servo question

OK Id like to activate a servo but not use an RC channel to do so…

could i wire up a servo to be multi position using a switch. Ie apply voltage, to move the servo, reverse the voltage to make it go the other way???


A few years ago my wife bought me an HPI Savage truck that was fitted with reverse on the gearbox. The radio system used was a Spektrum DX3S, which has 2 proportional channels and 1 ‘switching’ channel. This 3rd channel actuated the gearbox servo from full throw to full throw dependent on the position of a two way switch on the transmitter.

Obviously, this doesn’t get around your not wanting to use an additional channel but if you don’t intend to modify the servo then it’s not quite as simple as reversing the voltage. In the Spektrum/Savage system the servo control didn’t receive the nomal pulse (1500 micro secs for neutral), for 1st position the signal would be around 800 micro secs and for 2nd position it would have been around 2200 micro secs. To my way of thinking this would indicate that you’d need a simple servo control circuit which could be controlled by a multi-position switch to give the servo output you’re after.

Any hints as to what you’re trying to do? Only ask as it may have been done before…



A long time ago, they used a momentary toggle switch to operate the sail servo, but the switch was operated by a servo. It was how you could make a very high-torque sail servo.

If I understand your question, Marc, you want to activate a servo without using your radio. The signal going to your servo is a pulse, not a voltage, so the simple answer is not really. A servo bench tester does what you want to do, but it is large, proportional and perhaps more than you want to spend. A simple circuit could be designed or exists that could generate the two pulse conditions that you want, but it would require a chip and some soldering. You could ask about it over in the RCG DIY section.

Here is something that would be a start:

I don’t really understand what Marc is getting at. Do you want to operate the servo in the boat without using the transmitter/receiver?

I made my own analog servo driver. It uses the 555 timer and was not hard if you can make printed circuit boards. I soldered a 12-inch extension wire directly to the board to reach most anywhere on a boat. You can buy an analog servo tester cheaply. They are about the size of a pack of cigarettes. I assume the testers for digital servos are slightly more complex.

If you just want a switch instead of a servo, from dimension engineering will work.

hmmm Ok I divulge a little…still trying to keep it under wraps somewhat…

Id like to use the boats physics to trip a servo. boat heels one way, servo trips, boat heals another way servo trips

My though was to use a mercury switch…

and the rules state the the two channels can only control sheets and rudder… so using a sail servo or rudder servo to activate a switch to trip the third servo would, IMO be a rules violation…


I thought about using a potentiometer on the boat, with a little plumb-weight to turn the shaft and simulate moving the radio knob. If you adjusted the resistance to be at center on the servo, then when the boat heals, the plumb-weight would move accordingly and control the servo. I think that would still need a signal from a receiver, but the weight would do the rest for you

How about a gyro? Not that I ever thought about doing it

Hi Marc

I have fittled with the same thoughts. Not for any any class boat boat, so my intention was still to have radio contact with the boat, but it should work anyway.

Have a look at youtube, I have a small video of a “proof of concept” model.

Basically i took out the potentiometer from a servo, extended the wires, so I could place it firmly to the boat and attach a weight(pendulum) from it. Now the servo will rotate untill the boat is back to horizontal, or if it hits the limit switch. If the ervo is still hooked up to the transmitter, I will be able to “offset” the horizontal meaning the boat would be heeling. I also have a small very cheap gyro in the system, to make the system react on emidiate movements.

I guess an expencive gyro could also du the trick, but i think it still need the transmtter. At least it is more tricky to make it work without RC…

Enough words, have a look at the video, I think it will make sence for you then.



If you remove all the electronics from a servo, then you have an electric motor driving some gears.
Wire up the pendulum to provide the +/- movement of the motor. Add a battery.
Does that sound right?



the pendulum would do what I need it to do… I still want the servo to have its on built in limits the servo will be fixed…

so how did you wire in the pot on the pendulum to drive the servo…


It is the pot meter from inside the servo. Actually just extended the wires. But this way the servo wil continue to rotate, until the pot meter is back to its “zero”, so there would be no stopping it.

I think the easiest way to do what you want without connection to the transmitter would be to do as “Islander51” is saying, and make a switch based system.

Pendulum to trigger a switch to activate a servo(motor and gear only), and a limit switch to stop it again.

I don’t know this for sure, but i bellive it is also possible to make the servo move by increasing the resistance in one of the wires to the servo pot-meter(not the middle one). This is a bit tricky, because the position of the servo horn, deppends on the delicate resistance balance between the centerleg on the pot-meter, and the two other legs on the pot-meter. Increase the resistance on one of them, and the servo will move som degrees. It could be possible to offset the balance with som switches.

My best advise is to find yourself an old or cheap servo, take it appart, and play around with it. Then ideas will come flying through your brain.

P.S. The above is only what i found from playing around with an old servo, and therfore not excact sience… but i think i got it right :lol:


ok so I take apart a servo, remove the pot, add extension wires to the pot. add power to the servo. and dial the pot right and left to get the servo to move right and left…

the only problem is if the pendulum continues to move/gyrate then the servo arm will also move/gyrate…

I did buy one of these on ebay…I figure for 4 bucks… I could experiment with taking the switches out and replacething them with merc switches

Yes, that would properly be the simplest solution.

This is from the manual of that servo tester:

Connect the battery: the Servo Tester will go automatically into Manual mode.
To change mode, press the button: the Servo Tester will cycle through Manual, Neutral and
Automatic modes, in that order.
In Manual mode the DEC or INC buttons can be used to reduce or increase the width of the pulse
in steps of 0.01 ms. Holding one button will cause the pulse width to change continuously until the
limit of 1 ms or 2 ms is reached; at that point the LED will flash.
In Neutral mode, the width of the output signal is a fixed 1.5ms.
In Automatic mode, the width of output signal changes smoothly and continuously between 1ms
and 2ms.

They also have one with a pot control:

You could use the pot version with a plumb weight hanging from the dial to activate the servo to counter the heel.

i saw the pot version, but after I orderd the push button, when I saw anders video

eh, well see…

Hey Marc,

Having futzed around with merc switches for a project that used angle of heel for automatic control of a trim tab on a keel fin (now almost universally banned in model yachts after I debuted a mechanical version at the Worlds in Canada on the early '80’s) I should warn you that while on the bench something may work just fine, the real world is another situation entirely. Mercury switches are very sensitive and model boats travel through the water rather chaotically (even though they look to us on shore as if they are sailing smoothly). Add a bit of chop and … on-off-on-off and on and on like that as the mercury sloshes about.

Any automation that you incorporate into your boat needs to be dampened (perhaps with adjustable dampening, tunable to conditions).

Also, if your automation proves to be a decisive advantage don’t be surprised if it is banned, as my trim tab was with the “no movable keel rule”. Real breakthroughs are excluded for the good of the class as a whole, or so I was told back then. Model boats are expensive enough, if a game changer comes along and forces everyone to upgrade to stay competitive then it can be a death nell for the class. Such has happened to the M in the US. The Skalpel, with its advanced (and expensive) rigging system was so successful here that a majority of sailors quit and moved on to other classes. It is understandable, not many guys have an autoclave to cook their masts in, or government backing to set up shop.

Anyway, I wish you luck with your project and await the unveiling thereof.