As I made the control box and most of its wiring I started experimenting with it on the the desk with 1kW lights connected as the load instead of the motor. I found that I can control the lights with the throttle quite ok but sometimes they go off and sometimes they don't when I release the throttle. I thought that it might be Kelly controller glitches as I didn't trust it.
But then I looked at the voltage of throttle output signal through oscilloscope. And I didn't like what I saw - at released throttle was sometimes producing rippled voltage of sawtooth shape oscillating between 0.9 and 1.2 V at around 30Hz frequency. Sometimes it was stable at 0.9V. I suspected that 5V supply to the pedal could be noisy causing hall chip to generate this ripple but when analyzed it with oscilloscope I found it pretty normal 5V power supply noise ripple of only around 0.1mV which should not be an issue at all. Anyway I added extra 0.1uF capacitor to measured the 5V supply voltage which didn't change anything - I was still seeing the nasty ripple on released throttle. In addition to that the movement range from low throttle to the moment when the output voltage started rising was quite big - around 6 degrees out of ~35 degree range. I also noticed that full open throttle was giving only around 3.8V. This is certainly not even close to 5V stated by Kelly.
After finding that I contemplated on two options: send the throttle back to Kelly for repair or repair it myself. First option looks logical when you are dealing with respectable company in your country which will provide good customer support to resolve the problem. With Kelly it is different story - I've read many posts about terrible support of Kelly in the forums and it could be likely that I would spend additional money and time for shipments of repair/replacement with likely the same end result - throttle not working in full range. It would also be little use trying to claim the money back as Kelly is in China where business ethics in some companies could be close to jungle and their government does nothing to protect foreign customers. So I decided to look what's inside and correct the throttle box myself.
I opened the box using Dremel-style grinder cutting through black potty on the bottom to get to the bottom cover. Finally I opened the bottom cover. I found crudely hand-made marginal quality mechanics that is typical for Chinese products with magnet rotating against hall sensor. The hall sensor is AH49E which is produced by several manufacturers with similar characteristics. Datasheet could be found on http://www.bcdsemi.com/upload/datasheet/AH49E%20P1.1%2020080619.pdf. Datasheet shows that this device has output operating voltages range from around 0.9V to 4.2V so it can never output the voltages close to 0-5V range. This confirms that trying to send the throttle for repair would be waste of time and money as they would likely be replacing it with another AH49E. Replacement could possibly fix auto-generation sawtooth ripple on the output but not the total range.
I understand that the range of 0.9 to 3.8V could also work by adjusting the throttle lower and upper dead limits but such solution was not appealing to me at all.
So I replaced the AH49E with Melexis MLX90215 programmable hall effect sensor. I calibrated the sensor's gain and offset by programming it's parameters and I got stable voltage in 0.2V-4.8V range for full throttle movement range from moment when throttle switch clicks from idle till the throttle is full open. This is what I would expect from product which states that it's range is 0-5V
Conclusion: Kelly KP Series 0-5V Throttle can only give you voltage ranges from 0.9V to 4.2V in best case if you are lucky getting the one which was somehow adjusted to give these voltages range over full throttle movement. But in reality I would expect the range of usable movement and voltages to be even worse. In addition I am not sure self-generation of ripple would not be seen on other units as well. If I was a manufacturer I would not dare to put such specifications for cheating customers. I wish I read something like that before buying anything from Kelly.