Solid-state relay
Fig. 1: Solid-state relay circuit In its most fundamental form, a relay is a switch that, when activated by an electrically separated, low-power control signal, turns on power to a load.…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
Solid-state relay is a electronics project. Fig. 1: Solid-state relay circuit In its most fundamental form, a relay is a switch that, when activated by an electrically separated, low-power control signal, turns on power to a load.…
- Source pages
- 162-163
- Named parts
- 3
- Build goal
- Working, tested prototype
Tools you need
- Digital multimeter
- Wire stripper and side cutters
- Soldering iron with a fine tip
- Current-limited bench supply
Use eye protection, good lighting, and a clean insulated surface throughout the build.
Safety precautions
- Disconnect every power source before changing a connection.
- Check component polarity, pinout, and supply voltage twice.
- Use a current limit for the first power-up.
- Keep liquids, loose metal, and uninsulated wires away from the bench.
Gather, identify, and understand every part.
Use the standardized inventory, then open What's this? to learn each part's role, advantages, limitations, handling, and specifications.
What's this?Image, role, pros, cons, handling & specifications
SEMICONDUCTOR LEARNING VIEWBC547
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for BC547; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
SEMICONDUCTOR LEARNING VIEWBC557
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for BC557; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
IRF540 - MOSFET
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for IRF540 - MOSFET; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
Connect one verified path at a time.
Explore the named components and standardized signal flow, then use Source Check to verify exact physical pins, values, and topology before applying power.
Trace this circuit.
Solid-state relay: interactive parts, standardized terminals, responsive anchored wires, student explanations, and the original circuit reference in one shared system.
Trace before wiring
Follow power, ground, inputs, processing, and outputs in that order. Never guess a pin from package shape alone.
Connect with power off
Make short, labelled connections and share a common ground only where the schematic requires it.
Inspect every joint
Check continuity, polarity, adjacent shorts, and loose connections before the first power-up.
Open all source diagrams and build views 2 visuals
Confirm the hardware-only control path.
This project does not include firmware in the source. The circuit itself provides the required behaviour.
How to connect
- Match every controller label to the circuit view and source pin map.
- Join grounds before signal wires when separate low-voltage supplies are used.
- Keep motors, relays, pumps, and other loads on a suitable driver and external supply.
Common mistakes
Reversed VCC/GND, board-label versus GPIO-number confusion, missing common ground, and charge-only USB cables.
Troubleshoot
Disconnect loads, continuity-test one path at a time, then test with a current limit.
Software preparation
No IDE, board package, library, or firmware upload is required for this project.
If you add a programmable controller as an extension, document its pin map separately.
How to upload code
The original design is implemented entirely in hardware, so proceed after verifying the circuit and supply.
Assemble, deploy, test, and troubleshoot.
Use the complete source notes in build order, then pass the final checks before calling the project finished.
Build in functional stages
- Power and regulation
- Controller or processing stage
- Inputs and sensors
- Outputs and loads
- Enclosure and strain relief
Power up safely
- Inspect unpowered continuity first
- Apply the lowest safe current limit
- Measure supply rails before signals
- Add one load at a time
- Record expected and actual results
Work from simple to complex
- Confirm power, ground, polarity, and orientation
- Compare each pin with the source
- Test inputs separately from outputs
- Replace only one variable at a time
- Power off before every correction
Follow the documented instructions.
These notes come from this project's source and remain in their original order.
Project overview
Project build noteFig. 1: Solid-state relay circuit In its most fundamental form, a relay is a switch that, when activated by an electrically separated, low-power control signal, turns on power to a load. Up to this point, the most reliable components for carrying out this duty have been electromechanical relays. The development of a solid-state relay can be attributed to the progress that has been made in the field of semiconductor technology.
Fig. 2: Pin configurations of MOSFET IRF540, transistors BC547/BC557 and MCT2E Solid-state relays have many benefits, including almost infinite switching, bounce-free operation, immunity to electromagnetic interference (EMI), higher operating speeds, low-voltage control, small package size, and multifunction integration. Solid-state relays also have the advantage of being able to integrate multiple functions. In this project, we will explain a DC-operated solid-state relay that makes use of components that are easily accessible (see Fig. 1). It can run off of a TTL compatible PWM input or a battery with a voltage of 3V DC. In Figure 2, you can see the several pin configurations for the MOSFET IRF540, the transistor BC547/BC557, and the opto- coupler. For a current of up to 10 amps to be driven through the load, the load voltage can be supplied by a tubular battery or a car battery with a voltage ranging anywhere from 24V to 96V. A battery operating at 24 volts DC has been utilized here. The value of the series resistor R6, which is currently set at 330 ohms, will alter based on the voltage of the battery in order to produce a current of 30-35mA. Calculating the value of resistor R6 can be done as follows: R6 equals 1000 times (Bat.2 V - 12V) divided by 35, and its power dissipation equals 35 divided by 1000. 2 x R = 0.001225R Note: It is recommended that R6's wattage be at least twice as high as its power dissipation. When the S1 button is pressed, the logic 1 state of the opto-coupler input is set, which causes the MOSFET to trigger and turn on the load. The input of the opto-coupler is low (logic 0) while S1 is open, which prevents the MOSFET from being triggered. As a direct consequence of this, the load has not been activated.
You built Solid-state relay.
You followed the full workflow from understanding the mission to testing the finished project. That is a real engineering achievement - well done.
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