Do the Arduino Nano 5V and Mini Pro 5V have different rated and maximum current specs for the processor and the IO pins? I haven't had much luck finding the specs. My circuit pulls the occasional 200 mA for a split second and the servo (the component with the heaviest current draw) doesn't move when the circuit is connected to the Pro Mini.
Thanks.
Both (can) have the same processor, but I think fewer pins are brought out on the ProMini.
A big difference is the supply.
The Nano has a 5volt/~500mA regulator, and a ProMini has a 5volt/150mA regulator.
You should ofcourse never power a servo from the MCU supply.
Post a picture/diagram showing how you power the setup.
Leo..
I have been powering 1 small servo, the common blue 9g type from the MCU supply. Why do you say never?
So the pro mini is only good for 150 mA as a maximum? Maybe that's why the servo doesn't move? I did however remove the regulator from the pro mini based on the advice from a few websites on how to greatly reduce the current usage in sleep mode.
I have attached the circuit diagram.
Use a different power supply for the motors that the one you use for the Arduino.
Looks like I'm being told. Do you mean for all servos and all Arduinos? I just have 1 9g servo, 2 LEDs and 1 PIR sensor. The circuit works for the nano but not for the pro mini. I'm wondering why not because they both use the ATMEGA328 chip. Note that I removed the regulator on the pro mini. I thought the nano was the same thing as the pro mini except for the USB port, making the nano able to pass less current from external components because the USB port requires some current. I was hoping to use the mini so that the sleep mode uses less current due to the lack of USB port.
So you just forgot to mention that you had the regulator removed.
That makes part of my answer invalid.
Your setup can never work the way you have drawn it.
With removed regulator, the RAW pin is not attached to anything anymore.
Unless you forgot to mention something else.
If you are going to power both MCU and servo from a single set of batteries, then use good power supply decoupling.
I suggest 1000uF or more.
Leo..
bodkin77:
Looks like I'm being told. Do you mean for all servos and all Arduinos? I just have 1 9g servo, 2 LEDs and 1 PIR sensor. The circuit works for the nano but not for the pro mini. I'm wondering why not because they both use the ATMEGA328 chip. Note that I removed the regulator on the pro mini. I thought the nano was the same thing as the pro mini except for the USB port, making the nano able to pass less current from external components because the USB port requires some current. I was hoping to use the mini so that the sleep mode uses less current due to the lack of USB port.
When you modify a board, you will (unpredictably) change the operation of that board, so bear that in mind when looking for the problem. Also, what's with the two power switches? If all eight of your double A's deplete at the same rate, it will make changing them easier.
If you use a second set of batteries to supply power to the motors (the blue wires in your diagram) and connect the grounds (black lines) between both sets of batteries, your motors should work as expected with most (properly powered) microcontroller boards
Wawa, that is unless the regulator was bypassed, in which case 6 v was applied to the 5 v pin of the processor. I believe it is rated at 5.5 v max.
AFAIK absolute max for that MCU is 6volt.
There is a 1N4001 diode in series with the battery.
That diode drops about 0.7volt.
Not sure why the regulator (~100uA idle current) was removed.
A servo uses AFAIK way more idle current than that.
Leo..
You are saying that separating power supplies might be the way to get the total current usage low enough to make the servo work? I would prefer to use the same power supplies and I am wondering why it is working with the nano, but not the pro mini. I thought the only difference was the USB jack. Sorry to repeat that, but that is my main question I guess.
I did forget to mention the removed regulator. 
Another thing I forgot was in the drawing - to add the decoupling capacitor to the servo. This was required to make the servo non-jittery (with the nano). In fact a 1000uF capacitor was not enough. The next size up I had was a 2400uF, which smoothed out the servo and PIR operation nicely.
According to the site (http://www.home-automation-community.com/arduino-low-power-how-to-run-atmega328p-for-a-year-on-coin-cell-battery/), removing the regulator will save only 18 uA in sleep mode, so yes maybe I shouldn't remove it when my next minis arrive. However, the range of processor operating voltages is 1.8-5.5V with an absolute maximum of 6V, which means I should be able to power if off either the battery pack or the DC power pack because both are 5.3V after the diode.
Chris please note that I didn't have 2 battery packs in the diagrams, it is 1 battery pack and a DC pack.
I plan to rebuild the circuit as per either diagram A or B in post number 5 at:
http://forum.arduino.cc/index.php?topic=422160.0
However, I plan to incorporate a 3 pin DC jack in either a battery charging or battery elimination setup. Thing is, only the battery elimination part I understand so far. I would like to understand how to charge a LiFePO4 battery this way. Float charging a single or double cell up to 90%. I learned a couple of things from the following forum post about float charging a LiFePO4 battery as being similar to charging a capacitor and being able to leave the charger connected to the circuit.
http://forum.arduino.cc/index.php?topic=425890.0
http://electronics.stackexchange.com/questions/90529/what-to-do-with-third-contact-in-dc-barrel-plug-with-only-two-input-contacts
https://www.sparkfun.com/products/10785 (discussed in comments section)