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# Motor drivers
Power and motors · Driving a motor
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## Today
**We are learning to** a motor driver (an H-bridge) lets two pins choose the direction: IN1 HIGH and IN2 LOW is forwards, swapped is backwards, the same is stop. A third, PWM pin on ENA sets the speed with analogWrite. Two jobs, two kinds of signal.
- I can make a motor go forwards, backwards and stop with a driver.
- I can set a motor's speed with PWM on the enable pin.
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## Why it matters
> A transistor switches a motor on and off. But a robot has to reverse, and go slowly round corners. How does a pin make a motor go backwards?
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💬 Discussion
## Predict
**The motor goes forwards with IN1 HIGH, IN2 LOW and ENA at 200. How do you make it go backwards at the same speed?**
- Set both IN1 and IN2 LOW
- Set ENA to −200
- Swap them: IN1 LOW, IN2 HIGH, ENA still 200
- Set ENA to 55
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Fpower%2Fhbridge%2Fstep-2&code=widget%3A+motor%0Amode%3A+hbridge%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/power/hbridge/?full=step-2#step-2)
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✅ Check
## What happened?
* Swap IN1 and IN2. ENA only sets speed (0 to 255); it can't be negative. Both LOW stops the motor.
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💬 Discussion
## Direction, then speed
Now backwards at three-quarter speed.
$$\text{digitalWrite(8, HIGH)}$$
1) $\text{digitalWrite(7, LOW)}$ — IN2 LOW: forwards.
2) $\text{analogWrite(9, 128)}$ — ENA at 128 of 255: about half speed.
3) $\text{swap 8 and 7}$ — Backwards. ENA is unchanged.
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✅ Check
## Direction, then speed
**IN1 on pin 8, IN2 on pin 7, ENA on pin 9. Which lines make the motor go backwards at three-quarter speed?**
- analogWrite(8, 191); analogWrite(7, 0);
- digitalWrite(8, LOW); digitalWrite(7, HIGH); analogWrite(9, 191);
- digitalWrite(9, LOW); analogWrite(8, 191);
- digitalWrite(8, HIGH); digitalWrite(7, LOW); analogWrite(9, -191);
* Answer: digitalWrite(8, LOW); digitalWrite(7, HIGH); analogWrite(9, 191);
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✅ Check
## Find the mistake
**Sam's motor turns at full speed whatever Sam does. Click the line where Sam's sketch goes wrong.**
```
1 void setup() {
2 pinMode(8, OUTPUT);
3 pinMode(7, OUTPUT);
4 }
5
6 void loop() {
7 digitalWrite(8, HIGH);
8 digitalWrite(7, LOW);
9 analogWrite(8, 128);
10 delay(2000);
11 }
```
* Answer: line 9
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✍️ Activity
## Vary: change one thing
- **Brake:**
- **Coast:**
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/power/hbridge/?full=step-4#step-4)
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✍️ Activity
## Build it in Tinkercad
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✅ Check
## Two motors
**A robot has a left and a right motor. The left goes forwards and the right goes backwards, both at the same speed. The robot…**
- Goes forwards
- Stops
- Goes backwards
- Spins on the spot
* Answer: Spins on the spot
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✅ Check
## Speed
**ENA is at 64. What percentage of full speed is that (to the nearest whole number)?**
* Answer: 25
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✍️ Activity
## Make: a gentle start
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/power/hbridge/?full=step-7#step-7)
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✅ Check
## Exit ticket
**Write the three lines that make a motor (IN1 on 8, IN2 on 7, ENA on 9) go forwards at full speed.**
* digitalWrite(8, HIGH); digitalWrite(7, LOW); analogWrite(9, 255);