4s: How to control Brightness

I have a 4s.

The other day I had it connected via USB to my car and I was using a maps app to navigate.

I noticed that my screen went very dark. I thought my iPhone had powered off.

Finally, I determined that the screen brightness had changed.

So I turned the brightness up all the way and set Auto-Brightness to Off from On.

On my way home the same thing happened. Auto-Brightness was still Off.

It also seemed to happen once when I power cycled the phone.

Is there a way to keep the screen brightness set to one level?

Thanks.