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I recently purchased 2 Turnigy receiver controlled switches, neither worked. I searched for alternatives and could find none, but I then found a post on another blog that suggested using a servo, and turning the pot so that the power is off when the switch on the transmitter is in the ‘off’ position. This seemed like a great idea, but I thought I could improve on it a little and effectively make a switch which worked in the way that the Turnigy switch should. So I did and for anyone that is interested here is how it all works and how I made it.

First thing to do was to take apart a servo, we don’t need the plastic bits, gears or motor

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Take the top and bottom off, the motor and the control board come out quite easily and the gears come of the top with a bit of persuasion.

20140430_204104 To get the pot out I needed to smash the plastic casing

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Next step we need to cut the motor from the circuit, but we want the wires so cut it as close to the motor as possible

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This leaves the controller board and the pot. I have also trimmed the shaft of the pot since we only want it to set the center point of the switch, once this is set we don’t want it to change.

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So now we think a little about how the servo works. The servo is (was) made up of the 3 components, the controller board, the potentiometer and the motor. The potentiometer is connected to the arm connector and is used to tell the electronics board were the arm is. The board is then fed the signal from the receiver which tells it where the pilot wants the arm to be. If the arm is actually to the right of where the pilot wants it then the board will spin the motor to the left to move the arm to the left, if the arm is to the left of the motor then the board will apply a reverse voltage to the (brushed) motor and spin it to the right, if the arm is in the right place then no voltage will be applied to the motor. So if we move the potentiometer to the central position then when we move signal to it above and below its position we will get a positive and negative signal. See the following 3 pictures, the contraption is connected to the elevator channel, and I have connected a voltmeter in the place of the motor. When the elevator stick is in the central position we see zero volts on the voltmeter. When we move the stick down we see 3V on the meter, when we move the stick up we see -3v on the meter.

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We only want the switch to work in 1 direction so we can use a diode so that current will only flow in one direction, in this case I decided to use an LED (Light Emitting Diode) this works as a diode and also works as a signal to show if the switch is on or not. Before you solder the LED on its worth trying the LED each way round so that you can make the switch turn on when your switch is in the direction you want it to be on, if it is on in the off position (if that make sense) simply turn the LED round, then solder 1 leg. (This was the LED which I purchased for this http://www.ebay.co.uk/itm/271457390406?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649  they are significantly cheaper if you can wait for them to come from China such as http://www.ebay.co.uk/itm/200972608918?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649)

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This now should work as a switch but will only be able to deliver about 500ma at about 3v (maybe 5v maximum since this is the receiver voltage), but I wanted to ensure that this would allow me to switch more power, so I also connected a relay switch so that the LED and the relay are in series on the output of the servo board. I purchased this relay www.ebay.co.uk/itm/351041618488?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649, RS and other suppliers will supply suitable relays, a non latching relay with a 3v nominal coil is required. I found that the LED connected in series created too much of a voltage drop and failed to activate the relay correctly, so I removed the LED and found that this relay is actually polarized so it works only in 1 current direction so the diode was not necessary, I then initially connected the LED in parallel to the relay as an indicator, using the Normally Open connections on the really I connected the JST plug and the load connection.

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This is now ready, a bit of packaging with some insulation tape and its all good

20140504_093432[1] I actually thought that the LED was causing a further problem, so I removed it, but it turned out that the pot position had moved which was causing the problem, but this now all works nicely giving a full 12v output on the connector I attached when the switch is activated.

 

 

There is a lot of information here that you don’t really need to make the switch from a servo, so in summary what you need to do is:

  1. Remove the controller board, pot and motor from the casing, getting rid of the gears.
  2. Cut the motor away
  3. Connect the coil of a relay switch to the motor connections (the direction determines which throw direction the switch will be on at)
  4. Use the Normally Open connections on the relay output to connect your load and power source.

 

I hope someone understands this and finds it useful.

 

Update:

The relay that I listed here is rated at 2 amps switching current, it is double pole, I only used 1 pole and using it for the rocket igniters did blow the relay contacts, so if you are using this relay then connect the 2 poles together to give a better rating, or use a different relay such as this one:

http://www.ebay.co.uk/itm/251487355244?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

rated at 10amps switching current @125v but they are larger than the other ones.

  1. Mel Jones says:

    Nice article Chris, most modern relays have a diode fitted across the coil, it`s there to prevent back emf from destroying the electronics that are driving it, but I`m sure you knew that.
    Once the position of the pot is set could it be glued with a spot of C A?
    Money saving tips are always welcome.

    • Chris says:

      Buying as cheap and as small as possible I didn’t assume the relay had a diode installed, so it was good to find the one I got did have one.
      You can use CA glue to fix the pot, the whole metal plate on the top of the pot moves with the spindle, so you need to glue the edge of this plate, or the tape I used to package the unit should also do the same effect with this extra surface area to stick to, although other servo may have different pots which may not have this exposed plate? These hexatronic servos are the only ones I’ve opened.

  2. Scott says:

    That’s brill Mr Chris, will have a go at making one of them, you’ve made it sound simple enough even I could do it, I have a couple of broken servo’s that I could strip down thanks for putting the article up 🙂

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