Key Engineering Takeaways
- •Poles (P) define how many separate circuits a switch controls; Throws (T) define how many output positions each pole connects to.
- •An electromechanical relay uses a low-power 5V electromagnetic coil to mechanically pull high-voltage/high-current contacts closed.
- •Relays have three contact terminals: COM (Common), NO (Normally Open), and NC (Normally Closed).
- •Never connect a relay coil directly to a microcontroller pin: coils require 70mA+ and produce destructive 100V+ inductive flyback spikes.
- •Solid State Relays (SSRs) and Optocouplers switch loads with light beams (LED + phototransistor), providing 5000V electrical isolation.
Prerequisites
- • Transistors as switches and Diode fundamentals
Required Hardware / Tools
- • 5V Electromechanical Relay Module
- • 2N2222 NPN Transistor
- • 1N4007 Diode
- • SPDT Toggle Switch
- • Breadboard
Switch Configurations: SPST, SPDT, DPDT & Momentary
Mechanical switches physically open or close conductive metal contacts to route electric current:
### Switch Terminology:
- **Poles**: The number of independent circuits the switch controls simultaneously.
- **Throws**: The number of output contact pathways connected to each pole.
### Common Configurations:
1. **SPST (Single Pole, Single Throw)**: Simple ON/OFF switch with 2 pins (e.g. power switch).
2. **SPDT (Single Pole, Double Throw)**: 3 pins. Routes input (COM) to either Output A or Output B.
3. **DPDT (Double Pole, Double Throw)**: 6 pins. Two isolated SPDT switches linked by a single mechanical toggle (used for H-Bridge motor polarity reversal).
4. **Normally Open (NO) Pushbutton**: Circuit is disconnected until pressed.
5. **Normally Closed (NC) Pushbutton**: Circuit is connected until pressed (used for Emergency Stop buttons).
How Electromechanical Relays Work (NO, NC & COM Contacts)
A **Relay** is an electrically operated mechanical switch. It allows a low-power microcontroller (3.3V / 5mA) to safely turn ON a high-power load (240V AC / 10A motor or solenoid) with complete physical isolation.
### The 3 Output Terminals:
- **COM (Common)**: The moving center contact terminal.
- **NC (Normally Closed)**: Connected to COM when the relay coil is DE-ENERGIZED (OFF).
- **NO (Normally Open)**: Connected to COM when the relay coil is ENERGIZED (ON).
When DC current passes through the internal coil, it generates a magnetic field that physically pulls down an iron armature, clicking the contact from NC to NO!
Transistor Driver Circuits & Flyback Protection Diodes
A standard 5V relay coil draws 70mA - 100mA, exceeding the 20mA safety limit of Arduino and ESP32 GPIO pins.
### The Complete Relay Driver Circuit:
1. **NPN Transistor (2N2222 / BC547)**: Acts as a low-side switch controlling current to the coil.
2. **1 kΩ Base Resistor**: Limits current from the GPIO pin into the transistor base.
3. **Flyback Diode (1N4007)**: Connected in reverse parallel across the coil (+ cathode to VCC, anode to collector). When the coil turns off, the diode absorbs the massive inductive collapse spike, saving the transistor from destruction.
Flyback Diode RuleNever drive a relay coil or DC motor with a transistor without a flyback diode. The 100V+ inductive spike will destroy your driving transistor on the very first switch off!
Frequently Asked Questions
What is the difference between an Electromechanical Relay and a Solid State Relay (SSR)?
Electromechanical relays have moving mechanical contacts that make an audible click and can wear out after ~100,000 cycles. Solid State Relays use power semiconductor triacs/MOSFETs with zero moving parts, silent operation, and unlimited switching lifespans.
Tags:#Switches#Relays#Optocouplers#Electromechanical#Flyback Diode#Hardware Isolation