Pboard

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Pboard  Project

Pboard (short for "P-channel board") is the working nickname of a new driver board that I plan to use with the next version RLbC.  Whereas RLbC 2 only featured output relays, Pboard will make use of  Adafruit's PCA9853 based servo-driver board to provide PWM signals to the outputs via P-channel MOSFETS.  Doing so will allow for fading the lights.



Why use P-channel Mosfets?

I chose to use  FQP27P06 P-channel MOSFETS over the more commonly accepted easier to wire N-channel logic-level MOSFETS out there simply because I wanted high-side switching.  Call me weird or old-fashioned, but I have always designed my low-level voltage systems with Common tied to the "negative" side of a supply (split or dual-tap not withstanding).

However, going to P-channel switching does make the design a little more complex.
  • Buffering is required since I can not directly tie the output of a PCA9685 to Vss (12VDC) otherwise it will be damaged.  
  • Inverted signals: A level-high from the PCA9685 to the gate of the P-channel would turn the device off, while a level-low to the gate of the P-channel would turn the device on. 
To get around these issue,  I made use of a Darlington DIP array, ULN2803A (IC1 in the first diagram).  This handy chip has eight Darlington pairs inside, which can provide the buffering and inversion required for each of the 8 P-MOSFETS in use. 






The remainder of the board uses a 10KĪ© resistor array to provide pull-ups on the gate of each MOSFET


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