Voltage for Servo


I got a starter kit from OddWires. It comes with a basic Servo which I have already tested/operated using my arduino uno connected to the USB port. I connected the motor to the 5v.

I ran into a couple of doubts:

  1. I wish to connect and control an LED via a bluetooth module which also came in the kit (HC-05 RS232) and for that I need to disconnect the USB cable from the Arduino and have it run off the included battery pack which is for a 9V battery.

A - What does this mean in terms of the voltage coming in, since the board usually gets up to 5V from the USB cable but now its getting up to 9V from the battery pack?
B - This wont ruin my board?
C - How come?
D - How about the LED? I understand LEDs must be protected by resistors which normally is a 220ohm resistor. From what little I know about electronics, this means that whatever voltage (or current) is coming into the LED, this resistor will provide 220ohms of resistance, which will "sort of" reduce the voltage down from 5V to some other value, say 3 or 4V. But if Im putting in 9V now, will a 220 ohm resistor fail to protect my LED?

  1. I also have a servo motor which I want to control via bluetooth (once I get my BT issues sorted out). Im not sure yet what voltage is my servo, but I understand there are different types. Could my servo burn out from switching it from my 5V USB power source to a 9V battery pack? Or worst, what about my BT module?

Please enlighten me :slight_smile:

There’s a regulator on the Arduino, so putting in a voltage from 6-20 (7-12 recommended) means it still gets only 5V internally. The i/o pins supply 5V as a result of that, so if your LED was safe before, it’s still safe.

Your servo should never get its power from the Arduino anyway… always give a servo its (normally) 4.8 to 6V from a supply separate from the Arduino.

1.A The arduino Uno has a voltage regulator between the Vin input and the rest of the board that regulates the voltage down to 5 V so there is no problem.

B No.

C. See above.

D. The resistor restricts the current flowing in the LED. The LED drops about 2 to 3 V depending on the type. The rest of the voltage is dropped across the resistor and the current can be calculated from I = V/R. You need to know what current is needed for that LED.

  1. Most servos are designed for four cell battery use and thus have a maximum voltage rating of about 6 V. The BT module needs 3.3 V which you can get from the Arduino board.