I am using one of these to boost the voltage from two AAA 1.2V 900mah batteries to power an arduino and two servo motors. at first it wouldn’t turn the board on with the batteries. Then I read that you need a 10Kohm resistor. I used that and multimetered 9V output. If I take off the resistor the output doesn’t reach 9V so I left it on. Now the board seem to turn on properly but the motors wont turn when running the program I have. It works fine when using the USB cable or an 9V battery plugged in with the barrel jack. Should I go down to a 1k resistor between output voltage and ground?
I am sorry you are having trouble using that regulator. Once you set the output voltage on that regulator, you do not need to keep the resistor you used as a load in your application. (Whatever you are supplying power to will be your load.) Can you tell me more about your setup? Which Arduino are you using? Can you post pictures that clearly show your connections? Also, 9V seems pretty high for a servo. Can you provide a datasheet or link to a product page for the servos you are using? I am also concerned about how much current your servos might be drawing, since I do not expect the #791 to be able to do more than a couple hundred milliAmps of current in your setup.
I am only using the boost regulator to supply an arduino UNO. I am using the boards 5.5v supply to power the motors which are futaba S3004 motors modified to be continuous rotation.
I made a quick PDF drawing of my setup
my arduino reg setup.pdf (30.0 KB)
That regulator is appropriate for powering your Arduino UNO, but not appropriate for powering those servos. When you supply power from that regulator to VIN on the Arduino and supply power to your servos from the 5V pin on the Arduino, you are still using the regulator to supply power to your servos (it just gets regulated again from the VIN voltage to 5V by the Arduino’s onboard regulator). Unfortunately, we do not have a step-up regulator that can handle much more current from voltages as low as two AAA batteries connected in series, especially if you boosting up to 9V.
Standard size servos like the one you are using can generally draw about 1A at stall, and it is good practice to pick an appropriate supply that can handle that in case the servo stalls. For a system like yours, I recommend using a 4.8V or 6.0V battery pack to supply power directly to your servos, and you can use the same #791 regulator to supply power to VIN on your Arduino.
By the way, powering your servos from the regulated 5V supply on your Arduino is not a good practice. While it has been working for you in your tests, if either of your servos draw more current (for example, if either of them is stalled), it could permanently damage your UNO.
Can I get another regulator and have one power the UNO and the other power the motor?
In general, 2 AAA batteries is probably not an appropriate source for powering devices like multiple motors or servos. I did not mention it specifically in my last post, but if you were to get a replacement battery pack for your servos, you should consider a 4.8V or 6V rechargeable pack that uses AA batteries. The primary reason is because AA batteries can provide much more current than AAA batteries.
If battery packs using AA batteries are too large for your application, you might consider getting a LiPo battery. Those batteries are more energy-dense than NiMH batteries, can provide large amounts of current, and often have small form factors. If that is interesting to you, one way you could power your system is to use something like a 2S LiPo (which supplies 7.4V nominally and 8.4V fully charged) for your Arduino, and a step-down regulator like this one to supply 5V to both of your servos.