Visual AC Mains Voltage Indicator
If someone tells you that the mains voltage can change from 160 volts to 270 volts, you shouldn't be surprised. Even though most of our electrical and electronic devices have some kind of…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
Visual AC Mains Voltage Indicator is a electronics project. If someone tells you that the mains voltage can change from 160 volts to 270 volts, you shouldn't be surprised. Even though most of our electrical and electronic devices have some kind of…
- Source pages
- 293-295
- Named parts
- 5
- 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.
- This project may involve hazardous voltage. Work only with qualified supervision and proper isolation.
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
Transformer
A power component supplies, converts, stores, or regulates energy for the project.
What it does hereIt must provide the documented voltage, polarity, isolation, and sufficient current safely.
Buy / compare this part ↗Advantages
- Stable power improves reliability
- Current limiting protects first tests
- Regulation reduces resets and noise
Limitations
- Wrong polarity can cause immediate damage
- Underrated parts overheat
- Mains circuits require qualified supervision
Handling
- Measure output before connection
- Use a fuse or current limit
- Insulate exposed conductors
Specifications to verify
- Use the exact model, value, package, and rating listed for Transformer; similar-looking parts are not always interchangeable.
- Confirm input/output voltage, current, polarity, connector, isolation, and thermal rating.
What's this?Image, role, pros, cons, handling & specifications
SEMICONDUCTOR LEARNING VIEWLM339
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 LM339; 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
NETWORK
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for NETWORK; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
What's this?Image, role, pros, cons, handling & specifications
MAXIMUM
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for MAXIMUM; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
What's this?Image, role, pros, cons, handling & specifications
LED1, LED2, LED6, LED12 - indicator LEDs identified in the circuit
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for LED1, LED2, LED6, LED12 - indicator LEDs identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
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.
Visual AC Mains Voltage Indicator: 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 1 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 noteIf someone tells you that the mains voltage can change from 160 volts to 270 volts, you shouldn't be surprised. Even though most of our electrical and electronic devices have some kind of voltage stabilization built in, power fluctuations cause more
than 90% of the problems with these devices. This simple AC mains voltage indicator circuit shows the AC mains voltage from 160 to 270 volts in 10 volt steps. There are 12 LEDs with the numbers LED1 through LED12 that show the voltage level. All the LEDs stay off when the AC mains voltage is less than 160 volts. When the voltage reaches 160 volts, LED1 lights up, when it reaches 170 volts, LED2 lights up, and so on. With every 10 volts more, the number of LEDs that light up keeps going up. When the voltage coming in reaches 270 volts, all of the LEDs light up. AC mains voltage indicator circuit AC Mains Voltage Indicator A 12V regulator and three LM339 comparators (IC1, IC2, and IC3) make up most of the circuit (IC4). It runs on 12V DC that has been controlled. Step-down transformer X1 converts 230V AC from the mains to 15V AC, which is then rectified by a bridge rectifier made up of diodes D1 through D4, filtered by capacitor C4, and controlled by IC4. For controlling the level of the AC, the input voltage of the regulator is also sent to the inverting inputs of gates N1 through N12. The LED display circuit is made up of four op-amp comparators (IC1–IC3). All of the comparators get their inverting inputs from the unregulated DC voltage, which is proportional to the mains input. The non-
inverting inputs get their reference DC voltages from the regulated output of IC4 through a series network of precision resistors. The resistors R13 through R25 are chosen so that the reference voltage at points 1 through 12 is 0.93V, 1.87V, 2.80V, 3.73V, 4.67V, 5.60V, 6.53V, 7.46V, 8.40V, 9.33V, 10.27V, and 11.20V, respectively. When the AC voltage at the input changes from 160V to 270V, the DC voltage at the anode of ZD1 also changes. When the voltage coming in changes from 160V to 270V, the voltage between the filter capacitors C1 and C2 changes from about 14.3V to 24.1V. Zener ZD1 is used to drop a fixed 12V and give all comparator stages proportional voltages (inverting pins). When the voltage at the comparators' non-inverting input goes up, the LED at the output lights up.
Construction & testing
Project build noteSet up the circuit on a PCB that can be used for many different things so that all of the LEDs make a bar graph. Mark LED1 for a minimum level of 160V on the bar graph, LED2 for 170V, and so on. Finally, mark LED12 for a maximum level of 270V. Now, your test gadget is ready to be used. To measure the AC voltage, just plug the device into the mains AC measuring point, press switch S1, and look at the bar graph made up of LEDs. Let's assume that LED1 through LED6 are on. In this case, the voltage that was measured was 220V. Similarly, if all the LEDs light up, it means that the voltage is higher than 270V.
You built Visual AC Mains Voltage Indicator.
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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