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Introduction

This single channel fail safe unit is designed to be installed between any single receiver output and servo, and checks for loss of signal or coruption, and if detected drives the servo to a preset position and then waits for the signal to restore to a good condition before allowing the servo to be controlled by the receiver again.

The fail safe 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, if required I can supply a version of IC1 programmed for 1.7mS systems on request.

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 previous PIC based projects that I have done 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 previous projects, the development of this unit was fairly quick.

The software basically measures the width of the incoming pulse, then first checks to see if it is a valid pulse (>0.736mS and <2.235mS) it then checks the input for the next 10mS to make sure it does not see any input pulse (noise). If both of these tests are passed, then the input is deemed to be good. If either test is failed then a counter is decremented, if this counter reaches zero then the unit fail safes.

If the unit fail safes then the output pulse will be determined by the setting of SW1, this will give 16 output positions, in the range 0.869 to 2.059 mS.

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

This is a difficult thing to test fully but some checks can be made. plug the fail safe into the throttle output of the receiver and plug a servo into the fail safe unit. With all of the switches in the OFF position, switch the receiver on first, the servo should move to one end of its travel. Turn the transmitter on and the servo should move to take up a position determined by the stick position, check that the servo follows the stick movements. Switch off the transmitter and the servo should again move to the end position as before. If this is all OK then change the setting of SW1-1/4 and check this gives a new fail safe position.

You have now confirmed that a loss of signal will cause the unit to fail safe, the next test you can try if you have access to another transmitter with the same crystal fitted is to switch your transmitter and receiver on and check for correct operation of the servo, then switch on the second transmitter and check that this causes the unit to fail safe.

And finally

I do hope that this will be another useful item for you to use in one of your models, and will add you your range of electronics accesories from the RCM&E range. If there is some device that you have a need for then please let me know as anything is possible within reason.

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