It's pretty darn easy (& cheap, too) to build your own. It's also a great way to get into building some of your own electronic equipment. JM
My understanding was that you needed to provide a pulse stream (singal aprox every 20ms), not a steady level to the servo.
Just type "servo tester " into ebay. That's how I found mine, it was cheap, works well and has made ship setup/diagnosis a breeze.
Interesting.... from google: How do you communicate the angle at which the servo should turn? The control wire is used to communicate the angle. The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse Coded Modulation. The servo expects to see a pulse every 20 milliseconds (.02 seconds). The length of the pulse will determine how far the motor turns. A 1.5 millisecond pulse, for example, will make the motor turn to the 90 degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, then the motor will turn the shaft to closer to 0 degress. If the pulse is longer than 1.5ms, the shaft turns closer to 180 degress. It still looks like Pulse Width Modulation to me, which is how digital electronics make analog signals. I'd still be willing to bet a good old fashioned servo (not the digital ones) would respond to a analog voltage change.
You'll lose that bet every time, unless you define "respond" to include a pretty wide range of conditions that most people would consider errors. Reliably setting to some particular position isn't going to happen with an analog voltage level, I can guarantee you. You need to generate a repeating series of pretty short pulses, at regular intervals as noted above. It's not PWM, which is duty-cycle based (duty cycle is percentage of a timespan that the signal level is high/low; not necessarily, or even desirable, that individual pulses be even & regular. Example: 50% duty cycle means that, over some period of time, the signal will be high 50% of the time. However, if the signal is a perfect 50/50 square wave, you're going to get "singing" at the frequency of the pulses, which isn't very desirable. It's typical to "randomize" the widths so as not to produce any particular frequency. A 100% duty cycle (never actually produced by any ESC) would be DC. Proportional Pulse Modulation (PPM) is what hobby servo signalling is called, & the pulses need to be constant width to hold any position. By contrast with PWM, PPM is very regular & the "duty cycle" would be quite low (2.0 msec being the typical max. pulse width, represents a 10% duty cycle, effectively wasting almost 90% of your "bandwidth" to "communicate" speed to your motors). You may consider an ESC to be a "PPM-to-PWM Converter". For some pretty easy, basic servo testers include "555" in your search terms. ntxbg.org has a tech article I wrote several years ago, about how to build a dual firing board for use with solenoid valves, back before such things were available commercially, cheaply. It's hardly worth the trouble to build something like that anymore, except as a learning exercise. JM
