Download Diff.pdf, Acrobat version of orignal article.

Ordering parts or complete kit

Introduction
Over the past 5 years I have presented designs for several simple to build projects based on the
PIC range of micro-controller, these have been the Mk1 and Mk2 V tail mixers, servo slow, fail
safe and servo differential. The servo differential was miss named and should have been called
an intelligent Y lead, all of these projects were designed for those people who needed these
functions but did not have computer radio set that would provided them. They have all proved to
be very popular with fliers around the world and the most popular has been built by the hundreds.

I decided it was time to correct the original error and do a real aileron differential unit. Most
computer radio transmitters have an aileron differential mixing function and this is the function
that this unit replicates. Because the down travel of the Aileron creates more drag than the up
travel of the Aileron. In severe cases, this drag will cause a yawing tendency. Differential Aileron
mixing is used to reduce this action by reducing the amount of down travel of the Aileron.

The differential unit has 4 setting for the amount of down travel reduction, these are a
percentage of the up travel. They are 25%, 50%, 75% and 100% which gives no down travel at
all.

The Technical Bit

The differential unit measures the incoming aileron pulse to the nearest 5uS and then looks at
the 2 mode switches to see how much down aileron is required, then the input value is divided by
this amount and a calculation done to produce the 2 output values. Then the reversing switches
are checked to see if the outputs should be reversed or not. The last stage is then that the 2
output pulses are generated for the servo's, once this has been done the differential waits for the
next input pulse and the whole process repeats.

The PIC16C52

The micro computer used in the differential is a PIC16C52 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, 384 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.

Assembly

The assembly of the Aileron Differential unit is very easy, it only consists of 5 components but 2
of these must be fitted the correct way round, these are the PIC micro controller IC1 and the
resistor pack RP1 the orientation of these 2 items is shown clearly in the layout drawing.

The main component IC1 used in this design is a CMOS device and can be damaged by static
electricity. When handling this item it is advisable to take some basic precautions, do not wear
clothing which builds up a static charge, or handle the item until needed and before you touch it,
try to touch a water pipe which should earth any static charge you have built up. DO NOT
connect yourself directly to the mains earth.

Testing

Before anything else check the bottom of the PCB to make sure that there are no solder bridges
between pads that may short out the power supply or receiver outputs. Once this is checked and
all is correct make sure all of the switches sw1-4 are off, connect two servo's to the outputs and
plug the input lead into the receiver aileron output.

Switch on the receiver and transmitter, the servo's should move as you move the aileron stick on
the transmitter, this should make the servos move in opposite directions. Both servos should be
giving full output in one direction away from centre and a reduced amount in the other direction.
If SW1 and SW2 are both ON then you should only get the full throw in one direction and the
servo should say in the centre as the stick goes in the other direction.

You can now check that when you put either sw3 or sw4 on that each output starts to move in the
opposite direction. If all is operating as described then the differential is working correctly.

Setting the Mode Switch

As mentioned in the introduction the aileron differential has 4 different settings for the amount of
down aileron throw and the output reversing for each output channel, this is all set up using
SW1-4 according to the following settings.
Down % Sw1 Sw2
25% off off
50% on off
75% off on
No Down on on

Outputs Sw3 Sw4
1 Normal off .
1 Reversed on .
2 Normal . off
2 Reversed . on

And Finally

I hope that this aileron differential proves to be as popular as some of the original PIC based
units, 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.

Return to main page...