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Introduction

The servo differential (Active Y lead) described in this article is based on the V Tail mixer that was featured in the March 1995 issue. This unit can be built for little more than the cost of a standard 'Y' lead but includes two servo reversers.

The differential reproduces the function of a standard 'Y' lead with a servo reverser fitted to one output, it can also be used as a straight 'Y' lead or a reversing 'Y' lead depending on how you set the two reversing switches.

The circuit for the differential 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 differential 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, this unit will not work correctly with these sets.

The PIC16C54

The micro computer used in the differential 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 differential 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 pulse from the receiver is measured to the nearest 5uS.

When the input pulse has been measured the values for two output pulses are calculated, taking into account the position of the reversing switches, if both switches are in the OFF position the one output pulse follows the input pulse and the other output goes in the opposite direction giving the differential function.

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 pulse 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.

1. 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.

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

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

I recommend option 1 as the best all round solution. Another area which can be built in two different ways is the input connections from the receiver, and output too the servos.

1. 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 8.00 ukp.

2. You can purchase a servo lead (1.50 ukp) to use for the input 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 2.00 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.

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.

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 differential 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. With SW1-1 and SW1-2 in the OFF position, when the stick is moved then the servos should move in opersite directions.

By switching ON the reversing function using SW1-1 or SW1-2 then either of the servo can be made to operate in the reverse direction. Other than this, there is very little that can go wrong.

And Finally

I hope that this differential 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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