Tuesday, January 19, 2010

Rock-Mite mod

The trick with the Rock-Mite is the PIC (programmable interface controller) which provides keying, RIT and other functions. It is a 12C508 which is pin-compatible to the 12F629 and in a way also to the PICAXE-08M. The main modification would be to write a new firmware for the controller, hence the MEPT/RX would have an onboard beacon keyer.

Check out the schematics:

The Rock-Mite uses a simple oscillator (Q4,Y2) which is pulled (D6) for RIT. Some options here:
  1. replace the varicap diode by a rectifier or LED and FSK the oscillator with Q1
  2. leave the RIT alone (allowing for beat offset) and add keying voltage from GP2 (pin 5, PWM!) via some low-pass filter and resistors
  3. reroute Q2 to GP0 and add extra crystal pulling by using GP2's pseudo DAC (PWM)
I will probably go for option 3.
Option 3 opens this: Use ADC4 (pin 3) for a QRG control via a potentiometer.

The receiver will remain nearly untouched, it already employs a crystal filter and all things needed for grabbing. C1 and C2 however should be replaced by trimmers, so that the filter can be tuned.

With all that said, looking what still is left of the controller, GP0, GP1 and GP3.

When using a PICAXE, GP0 is "out" only. This could be used for RIT control, when using Q2, as indicated above.
GP1 is a digital input and GP3 is an analog/digital input/output. I figure that GP1 could seve as a receive/transmit toggle. Now we are left with one single analog/digital IN/OUT port. What about Johan's idea of including telemetry?

There you have it, the by far best kit to convert to QRSS. BTW: Programming the PICAXE chips is really simple. With a little bit more effort, the 12F629 offers more to play with, in particular more memory.
PICAXE: http://www.rev-ed.co.uk/picaxe/