Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of Make: Install the Trim Potentiometer
Insert the trim pot as shown then solder everything in place.

Step 10: Make: Solder in the Potentiometer

Picture of 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

Picture of 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

Picture of 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

Picture of 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.

divesh_malhotra made it!4 months ago
I've made the circuit... and checked their connections twice... but still my servo is not working even after rotating potentiometers.... please help me..!!
tisaconundrum made it!1 year ago

Sorta made it :D LOL


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.

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macman8084 years ago
what voltage is that battery?
Most likely 4.8 volts.
BOOJAN5 years ago
nooo...you 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.
W9GFO BOOJAN5 years ago
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
BOOJAN W9GFO5 years ago
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
Unit042 BOOJAN5 years ago
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.....
Or ypu could do what I do. Randomly grab something, solder it in, repeat.
yup...my 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...
Sandisk1duo5 years ago
how about a servo tester for 4-5 wire servos?
Need video

edfarina5 years ago
I modified the circuit to use batteries LiPo 2S or 3S, including a 4910 regulator taken fron an ESC burned.
W9GFO edfarina5 years ago
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
edfarina W9GFO5 years ago
 Exactly, Rich, use small servos that work with no more than 6 volts.
In 4910, I guarantee 5 volts to the servos.
BlueFusion5 years ago
Would anybody be interested if I made a board available, either as gerbers, or to purchase (at cost price naturally)?
paulm5 years ago
 Ok, wait a second here:

if they don't provide the "Custom PCB" information so we can etch one ourselves, this is just an advertizement

and its featured?

what the hell
W9GFO paulm5 years ago
You can use the schematic and lay out your own PCB if you want.

Rich H
kelseymh5 years ago
While I understand that the PCB layout/silkscreen is GG's intellectual property, would you consider making the circuit schematic available? That would allow hardcore electronics types to do their own (presumably different) board layout, or breadboard it, or whatever.
T2Pogi kelseymh5 years ago
Kelseymh, check this out. this is the basic 555 design that will work the same way. Cut in a potentiometer between R2 and the capacitor C1 to make the timing variable.

Thanks for this link! Another awesome schematic/explanation site added to my bookmarks. :)
Gadget Gangster (author)  T2Pogi5 years ago
That's the basic circuit - there are a few little things added there (like the polarity LED), but nothing crazy.
Gadget Gangster (author)  kelseymh5 years ago
It's actually owned by W9GFO. I'm not sure if he has anything in a share-able format, but I'll ask him. It's a really cool circuit and a cool use off a 555 timer!
danlab5 years ago
Does radio shack stock 555 chips? My normal electronics shop doesn't seem to have them.
danlab danlab5 years ago
Or should I just order them online?
Gadget Gangster (author)  danlab5 years ago
Really? I'm surprised that your electronics shop doesn't have 555's, they're really common. Try asking for a TLC555.
Ok i'll have to try that, is it an online only thing from radio shack?
danlab danlab5 years ago
So I checked the other radio shack (the one that is actually closer to my house) and they had several of them. Thanks for the input.
danlab danlab5 years ago
I asked one of the managers at the other electronics store and they hadsomeone come in and buy out their whole stock.
crapflinger5 years ago
a little blurb about how it does what it does for us Luddites would be neat as well (surely couldn't be hard)

i have NO idea why i never thought of the painter's tape idea when soldering junk onto something like this (again...me=electronics N00B10)...that's such a simple idea
Gadget Gangster (author)  crapflinger5 years ago
That tape is a super good idea. Servos are the little motors in R/C models and robotics projects (and a bunch of other things). You can use the Servo tester to make sure your servo is working correctly. By turning the knob, you can turn the servo.
I think "crapflinger" is asking for a beginners-level theory-of-operation. The pot changes the timing which drives the servo yada yada yada.
indeed, i know what servos etc are...i've done a lot of RC in my time...just wondering what the 555 is for and how the pots modify what it does etc..
the 555 IC is wired as an oscillator. the pot determines the pulse width of the output (pin 3) any where from 1.5 ms to about 2.5mseconds (?) ; this pulse widh determines which way the servo motor will turn (left or right) and how much it will turn. the potentiometer works w the timing capacitor and the 555 functions as an astable multivibrator.
Gadget Gangster (author)  crapflinger5 years ago
Sorry! I wasn't sure. The 555 timer generates the pulses to drive the servos. Using the potentiometer, you're varying the pulse width and changing the position of the servo. Check out the wikipedia article on 555 timers, especially the part on astable mode.

The 555 timer is one of the most classic, ubiquitous integrated circuits - the wikipedia article is a pretty good read.
AHA! so basically you could (in theory) use this to drive any PWM driven device assuming that the board/electronics can withstand the supplied voltage?
PKM5 years ago
Does this mean all servos use the same "protocol" (or what I would consider a protoco, being a software guy) to determine position? It seems odd that different servos with different angles of movement etc. would use the same timings, but on the other hand it probably makes things a whole lot simpler as well.
Gadget Gangster (author)  PKM5 years ago
Yeah, pretty much - the typical RC servos all use the same protocol. The Seattle robotics society has a good writeup here, but basically, the servo waits for a pulse every 20 ms. The length of that pulse tells the servo what angle it should be at. Check out the little graphic on the bottom of their page.
brunoip5 years ago
the circuit?