Tutorial: How to use a servo board as a brushed ESC or variable brightness LED switch

I often see people asking about brushed ESCs, mainly for very small micro RC cars around 1/64 scale, and also people wanting to be able to turn LED lights on and off, vary their brightness, and control other things from their transmitter. However there does not seem to be a comprehensive guide on how to use a servo board to achieve these results, which is actually a very cheap and effective method.

A few people have touched on the subject, and briefly mentioned how it can be done, but I thought I would make this guide to help people out.

Step 1: Find yourself a servo
You can use any servo that you have, the larger servos will be able to handle higher powered motors, but for all of my purposes I have found that a cheap $3 servo from eBay is perfect.

This is the one I will be using in this guide, a cheap 9g micro servo.



Step 2: Take the servo apart
It should be pretty easy to figure out how they come apart.



This one has 4 screws, but other servos will be different.



Step 3: Find the centre point of the servo
The easiest way to do this is to connect it to a receiver and turn it on. Do not worry if you cannot do this step, it just makes it a bit easier.



Step 4: Find out what resistors you need
To do this you need to have a multimeter to measure the resistance of the existing pot.

Firstly, locate where the 3 pot wires are connected to the board.



Set your multimeter to measure resistance, I set mine to 20K as most servo pots will be around 5K.



Now test the middle point with one of the outside points, I chose to test both, which you will need to do if you did not centre your servo earlier.

First side.



Second side.



The resistor value you want is either the same as the value you found with a centred servo, or if you didn't have a centred servo, add up both values and divide by 2.

Of course you can't get that exact value unless you are lucky, with mine I went for 2.61K resistors.

All servos should be around the 5K mark, so require resistors around the value of 2.5K.

Out of interest I tested to see how far the trim went on my FlySky GT3B controller, the values I got were 2.81k and 2.25k, so as you can see there is a small amount of tolerance for choosing resistor values as you can just adjust the trim on your transmitter.

You can choose two different value resistors to use and then change the trim setting, but it is best to get two of the same value.

Step 5: Remove the pot

Pot removed




Step 6: Solder the resistors on
You need to connect one leg from each resistor together and solder the join to the middle of the three points, then solder the remaining two legs to the other points, each to a different one.

I chose to use surface mount resistors, but any size can be used.

Surface mount resistors in place of the pot.



Step 7: Test the board
Notice at this point the motor is still connected, this is so you can test the board and see if the trim needs adjusting.

Connect the board to your receiver and check that the motor does not spin when the transmitter is centred, if it does then adjust the trim until it doesn't move unless you turn the wheel on your transmitter (assuming the board is connected to the channel used for steering).

Step 8: Remove the motor
If you are going to use LEDs, or anything that requires a specific voltage, make sure to measure the maximum voltage the board provides, do this by turning your transmitter wheel all the way to one side and use a multimeter set to volts to measure the voltage.

Motor removed.



Here is the voltage reading I got with the wheel turned all the way.



Step 9: Connect your electronics
Now you need to attach the electronics you want to use to where the motor was connected.

In my case I connected a JST connector so that I can change what is plugged in, my main use will be for some LEDs though.



Step 10: Complete
And there you go, one finished transmitter controlled device.

It may also be a good idea to put some heatshrink on the board before using it in your RC device.

Here is a short video with the motor still attached and the rotation limiter removed, meaning the board is acting as an ESC.

How to use a servo board as a brushed ESC (0 min 38 sec)

Here is a short video demonstrating the variable brightness on/off LEDs, I admit the brightness range is not great, mainly because I am using a random resistor for the LEDs so they do not reach their brightest level.

How to use a servo board as a variable brightness LED On/Off controller (0 min 31 sec)

If you have any questions please feel free to ask.

Using some LEDs in parallel( one "this" way, one "that" way), you can use this method to make turn indicators that blink when you turn too

Originally Posted by Bugman Jeff

Using some LEDs in parallel( one "this" way, one "that" way), you can use this method to make turn indicators that blink when you turn too

Good tip, thanks.

Just to expand, you can use a splitter and connect this to the same channel as your steering to get turn signals, but you would need flashing LEDs to make it look right.

You could also do the same for reversing lights, but realistically by the time you've got all the separate boards it's probably cheaper and easier to go for some sort of kit.

Originally Posted by x5252x

Good tip, thanks.

Just to expand, you can use a splitter and connect this to the same channel as your steering to get turn signals, but you would need flashing LEDs to make it look right.

I forgot about the blinking part of the LED blinkers

Would it be possible to have 2 of these servo boards connected to 2 receiver ports but with only one common signal wire.

Then the output wired in series, will it double the voltage?

Thanks

Originally Posted by XFM

Would it be possible to have 2 of these servo boards connected to 2 receiver ports but with only one common signal wire.

Then the output wired in series, will it double the voltage?

Thanks

No, I believe that would basically be creating a short, the same as if you connected the positive to ground on the receiver.

Please someone correct me if I'm wrong on this though.

This is a great tutorial! Thanks for the info!

question : instead of performing steps 4 , 5, and 6, would it be possible to simply center and glue the POT to a fixed position ?

Originally Posted by vinnieg

This is a great tutorial! Thanks for the info!

question : instead of performing steps 4 , 5, and 6, would it be possible to simply center and glue the POT to a fixed position ?

That's what I did. If you don't get it glued perfectly centered, you can correct it with the trims in your Tx.

when i found out about this a few years ago i was very intrigued. an old rc buddy of mine showed me how he used the servo board as an esc, he used one in a small tug style boat. i did the same thing but in an airboat application in my bathtub

ive also been using this trick for some time now but dont bother removing the pot anymore. its much simpler to leave it in and because these tend to drift with temperature and voltage makes zeroing easier. in fact i often leave the motor attached too for things like nanobot and boat applications.

sometimes i do change the value of the deadband resistor so full stop is not so critical.

will this work if you have an airplane tx. I want to use this application on a micro airplane but the voltage only goes from zero to full volts in half the stick movement. I need for the voltage to be at zero when the stick is all the way down and at full voltage when the stick is all the way up. Does any body know how to do this?

You may be able to turn down your endpoint or dual rate settings on that channel to get the voltage where you need it.

I was able to center the channel all the way to one end and then adjust my travel to 50% each way and that seems to be working well enough. Since I don't need reverse, I was trying to get rid of the ability for the servo board to be able to put out neg. voltage but could not figure it out. I just adjusted the stick trim to not be able to go as far as to get into the neg. voltage.

Originally Posted by flybyjohn

will this work if you have an airplane tx. I want to use this application on a micro airplane but the voltage only goes from zero to full volts in half the stick movement. I need for the voltage to be at zero when the stick is all the way down and at full voltage when the stick is all the way up. Does any body know how to do this?

I'd try connecting this up with the pot on, leaving the stick at the bottom, then rotating the pot so that the servo board gives out 0v, then check the max voltage by moving the stick all the way up, then adjust the end point so that when the stick is pushed all the way up, it gives out slightly below the max voltage, then increase the end point by a very small amount so it will give the max. That should provide you with what you need.

is there anywhere i might find a 2.5k ohm resistor in old electronics or what not?