creating a switch using arduino's digital outputs

Question

I'm trying to charge a simple servo that only has + and - pins. Usually I would connect it's - pin to the ground and + pin to the digital output from Arduino. This works, but the servo turns weakly and can be stopped by hand. If i connect the servo directly to the battery or to 3.3 pin from the Arduino it turns strongly and i cannot stop it.

I tried an op-amp amplifier and a PMOS transistor in order to let the servo get power from 3.3V or the battery, but didn't work out.

Anyone has already did something similar and can share with me his/her design?

----- edit: this is the servo i'm talking about. https://solarbotics.com/product/gm20/

----- update: Now that I have worked on more projects, I think this kind of task should be done through an H Bridge as explained here: http://www.instructables.com/id/H-Bridge-on-a-Breadboard/ I tried it (obviously for another project) and it worked like a charm!

Answer 1

That's not a servo, that's a straight up DC motor. Also @jippie's comment wasn't about burning out your motor, rather your Arduino.

I'm not sure what you mean by "Charging" it. If you want to control it in an on/off mode using your Arduino you can use the circuit below:

^{simulate this circuit – Schematic created using CircuitLab}

The nice thing about this circuit is that you can drive higher voltage motors with it too. The PMOS is the ZXM part. If you want to get another PMOS make sure it's VGS is about 1/3 to 1/2 of your motor voltage (in this case 3.3V) and that it can handle 30%-50% more current than your motor needs. The NMOS is used to drive the PMOS since your motor voltage might be higher than 3.3V -- for some other use case. The only thing when selecting the NMOS is making sure it is rated for higher than your motor voltage and that it's VGS is less than 2V (since you are driving it from a 3.3V Arduino).

The diode is to protect your PMOS and your circuit from your motor's emf induced voltage.