When buying a resistor do you have to have it exactly what the resister number is or just close? The resistence on my key was like 523ohms and the closests resistor we could find was 580ohms and it did not work. Any suggestions on where to find a closer resistor
TIA

 

There are 2 solutions:

1) Buy a variable resistor

2) You can put 2 or more resistors in series (one after the other). Add the values of each until you get the value you need. For example a 470+47=517


This is what i mean by in series:

----(~~~)--------(~~~)----

 

Luke's suggestion (as usual) is a good one, and you should be aware of another, slightly more mathematically challenging option. That is to parallel two resistors to get the desired value.

R1
|-------^^^^^^-----|
|-------^^^^^^-----|
R2

Formula for the resulting resistance is:

R1*R2
-----------
R1+R2

Thus, if R1 is 1000 ohms and R2 is 2000 ohms, we get

2000000/3000 = 667 ohms

Note that you can solve for the second resistor's value if you want to use a little algebra :-).

Paralleling the two resistors of the same value is a special case of the above where the resulting value is half the value of the one of the resistors. Example two 4ohm speakers in parallel gets you 2ohms.

Bet this was more than you needed to know :-).

Curious why you are needing so precise a value? VATS?

Also be aware of resistor tolerances and wattage requirements when selecting.

 

Radio Shack stocks (1/4 watt, 5%) 470 ohm resistors (271-1317) and 100 ohm resistors (271-1311).

Put two 100 ohm resistors in parallel (50 ohms), and then put those in series with the 470 ohm resistor. That will put the resistance near 520 ohms +/- 5%.

Also, for multiple resistors in parallel:

R total = 1 / (1/R1 + 1/R2 + .... + 1/Rn)

 

Follow the solutions above. Just so you know, you are alowed a variation +/- 5% of the resistor on the key. So you have to get it pretty much dead on.