XRP Robot Running Lights or LED Indicators

Order the PCB

LED_XRP_Robot_Eye

Discussion

This project requires the construction of a printed circuit board (PCB).  The board controls an LED using either of the servo motor connectors or one of the Qwiic expansion connectors.  The board input has a low pass filter so the presence of a PWM signal on a servo connector is interpreted as a “1” or a “1” can be output to the GPIB IO port of an expansion connector.  Likewise outputting a “0” or lack of a PWM signal turns off the LED.  Note: if you plan on powering the board from a Qwiic expansion port you won’t need to include the filter capacitors C1 and C2 when constructing the PCB.

The link for ordering the PCB opens the OSH Park website for ordering a standard 1.6mm thick printed circuit board.  You will get 3 boards for $1.00.  Refer to the Printables model “Rainbow LEDs in a Ring of Power” for instructions in assembly if you are new to building PCBs.  The BOM text file indicates the DigiKey part numbers for the electrical components and a selection of low current LEDs.  Go to DigiKey.com and copy and paste the DigiKey part number into their web site for ordering. 

The voltage supplied from the Qwiic expansion connectors is 3.3v.  Select the current limiting resistors (R4 and R5) from the Bill-Of-Materials based on the LED color chosen.  If the LEDs will be powered from a 5v supply then change R4 and R5 to 3000 ohms.

Printing the Bracket

I used a 0.25 Z-step, 15% honeycomb fill, 2 perimeter shells and 4 top- and bottom layers.   You may need to add a couple millimeters of brim if you are having adhesion problems.  The two versions are mirror images so the PCB may be inserted from the left or right.

Assembly

Use the Bill-of-Materials list for guidance in ordering parts.  The quantities indicated are for constructing three PCBs.   The resistors and capacitors are 0402 size and the quantity is listed as 10 to take advantage of a price break (you will also have spares in case you lose any). The BSS138 N-FET is in a SOT23-3 package to fit the pads on the board and the BAT54GWJ Schottky diode is in a SOD-123 package to fit the footprint on the board (Note: the bar across the end of the part is the same orientation as the bar in the symbol on the PCB).

Assemble the board and solder in a pigtail (lead wires with the desired connector) for the XRP port you plan on using.  The board needs three connections to operate: Ground, Power, and Signal which are orientated in the same order as the servo connector pinouts.  I would recommend the purchase of some Servo Extension Cables (search Amazon or EBay).  Clip off the end with the male connector and solder the wires into the LED board from the back side (note: the color code) and the ground orientation).  The bent over wires will help hold the board in place inside the bracket.  If the board is still lose, a bit of CA glue will hold it in place.

Note that: two LED light boards can be operated from a single “Qwiic Expansion Header”.  If you choose this approach construct a “Y” cable harness.   Purchase one of the Qwiic Cables and cut it in half.  Cut off the other connector from the servo extension cables you just soldered to the LED PCBs (seems a waste but you only need the 3-wire cable).   There are two GPIB IO lines available in the Qwiic expansion connectors so solder all the GND and PWR wires together and connect one Qwiic IO wire to each LED board control wire.  Remember to insulate the connections with some electrical tape or maybe some heat shrink tubing. 

Coding Hints

If using a Qwiic expansion port remember to set the GPIB IO lines to data output in the set-up portion of your robot control program.  Then outputting a “1” to the IO line turns on the LED and outputting a “0” turns it off.