An overhead projector should have an on/off switch. This switch should automatically allow the cooling fan to continue running even when the switch is off so that the bulb can cool properly. Overheads also allow you to get the best view of your overhead transparency by offering different methods of adjusting the focus. You can raise and lower the head on many models with a knob or lever. Other models have a keystone feature that allows you to adjust the image if its not projecting square with the screen. Some models of overheads may have bulb changers that make it easy to swap out used bulbs.