The M5 Stuart Project - Rigging the Electronics

  WARNING: This stuff worked for me, but I can't guarantee it'll work for you. If you decide to start hacking your tank, be aware that you can really screw it up if you're not careful or if you don't understand what you're doing! I can't (and won't) be held responsible for your actions!  

  The Stuart's receiver board is the large board just below the speaker in this picture. The board consists of the RF receiver and decoder for the control signals. The signals are delivered to the power and drive board via flex cable.

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.

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.


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.

BASIC Source Code

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.
Assembled HEX Code

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.

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.

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.

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.

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.

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.

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