360 servo mod

Before getting started on the turret mechanism, I dusted off the modified servo and gave it a test-run. Uh-oh! it was broken! One of the solder points inside broke loose, so I had to go back in and fix it. Perfect time to show how to modify a servo to move 360 degrees, eh?

OK, so a little walkthrough: The potentiometer (black circle far right) is getting disconnected, but first I made note of where the wires went. In this pot, the yellow wire was the 'center', or 'neutral' wire, and was the common point between the resistances both clockwise (red) and counterclockwise (green).. Representing 'Zero'. Servoes match the radio transmitters' thumstick by trying to constantly 'zero out' with the pot reading from that transmitter channel.

This little explanation is important for really only one reason: you need to know which solder point on the circuit board is the 'common'. In this particular servo: the common (yellow) wire went to the point of the <

Heres a better view of the circuit board, and how the resistors were situated. This was a particularly roomy servo, yet was small- making it perfect for this sort of modification. There was room to cram the pots wires back in the cavity, and STILL have room for the resistors. Side note: I cant remove the pot from the servo, since its shaft, which is geared into the control arm gear, also holds another gear in place on its shaft, so it needed to stay.

The resistors are obviously the same rating, as the stripes match, but the important stripe for this application is the far right one: its gold (cant really tell in this pic, but it is). This is the Tolerance stripe, meaning that this resistor is within 5% of its designated rating. This is better than normal, which is 10%. All resistors have this 'slop', and I went through several with an ohmmeter to find two that were as close as possible. (because this balanced Resistor-bridge is what represents 'zero', the same as the pot in center position, and it aint zero if they are lopsided!) But thats all thats needed to modify a servo to spin coninuously one way or the other on command.

I guess I should explain WHY it works, since I went though the trouble of showing HOW:

The pot in the servo is linked to the poition of the servo control arm. as the Arm swings around it turns the pot. The servo is set up to stop when the pot reads 'Zero'. If the pot finds itself lessthan [<] 'Zero', its setup to drin one direction intil the pot reaches 'Zero'. same way the other direction.

Now, 'Zero' is in quotes, because its actually not Zero Zero, but what the radio transmitter channel is commanding, and its command varies depending on how far you move the thumbstick on your transmitter one way or the other. In this way, the servo matches the thumbsticks position pretty much instantaneously, checking probably thousands of times a second.

Thats all well and good on how the servo channel works, but how does adding the resistors make it work? Well, by cutting out that feedback number from the pot, and replacing it with a matched set of resistance, we are 'faking' that its always at 'Zero'. You would think that it would make the servo never move again, but thats not quite the case: The "am I [<] than Zero??" part of the microchip is still working, and listening to the radio channel. So when you move the thumbstick left, the microchip goes "OH! Im [<] from where I need to be! that means drive left.. not there yet? then keep going... keep going" and will continue spinning left until it believes it has finaly reaches Zero - which is actually when you released the thumstick to center. At that point the servo stops and stays there where it wound up.

Presto! a 360 Modified Servo! Hope I explained it clearly enough.

Back to the turret mechanism, now that I had a functional Servo again, I mounted it to the turret piece to give it a little test, and also to eyeball if I got he center hole right (looking for a wobble), and shot another little movie:

Looks good! Hehe Side note, hear the stereo in the background that suddenly goes quiet when the turret starts moving? Thats because my Futaba transmitter flooded out all other radio signals in the immediate area. And the stereo was receiving only "LEFT ON CHANNEL 4! LEFT DAMMIT!!!" to its antenna, and was unable to pick up any other signals... hehe thats what old CB-ers call a STOMMMMP! Even though my transmitter was on 72 mhz (khz?), it still flooded all weaker signals out of existance.

hmm maybe I should'nt hold it so close to my lap when filming...