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!

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)

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.

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

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

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.

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.

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.

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.

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

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.

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.

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.

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.

ALL DONE! ENJOY YOUR NEW TOOL!