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The 22 pin
IC labeled PT8A991AP is an RF decoder IC. The
pins which drive each function are labeled in this picture (click for
larger image). I removed the decoder and reinstalled it in a DIP socket
to allow me to experiment. Note that applying Vcc (~5v) to the labeled
pins initiates the associated function.
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There is one
function in the decoder chipset that the Stuart doesn't use (Turbo) and
the MG fire is only a sound effect. Therefore, the unused Turbo function
and the MG fire can be used to raise & lower the marker with one servo.
The existing cannon 'fire' can be used to fire the paintball marker with
another servo. The upgrade consists of adding a PIC
microcontroller circuit that will drive the two servos (elevate &
fire) from the existing decoder. The schematic is shown. Click the link,
save the image, and reopen it to see it full size.
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The PIC microcontroller takes three inputs from the Stuart's receiver and translates those inputs into servo movements. I tested the code with three servos (Futaba, Hitec, and Tower Hobbies) I had on hand but I won't guarantee it'll work with just any servos. You may need to experiment. The source code was written and compiled in BASIC using Celestial Horizon's CH Basic Flash Edition Compiler. It's available from Reynolds Electronics. The raw BASIC source is provided via the link at right.
The assembly
code was turned into HEX and tested using Microchip's
MPLAB IDE with integrated simulator. Finally, the code was flashed to
the PIC using a homemade programmer called NOPPP.
It's cheap and works great! The 16F84A PIC is available from Digikey.
A few modifications
to the transmitter & receiver are required. First the transmitter.
A pushbutton switch needs to be added to initiate the unused function.
I decided to move 'fire' from the existing button to the new button and
make the old 'fire' button move the marker down. I installed a small normally
open pushbutton just below the right drive 'stick' on the face of the
transmitter where I could reach it with my thumb.
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Here's the
view inside. The new pushbutton connects to IC1 pin 11 (fire) and to pin
3 which is ground. The PT8A990AP encoder uses
'lows' to initiate the functions.
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A couple of
PCB traces are cut (carefully with a dremel tool or exacto knife) to enable
use of the TURBO function and to reroute the 'fire' switch. Capacitor
C1 is removed and a jumper is added between the TURBO pin and SW6 to create
the new marker 'down' button from the old 'fire' button on the transmitter.
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Here's a view
of the opposite side of the board showing the capacitor to remove. That's
it for the transmitter mods that are required. The transmitter must be
reassembled carefully to get the sticks & stick springs back in the
right orientation so the sticks will work.
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Now for the
receiver. I only cut one trace as outlined in the photo. This was to disable
the 'kickback' when the cannon fires to improve accuracy of the paintball
shots. Otherwise, the wires are simply connected directly from the PIC
to the applicable pins of the PT8A991 decoder. 12V and ground is available
from the 2nd and 3rd wires of the ribbon cable on the right of the photo.
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This is a
shot of the complete PIC 16F84A controller. Simple breadboard construction.
I used a short piece of 8 conductor stranded wire to go from the PIC up
to the turret through the hole in the center of the turret drive gear.
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Here's a shot
of the PIC controller along with the receiver board. It works like a charm!
I would probably reroute the receiver antenna and tidy things up if I
was going to leave things as-is. Otherwise, that's about all there is
to it
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