Simple Servo Tester




A little bigger than a postage stamp, the Simple Servo Tester lets you control two digital or analog servos without using a transmitter or receiver, just plug in your battery pack to start testing.

Use it to check your servos before installing them into your models or to center your servos when setting up linkages. The Simple Servo Tester can also be tuned to precisely center your servos - Some manufactures consider 1.520 milliseconds to be center while others use 1.500 milliseconds.

Use it anytime you want to operate a servo but don't want to get out your RC equipment!

This project was designed by W9GFO. You can get the kit from Gadget Gangster and download a PDF version of this howto.

Warm up your soldering iron and get started!

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Step 1: Make: Parts List

Check to make sure you have the following parts:

Parts list

  • Potentiometer Knob
  • Green LED
  • NPN Transistor
  • Right angle pin header (9 pins)
  • 555 Timer Chip
  • 20k Right Angle Potentiometer
  • Trim pot
  • 2x Capacitors (0.1 uF)
  • Rectifier
  • Custom PCB
  • 220k ohm resistor (Red-Red-Yellow)
  • 3x 10k ohm resistor (Brown-Black-Orange)

Step 2: Make: Potentiometer Knob

Let's start easy, just press the knob onto the Potentiometer. Note that the the potentiometer shaft is shaped like a 'D', so the knob won't slip when you turn it.

Step 3: Make: 555 Timer

Insert the 555 timer chip. Be sure that the notch faces to the right as pictured here.

Step 4: Make: Prepping the Caps & Resistors

Bend the leads by holding onto the component with one hand and fold the leads together with your fingers.

Step 5: Make: Adding the Caps & Resistors

Insert the capacitors at C1 and C2, they are not polarized so it doesn't matter which way they go in. Insert the four resistors.

The 220k ohm (red-red-yellow) goes on the right. The other three 10K ohm go in the other spots. These also are not polarized - but I like to put them in with the gold bands at the bottom. It makes no difference at all - just personal preference.

Step 6: Make: Start Soldering

I like to hold all the components in place with some tape, then flip over the board and start soldering.

After soldering, snip the leads.

Step 7: Make: Install the Right Angle Pin Headers

Snip apart the pin headers so that you have three of them with three pins each.

Solder just the center pin of each header. Then pick up the board and reheat the center pin while applying pressure from the other side to snap it in place. This is an easy way to insure that the headers are flush and straight.

Don't forget to solder the rest of the pins when you are satisfied with the alignment.

Step 8: Make: Install the NPN Transistor and Diodes

Spread apart the leads on the transistor and insert it so that the flat side faces the 555 chip.

Insert the rectifier diode as shown with the band facing to the right.

Place the green LED so that the shorter leg goes in the lower, square hole.

Step 9: Make: Install the Trim Potentiometer

Insert the trim pot as shown then solder everything in place.

Step 10: Make: Solder in the Potentiometer

Place the potentiometer in position and solder just one pin at first - same as with the pin headers - so that you can adjust it to be straight and flush with the board before soldering it in for good.

Step 11: Operation: Hook Up Power

Always check for a green light before plugging in a servo.

The Simple Servo Tester has reverse polarity protection for itself but it will not protect an attached servo if you manage to hook up the power backwards. The green light will indicate that the polarity is correct.

Step 12: Operation: Hook Up Your Servos

Plug in your servos, the polarity is marked on the board. The Signal line is usually White, Yellow or Orange depending on which brand of servo you are using.

Test for proper operation. If there is erratic movement, or no movement at all it is most likely because of a bad solder joint or bridge. Unplug the battery and servos and inspect all joints. Re-solder any connections that looks suspicious.

Step 13: Operation: Adjust the Center Setting

Center the knob by lining it up with the line printed on the back of the board

Using a small screwdriver, adjust the trim pot until your servo is centered. I have found that 1/8 to 1/4 turn clockwise is needed to center the servo.


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45 Discussions


Question 8 months ago on Step 13

On the the circuit diagram you have a horizontal line below the circuit what does this represent? You also have a line at the top of the circuit, you have connected the emitter of the NPN transistor and two 100nF capacitors, the line carries on to the right is there anything else to be connected to this line? or is it just connected to the emitter and the two 100nF capacitors.
l loo


Question 8 months ago

Does anyone have a circuit diagram for a Servo Amplifier, I have a Navio PCB and it is being driven by a Raspberry Pi 3, the PWM signal from the Navio PCB to the Servo is 3 volts Peak to Peak at 1.5 milli seconds, the signal from the Navio PCB does not drive the Servo unless I move it manually, then when I run the the software that generates 1.5 milli second PWM signal it works OK. I now believe it needs an amplifier. I would be grateful for any help?


2 years ago

Interested in doing a project like this. But I have a few variables that need
to be considered. First I will be working with a 12VDC input voltage.
Next I will need it to operate three servos, probably 6vdc hi toque. Plus I will need to be able to set
the min max settings in each servo interdependently and may need to control the servo speed. I would like all
three serve to be operated by one single throw switch. finally the
controller needs to be as small as possible.

1 reply

Reply 2 years ago

Get an Attiny 85 development board (Digispark) and write a code to suite your application. Use proper buffers at the output of signals.


3 years ago on Introduction


An capacitor 100 nF to take HF spikes and an electrolytic capacitor 100uf (470uf) to take the fluctuations, both mounted from + to - will give a more smooth and stable movement, especialy by hi torque servo with high power consumption.


The construction of the board itself is very confusing, I don't understand fully how it's being wired. But here's my attempt at the GG PCB board. This is what I was able to put together. I sort of feel like it shouldn't be this complicated.

Untitled picture.png

8 years ago on Introduction

Here is an updated schematic. The old one did not show that pins 2 and 6 were connected.

Thanks to Dustin for pointing out the omission.


9 years ago on Step 3 are making a huge mistake. First resistors, jumpers and other small components, after that capacitators, diodes, IC's and finally potentiometers goes last. If you place the IC first you are risking to burn it while you are soldering other components.

5 replies

Reply 9 years ago on Step 3

Huge mistake?  I think that is a slight exaggeration.

I'll agree that it is good practice to save the chips until the end but for this board there are so few components that it really makes no difference. I also usually like to start with the lowest height components first, but again, this board is small enough that it is easier to just start in the middle and work outwards.

However, for anyone concerned about burning up the 555 chip, go ahead and put it in last, there is no reason why that wouldn't work just as well.

Rich H


Reply 9 years ago on Step 3

i exaggerate a little bit with that huge mistake, i was sleepy then and i've didnt know what i was writing, and i agree with what you are saying, first place the lowest height components, and then go bigger and bigger


Reply 9 years ago on Introduction

This is a nice project, but the schematic is quite confusing, perhaps you could add another schematic where the components are not all over the place? I mean, I don't intend to insult your efforts, it's just hard to read what with all of the 90 degree angle wire web....

Anyway, I just solder on an IC socket first, then transistors, because they have pins so close together. It's hard to solder around a tone of resistors, capacitors, etc, as my soldering iron tip is not quite what it sould be. (it hits other stuff because of its thickness, very annoying)

Just a thought.....


Reply 9 years ago on Introduction classmates are doing that...and they usually end up with burned ICs, a big smoke of capacitator, if it is a smaller value it will end up with huge BANG, burned transistors...etc...


9 years ago on Introduction

I modified the circuit to use batteries LiPo 2S or 3S, including a 4910 regulator taken fron an ESC burned.

2 replies

Reply 9 years ago on Introduction

The 555 chip used in the kit is rated for up to 16 volts. I have used it with a 2S LiPo but have not tested it with a 3s LiPo.  The thing to watch out for is the voltage going to your servos. Very few servos are designed to work on more than 6 volts.

In other words, the tester itself wouldn't need the regulator but the servos might. If you power the servo tester with a battery that is a higher voltage than your servos are rated for, you run the risk of damaging your servos.

Rich H


Reply 9 years ago on Introduction

 Exactly, Rich, use small servos that work with no more than 6 volts.
In 4910, I guarantee 5 volts to the servos.