Activity 4.1.1 — Sensors & Motors¶
Learning Objectives¶
By the end of this lesson, students will be able to:
- Distinguish between digital and analog sensors
- Explain how to interface various sensors with digital circuits
- Describe different types of motors and their control methods
- Use motor driver ICs to interface motors with digital logic
- Design simple sensor-motor interface circuits
Vocabulary¶
Vocabulary (click to expand)
| Term | Definition |
|---|---|
| Sensor | A device that converts a physical quantity into an electrical signal |
| Actuator | A device that converts electrical energy into physical motion or other output |
| Digital Sensor | A sensor with two output states: HIGH (1) or LOW (0) |
| Analog Sensor | A sensor with a continuously variable output voltage |
| Voltage Divider | A circuit that produces an output voltage proportional to input voltage |
| H-Bridge | A circuit that allows control of motor direction by reversing polarity |
| PWM | Pulse Width Modulation - rapidly switching to control average power |
Part 1: Introduction to Sensors¶
Sensors are input devices that convert physical phenomena into electrical signals that digital circuits can process.
Two Categories of Sensors¶
Digital Sensors (On/Off) - Output is either HIGH (1) or LOW (0) - Like a switch that is either open or closed - Easy to connect directly to digital circuits
Analog Sensors (Continuous) - Output varies over a continuous range - Need additional circuitry to convert to digital - Provide more detailed information about the physical quantity
Part 2: Digital Sensors¶
Common Digital Sensors¶
| Sensor | Symbol | Function |
|---|---|---|
| Pushbutton | Pressed = HIGH, Released = LOW | User input |
| Limit Switch | Activated = HIGH | Detect position/motion |
| IR Break Beam | Beam broken = HIGH | Detect object passage |
| Magnetic Reed Switch | Magnet present = HIGH | Detect magnetic field |
| Photointerrupter | Light blocked = HIGH | Object detection |
Digital Sensor Interface¶
Connecting a pushbutton to a digital circuit:
Pull Resistors: - Pull-down resistor: Connects to GND, keeps input LOW when button not pressed - Pull-up resistor: Connects to VCC, keeps input HIGH when button not pressed (more common)
Key insight: Pull resistors ensure predictable input states. Without them, the input "floats" and can pick up interference, causing unpredictable behavior.
Part 3: Analog Sensors¶
Common Analog Sensors¶
| Sensor | Output Range | Application |
|---|---|---|
| Photocell (LDR) | Dark: high resistance, Light: low resistance | Light detection |
| Thermistor | Temperature-dependent resistance | Temperature sensing |
| Potentiometer | Position-dependent voltage | Position/volume control |
| Flex Sensor | Bending increases resistance | Bend detection |
| Force Sensor | Resistance varies with force | Pressure/force sensing |
Analog Sensor Circuit: Voltage Divider¶
Most analog sensors change resistance. To convert this to voltage, use a voltage divider:
Formula:
Vout = VCC × (R_sensor / (R_fixed + R_sensor))
Example: Photocell Circuit¶
- In bright light: Photocell resistance is low (~100Ω), Vout is near 0V
- In darkness: Photocell resistance is high (~1MΩ), Vout is near 5V
Op-Amp Amplification¶
For small sensor signals, use an operational amplifier:
Non-inverting amplifier: Gain = 1 + (R_feedback / R_input)
Part 4: Motors as Actuators¶
Types of Motors¶
| Motor Type | Control Method | Application |
|---|---|---|
| DC Motor | Voltage/speed, polarity/direction | General motion |
| Servo Motor | PWM angle signal (0-180°) | Precise positioning |
| Stepper Motor | Step pulses, precise increments | Precision control |
DC Motors¶
Characteristics: - Speed proportional to applied voltage - Direction depends on polarity of applied voltage - No built-in position control
Speed Control: - Higher voltage = faster speed - PWM can simulate variable voltage
Direction Control: - Forward: Positive to terminal A, Negative to terminal B - Reverse: Negative to terminal A, Positive to terminal B
Servo Motors¶
Characteristics: - Precise angular position (typically 0° to 180°) - Internal feedback for position control - Controlled by PWM signal
Control Signal: - PWM with 20ms period (50 Hz) - Pulse width determines angle: - 1ms = 0° - 1.5ms = 90° - 2ms = 180°
Stepper Motors¶
Characteristics: - Moves in precise step increments (e.g., 1.8° per step) - Can hold position without power - Requires controller to energize coils in sequence
Types: - Unipolar: coils have center tap - Bipolar: four wires, no center tap (requires H-bridge)
Part 5: Motor Driver ICs¶
The L293D Dual H-Bridge¶
The L293D can drive two DC motors or one stepper motor with direction control.
Pinout:
┌─────────────┐
VCC2 -│ │- GND
1Y -│ │- 1A
1A -│ L293D │- 1Y
2A -│ │- 2Y
2Y -│ │- 2A
VCC1 -│_____________│- GND
Connections: - VCC1: Logic power (5V) - VCC2: Motor power (up to 36V) - 1A, 2A: Control inputs (from digital circuits) - 1Y, 2Y: Motor outputs
Truth Table:
| 1A | 1Y Output |
|---|---|
| 0 | Disabled (Hi-Z) |
| 1 | Enabled (HIGH) |
For bidirectional control with enable:
| Enable | Input A | Motor Output |
|---|---|---|
| 0 | X | Disabled |
| 1 | 0 | Forward |
| 1 | 1 | Reverse |
The ULN2003 Darlington Array¶
The ULN2003 drives stepper motors or high-current loads.
Features: - 7 Darlington pairs - Can sink up to 500mA per channel - Built-in flyback diodes
Typical Stepper Connection:
Part 6: Interfacing Sensors with Motors¶
Example: Light-Activated Motor¶
Goal: Turn on a fan when it gets too bright
Components: - Photocell sensor - Comparator circuit - L293D motor driver - DC fan
Circuit:
+5V ──[Photocell]───[10kΩ]─── GND
│ │
▼ ▼
Comparator ──────[Input]
│
▼
L293D Enable ───[HIGH when dark]
│
▼
Fan Motor
Example: Object Counter with Motor¶
Goal: Count objects and rotate turntable after each one
Components: - IR break beam sensor - 74LS90 decade counter - L293D driver - Stepper motor
Operation: 1. Object breaks beam 2. Sensor output goes HIGH 3. Counter increments 4. After 10 counts, stepper advances one position
Part 7: Practice Problem¶
Problem Statement¶
Design a circuit that: 1. Uses a limit switch to detect when a door is closed 2. Turns on an LED when the door is open 3. Turns on a motor (to close a mechanism) when the door is closed
Components Available:¶
- Limit switch (closed = LOW, open = HIGH)
- LED
- DC motor
- L293D motor driver
Draw the circuit and explain the logic.¶
Show Solution
Circuit Diagram:
+5V ──[Limit Switch]──────▶ Input Pin (with pull-down)
│
─┴─ Switch to GND (closed position)
│
▼
┌──────────────┐
│ Logic │
│ Input │
└──────┬───────┘
│
┌────────────┼────────────┐
│ │ │
▼ ▼ ▼
[Inverter] [LED] [Motor]
│ │ │
▼ ▼ ▼
Motor ON LED ON OFF when
when closed when open door open
(door closed (door open
signal) signal inverted)
**Logic:**
- Door open: switch = HIGH → LED ON, Motor OFF
- Door closed: switch = LOW → LED OFF, Motor ON
**Using L293D:**
- Enable pin = switch signal
- Input A = HIGH (for direction)
- When door closes (switch = 0), motor turns
Summary¶
- Sensors convert physical phenomena to electrical signals
- Digital sensors output HIGH or LOW (switches, limit switches, IR beams)
- Analog sensors output continuous voltage (photocells, thermistors)
- Use voltage dividers to interface analog sensors with circuits
- Motors convert electrical energy to motion (DC, servo, stepper)
- Use motor driver ICs (L293D, ULN2003) to interface motors with digital logic
- H-bridges enable bidirectional motor control
Key Reminders¶
- Always use pull resistors with digital sensors
- Analog sensors need signal conditioning (voltage dividers, op-amps)
- Motor drivers protect digital circuits from motor current
- Use flyback diodes to protect against motor voltage spikes
- Servo motors use PWM for position control
Custom activity — adapted from PLTW Digital Electronics