Arduino Rover Part 3: Motor Control

A few weeks ago, I picked up the Arduino Uno that I've had for a while, and started thinking about what I should build with it. I decided on modifying an old toy rover that I had, and making it wirelessly drivable... but with 2 way audio and a camera feed.

This is the third part of several posts. I will be explaining about my struggles to power the default motors with my Arduino. Prepare for math!

Links:
Part 1: Initial Plan and Reverse-Engineering
Part 2: Preparing the Rover Chassis
Part 3: Motor Control
Part 4: Bluetooth
Part 5: Mounting
Part 6: Batteries
Part 7: The Test

Now that the rover was gutted and is ready for installation, the next thing to do is configure a circuit to power the motors. It can't just be plugging the leads directly into the board. Prior to most of my research, I knew there were problems associated with taking too much current from the Arduino. After my research, I had a much better understanding and could eventually power motors through Arduino.

The maximum safe current from an Arduino output pin is 20 mA. That's 0.02 Amps. Ohm's law states that Current = Volts / Resistance. Using basic algebra to get resistance by itself, we get R = V/A. If we plug in the output values of the Arduino (5 volts, 0.02 A max), we get R = 5/0.02 = 250Ω minimum resistance.

I downloaded the Arduino IDE to my Linux drive (Yes, I use Linux and I love it) and started refamiliarizing myself with the IDE and programming language, which is a variant of C. I'm very familiar with Java (As is obvious from my Minecraft mods), so the learning curve wasn't too large.



I looked up a motor-control circuit, and found this. I recreated it with a motor from an electronics kit.

Measuring the motor's resistance, I got 100Ω, which is fine because the transistor from the same kit was protecting the Arduino. Then I hooked up one of the rover's motors...

Nothing happened.

I tried again: Nothing.

I left the blink program on and listened to the gearbox: Nothing.

Then I touched the transistor...

and promptly pulled my hand back. It was really hot.

I looked up the data sheet for that particular transistor and as far as I can tell, it's got a maximum current rating of 200mA. Measuring the resistance of the motor with my multimeter, I got... holy cow! 0.7Ω! That's supposedly pulling 7.14 A based on 5V/0.7Ω. I tried doing some math to find the right resistor to do the job. I got about 30Ω. I didn't have any of those handy, and figured that if they were designed for use with 4.5 volts from 3 AA batteries and 50Ω didn't work with 5 volts, then this transistor wasn't going to work.

Guess what. 2 100Ω resistors in parallel didn't work...

The headache intensified.

Finally, I gave up trying to use transistors. I went searching online for some relays. Luckily, I found some really good ones, the HE3621A0510, that were just what I needed (Hooray for Capitalism!)

They are rated for 5V, and have 500Ω resistance, thus requiring 10mA, and they only have 4 pins... That's exactly what I needed.

Here's the datasheet as of May 6th.

Due to the fact that it's the 510 version, it has an internal diode, (which in my relief I neglected to learn more about).

They came in the mail a day early, and I forgot to look in the giant yellow bubble-wrappered envelope for the bluetooth module (I'll talk about that in part 4).

I mapped all of the resistances with my multimeter. The two outer pins (Shown as green in the picture) had no conductivity, to be connected when electricity flows through the inner two, which had 515Ω. I plugged it into my project, and loaded the blink script...

Nothing. I got nervous, verified connections...

Nothing. The headache from the transistors returned...

I looked back and researched the exact model of relay... Mentions something about a diode.

I researched the Diode Suppressed Coil: A method of protecting delicate electronics from current surge caused by collapsing magnetic field in the coil. Thus, the relay only works one way. Here's more information on how relays and DSCs work.

I switched the direction of the relay, and... Poof! It worked perfectly! I've marked the negative and positive leads in the picture with black and red, respectively.

That concluded my motor-control headaches.

Part 4 will be about bluetooth serial communications and connecting to an Android phone (with a custom app)

Now for more DIY tips:

1. Do your research! I could have saved myself a lot of headache-hours and a burnt finger by just learning more about the transistor and relay I was trying to use.

2. Use Linux (Preferably Mint)

Edit: 6/1/16: Updated list of links

Edit: 7/8/16: Updated list of links
Edit: 8/25/16: Updated list of links
Edit: 7/6/17: Updated list of links