analog data goes through before entering the ADC in order to reduce potential noise from the long connection leading down from the phototransistor to the PCB. The 4.7k Ohm resistors act as pull-up resistors for the Alert, SDA, and SCL signals on the ADC. The two shunt capacitors are to block out noise in from the input voltage. light_sense.zip

PWM Control

Purpose

This PCB facilitates turning on, dimming, and turning off the IR LEDs for the AprilTags.

Breakdown of circuits

The circuit on this board is very simple. It consists of an NMOS transistor rated for 60V and 12A and two resistors. Since the 10k Ohm resistor is so much larger than the 400 Ohm resistor, the voltage drop across the transistor is high enough to turn on the transistor when we get a PWM signal. J1 and J2 are JST 2-Pin female connectors that provide power to the AprilTags. J3 is where power from the 12V rail enters the board (see Note below). J4 is where the PWM signal and ground from the LattePanda are provided. pwm_ctrl.zip

Note: There was no need for the second ground connection since the two JST 2-pin connectors for PWM and +12V use the same ground, so there is a dangling ground wire that is not connected to anything.