Servo Slow and Reverser
By Ken Hewitt

Originally published in the March 1996 issue of
Radio Control Models and Electronics


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.

Table 1.

        Delay Times in Seconds

	0 	1 	2 	3 	4 	5 	6 	7 	

SW1/1 	OFF 	ON 	OFF 	ON 	OFF 	ON 	OFF 	ON 	
SW1/2 	OFF 	OFF 	ON 	ON 	OFF 	OFF 	ON 	ON 	
SW1/3 	OFF 	OFF 	OFF 	OFF 	ON 	ON 	ON 	ON 	

SW1/4   OFF = Normal operation               ON = Reverse operation


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.


Parts List

C1              220nF
RP1             SIL 10k x 4
SW1             4 Way DIL switch
X1              4MHz Resonator 3 leg
Pin Strip Header                     

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
                                 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.  


