HS 311 Potentiometer Wires

[jforsmo]

Hello,

I'm new to these forums and I'm building the $50 robot with my son. I need to hack the HS 311 servos and I'm using the method of replacing the pot with two resistors wired in series. Can someone tell me which of the pots' wires is ground (yellow, green, red) please. There are photos online that show the pot standing on legs attached to the circuit board (the center of which is supposed to be ground), but that's not how our servos are. The pots on ours use copper wire and the center wire is yellow (assuming it is the ground).

Any help would be greatly appreciated.

TIA

James

[airman00]

http://www.societyofrobots.com/member_tutorials/node/82
the middle wire is always the common point , so its ground

 did you make sure to get precision resistors???

[jforsmo]

I just got a pair of 2k resistors from Radio Shack for each servo.

i hope this thing works!

[jforsmo]

Sorry, 2.2K

[airman00]

Sorry, 2.2K

but are they precision resistors? what color is the last band on the resistor?
 if you don't know how to read a resistor  -http://www.bcdxc.org/resistor_color_codes.htm

you want to have 2% tolerance , there are 1% tolerance also called metqal oxide resistors , those are the best to get.

Use google if you have any questions , and if your search ocmes up empty, jsut post here

[Webbot]

Just to flesh out what Airman has said - and why its important. If you have 2 resistors Ra and Rb in series then the voltage at the mid point is:
Vcc * (Ra/ (Ra+Rb))

So if both resistors are EXACTLY 2.2k then:
Vcc * (2.2 / 4.4) ie Vcc * 0.5 - which is what you want

However: a standard resistor has a tolerance of say +- 10% so your 2.2k resistors may, in the worse case, have values between 2.42k and 1.98k. So you could have:-
Vcc * (2.42/(2.42 +1.98)) ie Vcc * 0.55 - which is out by 10%

So if you are buying a resistor to use with an LED then 10% tolerance is ok - as it may make the LED shine slightly brighter or darker - but this won't be visible to the eye. But if you want to do something more exact, as in your case of dividing by 2, then you need to make sure that the 2 resistors have as an exact a value as possible - ie they have have a low % tolerance.

The main difference is price - the more exact the tolerance - the more expensive the item.