Take some time to play with a microphone prior to a lecture and hear how the sound changes as you move it towards your mouth or attach it to your clothes. Each microphone is different so a short experiment will help you determine its optimal positions. Be careful when setting the volume level because the speakers may not project into the podium as well as they do around the seats. It is useful to ask the class for feedback on the volume until you are used to judging the sound you can hear. Whatever equipment you use, make sure you know how to work it properly.
Most overhead projectors have a second lamp in case the first one blows.
If you have learned how to switch the bulbs, your smooth professional presentation can continue uninterrupted. In the lecture theatre, the lecturer should be in charge. It does not matter how many students are in the room or what they would rather be doing. They have come to learn. If the lecturer teaches professionally, the students will shut up and listen.