Micro-Computer V Tail Mixer
By Ken Hewitt

Originally published in the March 1995 issue of
Radio Control Models and Electronics


Introduction

The V tail mixer to be described in this article came about by a
chance remark from a colleague at work ( who should be reading
this in his new home down under ). Alan had a free plan for a
model called the 'Tiny Bit' which was a small flying wing which
required elevons, being small there was no room for the usual
sliding servo mixer, the original was meant to use an elevon
mixing function of a new type computer radio set. The request
was, would it be possible to build a mixer in to Alan's futaba
transmitter. It was decided that this was not a viable solution,
but an electronic airborne mixer was.

As I had been doing a lot of development work for the Mk2 speed
controller which is base around a small micro computer chip, I
had a lot of software routines already written that could be
used in a digital mixer. So as I was off on a couple of days
holiday I stated I would knock one up for something to do, The
unit presented here eventually took a lot longer than the couple
of days.

The mixer reproduces the function of the standard sliding servo
mixer, and can be used as a V Tail mixer mixing rudder and
elevator or as an elevon mixer by mixing aileron and elevator.
The circuit for the mixer is very straight forward as can be
seen from figure 1 it consists of a maximum of 8 components. The
main functions are all performed by the software that is
programmed into the micro computer (IC1) which is one of the PIC
range from Arizona Microchip, this software consists of in
excess of 300 lines of computer code. The mixer works with all
radio control systems having a pulse width of 1 to 2 mS with
centre being 1.5 mS, this is the majority of sets on the market,
but some of the older systems based around a centre setting of
1.7mS may not work correctly.

One novel feature of this design is the left and right servo
reversing switches which enable you to install the servos and
control linkages for the neatest installation, and then by using
your transmitter reversing switches for the two functions being
mixed, and the servo reversing switches in the mixer,  you will
be able to get the installation working correctly.


The PIC16C54

The micro computer used in the mixer is a PIC16C54 which has a
RISC like CPU, and supports 33 instructions. The chip contains
everything that is required to form a fully working micro
computer, it has 12 input or output pins, 512 program memory
locations and 25 bytes of RAM. This may not sound like much but
because of the RISC type architecture the resulting code can be
very compact. It also has a wide range of power supply limits,
2.5 volts to 6.25 volts at less than 2mA, making it ideal for
use in model avionics systems.

If the mixer had been built around conventional analogue
circuits it would have been a lot more complex, larger and would
have required a lot of setting up. In fact it is doubtful that a
home constructional project would have been practical.


Software Description

The software that is programmed into the chip is quite simple in
operation. First the input and output pins are defined and then
the width of the incoming pulses from the receiver are measured
to the nearest 5uS, this value is then divided by two, this is
so that if both inputs are at the maximum input pulse widht
(2mS), then the resulting output pulse is only at maximum (2mS)
and not twice the maximum value which would occur if the input
values were not divided by 2.

For the elevator input the resulting value is added to both the
left and right output values equally. But for the rudder or
aileron input the value is added to the right value and
subtracted from the left, or subtracted from the right and added
to the left, depending on whether you are turning right of left.

Once the output value for the right and left servo outputs have
been calculated then two loops are executed which take (5uS x
the calculated output value), this is done for the right and
left outputs, this generates the correct output pulse widths for
the left and right servos. After the output pulses have been
produced then the program loops back to the start and waits for
the next input pulses to arrive. 


Assembly

There is very little that can be said about the assembly of the
board for this unit. But there are a few areas of construction
that can be tackled in different ways. The first is the 4 Mhz
timing element (X1), this can be one of three different types.

The simplest and cheapest option is to use a 3 leg ceramic
resonator which has inbuilt capacitors so C2 and C3 can be
omitted from the assembly. 

Using a two leg ceramic resonator,  then C2 and C3 must be
fitted in the positions shown in the layout drawing figure 2.

Using a crystal, then C2 and C3 must be fitted in the positions
shown in the layout drawing figure 2.

Another area which can be built in two different ways is the
input connections from the receiver, and output too the servos. 

You can buy two servo extension leads (3.99 each) and cut them
in half to give you two plugs and two sockets, and use these as
the input and output connections. Cost approximately 8.

You can purchase 2 servo leads (1.50 each) to use for the
inputs and fit a 2x3 way piece of pin strip header ( purchased
in lengths of 36x2 ) as the output connections, this is the
cheapest solution. Cost approximately 4. 

If this second option is taken then a second opening will need
to be cut in the case to suit the plugs on the servos used.

Also if the servo reversing option is not required or you are
concerned that the switches may move in flight due to vibration,
then the switch can be omitted, and two wire links should be
installed in place of the switch for normal operation, or left
out for reversing the servo direction. 


Checking Operation


Once the unit has been assembled then the normal visual checks
should be made to ensure that there are no solder bridges on the
track side of the PCB, if all is well then the mixer should be
connected between a receiver and two servos. When the power is
switched on and the transmitter stick moved the two servos
should respond. When the elevator stick is moved both servos
should move in the same direction, and when the rudder or
aileron stick is moved then the servos should move in opersite
directions.

If the servos move but not as described above then the inputs
may be coming from the wrong channels or the servo reversing
switches if fitted may not be in the normal servo direction
position.

Other than this, there is very little that can go wrong.


And Finally

I hope that this mixer is of use to those of you who do not have
a computer Radio set that has mixing functions built in, and
that you have gained an insight into the possible uses of the
PIC range of micro computer chips in the world of model
avionics. If anyone is inspired into trying to develope PIC
based projects for themselves then please don't hesitate to
write to me for advice or information about this wonderful
little chip.


Parts List

C1              220nF
C2,C2           22pF

R1,R2           10k  1/4W
2way DIL Switch

X1              4MHz Crystal
        or      4MHz Resonator 2 leg
        or      4MHz Resonator 3 leg

Servo Lead	Available from local model shop	

IC1             PIC16C54XT/P     Available from author  5.25 UKP
PCB                              Available from author  2.50 UKP
                                 or 7.50 UKP for both items
                                 Post and packing is included in the price for
                                 the UK and Europe. Please add 1.00 UKP per
                                 order for the rest of the world
                                 Make Cheques/ PO payable to K.Hewitt.
Author

Ken Hewitt,
21 The Brambles
Welwyn
Herts, AL6 0PG
England

or
C/O RCM&E,Nexus House, Broundary Way, Hemel
Hempstead, Herts. HP2 7ST.  

