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Introduction

It has been about 3 1/2 years now since the first project I did for RCM&E was published, that was the Mk1 speed controller in the Sept 94 issue, it proved to be a very poplar project and about 250 were built, that was followed a year later by the Mk2 which was a Microprocessor based design. I have always been meaning to do an electric flight switch for speed 400 designs, and I have decided to try and do one which is very simple to build like the Mk1 speed controller, but have a lot of the features of the Mk2 speed controller which can be added by using a microprocessor in the design.

Description of features

As this switch is designed for use in speed 400 models it was decided that BEC was a must to help people keep the weight down by avoiding the need for a RX battery pack, so this unit has a BEC circuit and also an adjustable motor cut off, this can be reset in flight by moving the stick to the motor off position. It also has an arming button to stop the motor starting by accident, and it also requires that the stick is moved to the motor off position and left there for 1 sec. There is a fail safe feature which will shut the motor off if the RX loses the TX signal, this can be made to operate in two different ways, 1. the motor will shut down until a good signal is received again and then it will start on it's own or 2. the motor will shut down and not start again until the stick is moved to the motor off position. The unit also supports EMF braking which because it is done by the relay will give the best possible braking effect.

PIC12C508

The micro computer used in the differential is a PIC12C508 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 6 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. This chip is also one of the first from the company that has an in built oscillator, which means that an external component to do this job is not required.

Configuration of options

There are only two things that can be configured, these are fail-safe shut down mode and EMF braking ON or OFF. Fail safe mode is configured by cutting a thin PCB track which connects to pin 5 of IC1 the PIC12C508. Self restarting is enabled by cutting this track, by default with the track left intact the TX stick will need to be moved to the motor off position to restart the motor. EMF braking is enabled by soldering a wire link across the back of the PCB as shown in the accompanying drawing.

Identification of parts

There are not many parts that need identifying but those that do are described here, the capacitors C1 and C2 have their negative lead marked with a stripe. The BEC regulator should be installed with the metal tab away from the relay. Pin 1 of IC1 is identified by the small round dimple by it, pin 1 needs to be nearest to C1 and C2. The transistor Q1 should be fitted with its rounded side towards the edge of the PCB.

Construction

Some of the components (IC1) used in this design are CMOS devices and can be damaged by static electricity. When handling these items it is advisable to take some basic precautions, do not wear clothing which builds up a static charge, do not handle the items until needed and before you touch them 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.

There is little that can be said about the construction, just make sure that you do not short any PCB tracks out while you are soldering the components in. If you do mot wish to use BEC then just omit IC2 from the PCB. You can fit the finished PCB in a case or if you wish to save as much weight as possible just use it as it is being careful not to short out the bottom of the PCB to any wiring. If you do use the plastic case listed in the parts list, you will need to cut some holes in it. The top will require a hole to clear the relay and the metal tab on IC2. You will also need to drill some holes around the joint line to allow the wires for the servo lead, arm button, battery pack and motor to exit the case, these holes should be aligned next to the wire position on the PCB. The hole for the servo lead may also be best if it is elongated with a small file to form a slot.

Testing and setting up.

With the flight switch wired up to a motor and flight battery, and plugged into the RX channel that you want to use, first switch on the TX as normal and then the flight battery, at this point nothing should happen. Press the arming button if fitted and again nothing should happen, move the TX stick to the motor on position, the motor should start. If it starts but cuts out then you have got the BEC cut out set too high, try reducing it, if the preset RV1 is fully counter clockwise then this is the highest trip voltage, with RV1 fully clockwise then this is the lowest trip voltage position. Move the stick to the off position and then back to the on position. When you have the motor running try switching off the TX, the motor should cut out after about 1 sec, then function as per the setting of your fail-safe mode link. It is very important that you set the BEC cut out voltage correctly, if you try and set it to too low a voltage, then the unit will never cut out and you will be in danger of losing your model, make several tests on the ground to confirm that it is set correctly before using it in flight, you should not try to set it to cut out below about 5.6 volts.

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