Rev 1 - S2020 - Emily Adam

Owned by lwbantot@uwaterloo.ca (Deactivated)
Last updated: Apr 15, 2021
2 min read

The LED Matrix is a board consisting of 32 RGB LEDs connected in parallel that light up together to display the current status of the robot, such as if it's running autonomously, being teleoperated, etc.

The LED Matrix receives 24V power from the Power Distribution Board, as well as 3 PWM signals from the PDB's microcontroller. These signals control the red, green, and blue lighting of all the LEDs respectively. 

*Errata: the pull-down resistors in this schematic are 1k ohms, instead of 10k ohms

Calculations

Supply Voltage: 24V

Current to be drawn by each LED: 20mA

  • Red LED voltage drop: 2.1V
    • R = (24V - 2.1V)/20mA = 1095 ohms
  • Green & Blue LED voltage drop: 3.1V
    • R = (24V - 3.1V)/20mA = 1045 ohms

Functions

  • Uses a 3A fuse on the 24V supply to prevent an overdraw of current that could damage the board's components
  • From the Power Distribution Board (PDB), the LED Matrix receives three Pulse Width Modulation (PWM) signals. Each one controls either the red, blue, or green LEDs that compose the 32 RGB LEDs on the board
    • On the schematic, these PWM signals are labeled as RED ENABLE, BLUE ENABLE, and GREEN ENABLE
    • PWM signals are used so that we can vary the brightness of an LED by providing a lower, average voltage
  • The PWM/ENABLE signals switch on and off the LED using low-side switches
    • The switch is composed of N-type MOSFETs, or NMOS's. MOSFETs act as variable resistors. For the N-type, the higher the voltage applied to the NMOS's "Gate" (/the closer the voltage applied is to the NMOS's Gate Threshold Voltage), the lower the resistance becomes between the NMOS's "Drain" and "Source". Thus, when a HIGH signal is sent to the NMOS, it connects the circuit between 24V RED/24V BLUE/24V GREEN and GND. Otherwise, a LOW signal results in an open circuit, essentially, due to the high resistance on the NMOS
    • A pull-down resistor is also required in parallel to the ENABLE signal, before the NMOS's Gate. These divert the signal away from the NMOS – due to its lower resistance in comparison to the NMOS in its "off" state – when the signal is LOW. The resistor itself is necessary to prevent a short, i.e. from power being connected directly to GND without any load in between
  • Has capacitors in parallel between the voltage supply and GND for decoupling the supply and bypassing electrical noise
    • Electrical noise is any unwanted variation in voltage or current, usually of a low amplitude
    • The capacitors will smooth the voltage supply by discharging some power when the supply dips
    • The capacitors will also conduct high-frequency noise directly to GND, due to their reactive impedance 

Designers

  • Kyle Hong - Original Schematic Capture and PCB design
  • Tony Liu - Original Schematic Capture and PCB design
  • Emily Adam - Updates to Schematic and PCB design
  • Cindy Li - Updates to Schematic and PCB design