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# Timers
Arcade Foundations · Collide and score
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## Today
**We are learning to** use a timer to make a new enemy every second, from a random place, for as long as the game lasts.
- I can use a timer to make a new enemy again and again.
- I can choose how often a timer runs.
- I can give each new sprite a random place.
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## Why it matters
> Enemies that keep on coming, raindrops that never stop: a game that runs for minutes is made by a few blocks that run again and again.
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💬 Discussion
## Predict
**The timer runs every 1000 ms and makes a rock each time. You watch for 5 seconds. How many rocks have appeared?**
- 0
- 1
- 5
- 1000
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✅ Check
## What happened?
* 5. The timer runs once every second, and each time it makes a new rock: 5 seconds, 5 rocks.
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✍️ Activity
## Run it
Open the lesson at this step and try it together.
[studio.learningwaypoints.com/tutorials/code/arcade/arcade-timers/?full=step-2#step-2](https://studio.learningwaypoints.com/tutorials/code/arcade/arcade-timers/?full=step-2#step-2)
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💬 Discussion
## Investigate
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✅ Check
## How many?
**The timer runs every 500 ms. How many rocks has it made after 10 seconds?**
* Answer: 20
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✅ Check
## Ava's rocks
Ava's program:
**Ava picks a random height once, in on start, and stores it in y. Every rock in her timer uses y. What does she see?**
- Rocks at every height at once
- No rocks
- Each rock comes at a different height
- Every rock comes at the same height
* Answer: Every rock comes at the same height
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✅ Check
## How often?
**Match each timer with how often it runs.**
| | |
|---|---|
| `every 1000 ms` | A. once a second |
| `every 500 ms` | B. twice a second |
| `every 2000 ms` | C. once every two seconds |
* Answer: every 1000 ms → A. once a second; every 500 ms → B. twice a second; every 2000 ms → C. once every two seconds
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✍️ Activity
## Clap the timer (3 min)
The class claps once a second. On each clap, one student rolls a dice and calls out a height.
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✍️ Activity
## Modify
- **Faster rocks:** Make a rock come every **half a second**.
- **Random heights:** Give each rock a **random** height, so they don't all come through the middle.
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade/arcade-timers/?full=step-4#step-4)
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💬 Discussion
## Investigate: Mix it up
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✅ Check
## Mix it up
**The player starts with 2 lives. Fill in the lives after each thing happens.**
| what happens | lives after |
|---|---|
| (start) | 2 |
| collects a heart | ? |
| a rock hits | ? |
| a rock hits | ? |
* Answer: 3, 2, 1
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✍️ Activity
## Make
Make a **30-second dodge game** (bats in a cave, asteroids, traffic):
- the player starts with **3 lives**
- a **countdown** of 30 seconds (**start countdown**, in Info): survive it to finish
- every second, a new **enemy** flies in from the **right edge** at a **random height**
- each touch takes **one** life
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade/arcade-timers/?full=step-6#step-6)
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✍️ Activity
## Change a rain dodger
Someone is walking home in the rain without an umbrella. Make the rain heavier (more drops, falling faster), then give the walker 5 lives and a 40-second walk home.
[Open this step](https://studio.learningwaypoints.com/tutorials/code/arcade/arcade-timers/?full=step-7#step-7)
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✅ Check
## Exit ticket
**A timer runs every 2000 ms and makes one enemy each time. How many enemies after 10 seconds?**
* 5. It runs once every two seconds.