Auto Reset Over/Under Voltage Cut-Out
This over/under voltage cut-out will protect your expensive electrical and electronic appliances from the harmful consequences that can be caused by mains voltages that are extremely high…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
Auto Reset Over/Under Voltage Cut-Out is a electronics project. This over/under voltage cut-out will protect your expensive electrical and electronic appliances from the harmful consequences that can be caused by mains voltages that are extremely high…
- Source pages
- 305-307
- 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
Power supply
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 Power supply; 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
NEGATIVE
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 NEGATIVE; 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
T1, T2 - transistor stages identified in the circuit
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 T1, T2 - transistor stages identified in the circuit; 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
LED1, LED2 - 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 - indicator LEDs identified in the circuit; 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
PART LEARNING VIEWVR2 - preset potentiometer 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 VR2 - preset potentiometer 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.
Auto Reset Over/Under Voltage Cut-Out: 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 noteThis over/under voltage cut-out will protect your expensive electrical and electronic appliances from the harmful consequences that can be caused by mains voltages that are extremely high or extremely low. The circuit has an automatic reset and uses parts that are easy to find. It uses the comparators that are built into 555 timer ICs. To make
relays and control circuits work reliably, power is taken from different parts of the power supply circuit. The circuit is controlled by comparator 2, while the output of comparator 1 (which is connected to reset pin R) is kept low by connecting pins 5 and 6 of the 555 IC together. The voltage on the positive input pin of comparator 2 is 1/3 of Vcc. So, as long as pin 2's negative input is less than 1/3 Vcc, the output of comparator 2 is high and the internal flip-flop is set, meaning that the Q output (pin 3) is high. At the same time, pin 7 is in a high impedance state, so the LED that is connected to it is off. When pin 2 is taken to be more positive than 1/3 Vcc, the output (at pin 3) changes direction and goes low. At the same time, pin 7 goes low (because the flip-Q flop's output is high) and the ED connected to pin 7 lights up. Both timers, IC1 and IC2, have been set up to work the same way. The cut-out for low voltage (let's say 160 volts) is set by noticing that LED1 just turns on when the mains voltage is a little higher than 160V AC. At this setting, the output of IC1 at pin 3 is low, and transistor T1 is in a state called "cut-off." Since RESET pin 4 of IC2 is connected to Vcc by 100 kilo-ohm resistor R4, it stays high. Setting VR2 for an overvoltage cut-out (let's say 270V AC) is done by noticing that LED2 goes out. When the voltage from the mains is just below 270V AC. Pin 3 is also high when RESET pin 4 of IC2 is high. So, transistor T2 conducts and turns on relay RL1, whose N/O contacts connect the load to the power supply. As long as the mains voltage is more than 160V AC but less than 270V AC, this is the case.
When the mains voltage goes above 270V AC, pin 3 of IC2 goes low. This turns off transistor T2 and turns off relay RL1, even though pin 4 of RESET has always been high. When the mains voltage drops below 160V AC, pin 3 of IC1 goes high, turning off LED1. When pin 3 has a high output, transistor T1 is turned on. As a result, both the collector of transistor T1 and RESET pin 4 of IC2 are pulled low. So, the output of IC2 goes low, and T2 doesn't conduct. Because of this, relay RL1 is turned off, which disconnects the load from the power supply. When the mains voltage goes back above 160V AC but is still below 270V AC, the relay turns on to connect the load to the power supply.
You built Auto Reset Over/Under Voltage Cut-Out.
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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