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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 10uS, 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 (10uS 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 ukp 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 8.00ukp

You can purchase 2 servo leads (1.50 ukp 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 4.00 ukp.

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.

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