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Introduction

This project was once again inspired by watching the discussions on the internet, someone asked about reversing the direction of operation of Futaba 148 servos. There were several solution put forward including dismantling the servo and swapping the inside around. Several people came up with the solution to use a PIC micro-controller, but were shot down by others saying it was a sledge hammer to crack a nut solution, and too expensive.

At About the same time a project appeared in a UK magazine for a servo slow unit, this used 5 ICs and was built on a board of approx 3.5" x 1.5", as I had been thinking of doing one of these myself based on PIC. I though why not combine the two items to give a unit that would be very small and cheap to build, and would function as a servo reverser, a servo slow unit or both functions together.

Why have a unit to slow down servo's when the manufactures are bringing out faster servos all the time. Well the most obvious use is for retractable undercarriage, and if you were to use two servo slow units you could then set the delays to different speeds to give the effect of the main gear retracting one after the other as seen in old war time film. I am sure that people will think of many other uses, as has happened with past projects.

And what about the reversing function, you may think that this is of little use with modern equipment that has reversing switches on the transmitter. Well what if you are using two servos on one function and need to have one of them operating in the opposite direction to the other one, you can not use the transmitter reversing switch, because this will reverse both servos, so you need a unit to put in one servo lead to do the reversing for you in the model.

The Technical Bit

As can be seen the unit is very simple to build consisting of only 5 components, in fact if you did not wish to make it adjustable, and hardwired the functions, it could be built with only 4 components. As with the V tail mixer I did before, all of the hard bit is done in the program that is blown into the PIC16C54 micro-controller, as I had a lot of software routines written tested and working from the mixer, the development of this unit was very quick. Infact I had the main software written and tested in a unit in only half a day.

The software basically measures the width of the incoming pulse, then first checks to see if the servo slow function is to be used or not, if not the output pulse is generated, this output routine will check to see if the output pulse needs to be reversed or not. If the servo slow function is selected then the rate of change of the output pulse will be determined by the setting of SW1, this will give a range of 1 to 7 secs.

The rate of change of the output pulse is linked to the frame rate of the incoming pulse, if this frame rate is not 20ms then the times given in the table for the settings of SW1 will be a little bit out. Also you will notice that when you use the slower settings, that the output moves in series of small steps, this is unavoidable as the servo can move to the new position faster than the servo slow is generating positions.

Assembly

As I have already said the unit only consists of 5 components, so there is very little that can go wrong. The only two components that need to be fitted the correct way round are IC1 and RP1. IC1 is fitted with the notched end towards SW1, and RP1 is fitted with the end with a dot on it nearest the edge of the PCB. SW1 can be fitted any way round by if not fitted as shown in the component layout drawing then the switch setting table will not make sense, so it is best to fit it as shown in the drawing with SW1/4 nearest the edge of the PCB.

There are different ways of building the unit, you can either fit it into the small plastic case shown or just shrink wrap it in some heatsrink tubing as used for building battery packs, and available from advertisers in this magazine. The other option you have is not to fit the output connector to the PCB, just cut a servo extension lead in half, using one half for the input and the other half as the output.

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

With the servo slow unit plugged into a switched channel on your receiver ie. the Gear function, and a servo plugged into the output. Switch on the receiver and the transmitter. With all of the switches in SW1 in the OFF position the servo should be operating at full speed. Switch SW1/4 ON (Reverse Switch), the servo should now be operating at full speed but going in the opposite direction. Now try different combinations of SW1/1-3, you should now have the servo operating at different slow speeds. The times given in table 1, are the approximate times it will take the servo to travel from one extreme to the other.

And finally

I do hope that this will be another useful item for you to use in one of your models to ease installation problems, or enhance the appearance of the model in flight. The times I have chosen, were just the range of values that I though would be of most use. If you have a need for any different speeds then please don't hesitate to write to me via RCM&E, and let me know your requirements. Anything is possible within reason.

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