5V SPDT Relay (Bare, No Driver Board)
A standalone 5V electromechanical relay with no onboard driver circuitry, requiring the builder to add their own transistor driver and flyback diode — the discrete-component path the pre-built relay module in this category exists to avoid.
How this component works
A relay is an electromechanical switch that allows a low-power circuit to control a completely separate high-power circuit. It contains a small electromagnet coil. When a small current energizes the coil, the magnetic field physically pulls a set of mechanical switch contacts closed (or open).
Relays provide total electrical isolation (galvanic isolation) between the control circuit and the load. This allows a delicate 5V microcontroller pin to safely turn on a 120V AC appliance without any direct electrical connection between the two systems.
Deep Dive: Bare 5V Relay
The core fact that shapes how this relay must be used is its coil's current draw. With a typical coil resistance around 62-70Ω at a 5V rating, Ohm's law gives a coil current of roughly 70-80mA (5V / 70Ω ≈ 71mA) — comfortably more than the 20-40mA most microcontroller GPIO pins can safely source. Attempting to drive the coil directly from a microcontroller pin risks exceeding that pin's current rating, which is exactly the problem the driver transistor in the pre-built module (the sibling entry in this category) exists to solve by letting a small base current control a much larger coil current.
Building that driver from discrete parts means the flyback diode is no longer optional infrastructure hidden on someone else's board — it becomes a deliberate design decision the builder has to make and implement correctly. Because the relay coil is inductive, suddenly interrupting its current (as happens every time the relay switches off) induces a brief voltage spike in the opposite polarity, which without a diode to safely clamp and dissipate it can easily exceed the driving transistor's breakdown voltage and destroy it. Choosing this bare relay over the pre-built module is the right call when a project needs a specific footprint, contact rating, or coil voltage the module doesn't offer, or when the goal is explicitly to learn how to design that driver stage — but it does shift that responsibility onto the builder.
Common Use Cases
- Custom PCB designs where the driver transistor, flyback diode, and any isolation are laid out and chosen deliberately rather than inherited from a pre-built module
- Projects needing a specific relay footprint, contact rating, or coil voltage not available on an off-the-shelf module
- Teaching or learning exercises specifically about relay driver circuit design, since the driver has to be built rather than used as a black box
Frequently Asked Questions
Why can't this relay be wired directly to a microcontroller GPIO pin?
Its coil draws roughly 70-80mA at 5V (from V/R with a typical ~70Ω coil resistance), which exceeds what most microcontroller GPIO pins can safely source (commonly 20-40mA). Driving it directly would risk damaging the microcontroller pin.
What is the flyback diode for, and is it really necessary?
Yes — a relay coil is an inductor, and when current through it is suddenly switched off, the collapsing magnetic field generates a brief high-voltage spike in the opposite polarity. A diode placed across the coil (cathode to the positive supply side) gives that spike a safe path to dissipate, protecting the driving transistor from being destroyed by it.
How is this different from the 5V relay module also in this category?
The module bundles this exact relay (or one very similar) together with a pre-built driver transistor, flyback diode, and often an optocoupler on one board. This bare-relay entry is the same core relay used on its own, requiring the builder to design and add that driver circuitry themselves.
Comparison Notes
Compared to the pre-built relay module in this category, this bare relay requires the builder to add their own driver transistor and flyback diode, offering more control over the exact driver design at the cost of more work and more room for a wiring mistake (most commonly, forgetting the flyback diode).