If you realized your audience are not taking attention or not understanding your topic and feeling bored, so in this case what will you do?

maybe i can offer a small mental test , to add some fun to my lecture

or i can change my voice tone , loud and quiet to keep attentions . 

give them 5 min break may be the lecture is so long 

1. Ask for their attention Politely.
2. give your pitch in a story telling fashion. A n example of story will also drive our point.
3. Use your head and eyes in a light house fashion. Look into each persons eyes, but be sure not to stare. keep eye contact for a second and then move on to next person.
4. keep data and figures relevant and easy to understand. graphs and pics will do. a comparison to simple things like, the figure we arrive will be more than height of burj khalifa etc will do fine.

then I will change the way to give lecture or try to take attention

the better thing is to bring everyone's mind by saying a fake or comic suspense story. everyone will become interested. it was doing by my physics sir when i was in 10th. I experienced it. So I said it

Get their involvement by asking small & interesting questions relevant to the topic

This is what i read about public speaking and am just sharing what i read---

1. Use pictures,hd images,ask questions like - do u know,can u tell me,can anyone tell,have you heard about so so ..... Whatever topic you have search for any interesting incident or story that would help emphasis the explanation or importance  of topic .
2. Please make sure u know the topic thoroughly  because knowledge gives confidence and everyone loves to listen to a confident speaker.
3. And remember when u are going to speak u are the only one who knows the topic so be confident and smile and talk as if u are going to reveal something important.
4. We human beings are bound to pay more attention when we see profit so do let them know the benefits of knowing ur topic BEFORE starting the main seminar.
5. Regarding speech- change the pitch,speak in low tone sometimes and speak loudly sometimes to highlight points. Sudden change in pitch does grab attention.
6. Do not  announce just Take short smart breaks like u r adjusting papers,searching for a file etc.
7. Hope that helps.All The best  (:-) 

Give a small break and return while changing the style of lecture.

Thanks for the invite I am waiting for answers to the experts to take advantage

Thank you for the invitation

You must have the ability to convince the audience with an attempt to simplify an idea so that they can understand it and learn its meaning in a thrilling way

Incoporating of visual aids into the lecture can be good. Meanwhile also keeping the audience involved by asking them questions in between, related to the topic of discussion. Taking a few minutes break is also ideal if the lecture is longer than an hour. 

Engage the audience by asking questions and sharing their experience on the topic. Short group activity is always the energy booster.