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MPX Pico trainer UHF wireless

All RC plane instructors know the feeling of having trouble with the trainer cable -
This cable goes from the student RC transmitter to the instructor RC transmitter.
I am a quite active instructor of my local club, so I modified our good old trainer system into wireless
(UHF - Ultra High Frequency is the frequency band from 300 to 3000 MHz)

Just a picture to explain what this is all about.

The TX (Student) part:

Here you see my motor regulator kit modified so it can be used as power supply and base for an Atmel microcontroller
and a Micrel 620 radio module. The power supply was modified for under 3V operation, and the gnd pin for the original PIC12
was isolated so I can use this pin 1 as IO pin.

The schematic is quite simple, I only made a fast hand drawn version, I choose this construction with an Atmel AVR microcontroller Tiny45 or whatever,
and a Micrel MICRF620 transceiver module. (Transceiver = A single unit that both transmits and receives radio or electronic signals.)
The Tiny45 only setup the MICREL module, it does not handle any communication, The PPM signal from the Student PICO transmitter
is feed into the data pin of the UHF module set up in TX mode. It is level converted by a 3k9 and 4k7 resistor to go from 5V to 3V.
The two channel switches gives 4 possible channels, preset in the firmware I made.

The AVR run at 3.0V made by a LM2941 low drop regulator (LDO)
On the 3 SPI (Serial Protocol Interface) lines from the AVR goes into the MICRF620
The TX board DIP SW:

1 2 FREQ:
off off secret1.800 MHz
on off secret1.400 MHz
off on secret1.000 MHz
on on secret0.600 MHz

The transceiver module has 7 power levels, I use the most power full 10mW.
Range is about 80meters before we see trouble, more than adequate for a wireless trainer system !
If the data was encoded and converted into a digital stream with checksum, the range would be much longer,
the analog way to transmit the PPM pulses are much more sensitive to noise and trigger level changes, but much easier to make.

Here the wireless unit is mounted, it is programmed into TX mode for the Student RX transmitter.
The UHF antenna is a 17cm long cobber wire with 2 turns, to make it fit nicely inside at the top, free from hand area.
The 35Mhz part is simply disabled by removing the crystal, nothing else.

The RX (Instructor) part:

This little circuit board contain exactly the same parts as the TX board. But this one is kept in RX receive mode,
I ran out of Tiny45 so I simply used a good old AT90S2343 they are pin and (almost) software compatible. but the 2343 runs much slower,
but we don't care about this in this case, since we only need this controller to setup the radio unit.

The data output looks exactly as the PPM pulses from the TX board, (good luck for that)
(PPM = Pulse Period Modulation) 

The PPM from the UHF module goes into a 4093 nand gate, to buffer it up to 5V levels (the PICO CPU and stuff runs at 5V)
so I figured why not use the 4093 as a PPM signal switch, I cut the PPM track from the PICO micro controller (Zilog Z8 by the way)
and soldered two wires to each end of the broken track, "PPM instructor" and the "PPM to 35MHz TX system" on this drawing.

Note the UHF antenna in the middle, kept away from hand holding area, and 35MHz power area.
note the 4093 and wires to Student / Instructor switch. The RX board DIP SW is codes as the TX board.

Thomas Scherrer OZ2CPU - May 2006