does anyone know how to modify a servomotor to make it turn 360 degrees.. i searched some sites but i cnt seem to understand how to repalace the potentiometer with the resistors? can snyone help me with the pwm circuit
If you mean continuous rotation, you can't adapt a potentiometer position servo to do that, it requires a different type of control. If you mean extending it's range to 360 degrees or more, there are a couple of ways you can do that. The easiest is to substitute a multi-turn pot for the one that is there.
Actually many RC servos can be moidifed for continuous rotation, but not all can. I have several here at home that I did myself and put in different robots. They are popular in a lot of the mini-sumo robots, for example.
The basic method is to remove the limit stops and disengage the potentiometer from the output shaft. Many servos have a snall plastic clip that connects the output shaft to the potentimeter. Now you use the pot to center or stop the servo output shaft from rotating when you send a 1.5ms pulse train to it. Then 1ms causes the servo to rotate one way continuously and 2ms it rotates the opposite way continuously.
Some people remove the stops and bypass the pot using a couple of resistors. I have found that a small micro pot works better and gives you the ability to center or stop the servo much more easily. Otherwise you have to change your 1.5ms pulse width slightly to compensate for it. measure the pot's resistance and either use two resistors that add up to the pot's resistance or use a equivalent miini or micro pot to do the same thing.
Here's a super-easy mod. for Futaba S3004/Tower T-53 - the "standard" servos that you'll find included with most R/C kits:
Open the case & remove the black output gear.
Use a utility knife to cut the stop off of the gear. This allows for physical 360-degree rotation.
on the underside of the output gear, drill out the "X". I don't know the correct drill size off-hand - you'll have to figure that out. Don't drill all the way through the gear, only enough that the gear won't be able to engage the potentiometer shaft.
Before reassembling the servo, plug it into your radio, turn it on, & adjust the potentiometer so that the servo stops when the control is centered.