More project photos (3)
The problem
Every appliance nameplate carries a voltage rating, and every blown charger or dead fridge compressor in a voltage-fluctuation zone is what happens outside it. Commercial voltage guards exist but are sealed boxes; a student who builds one learns the full protection chain — isolated voltage sensing, RMS computation, threshold comparison with hysteresis, relay/contactor switching, and auto-reconnect timing. This project builds that chain as a transparent prototype: a ZMPT101B-class isolated voltage sensor feeds an Arduino, which computes the RMS line voltage, compares it against potentiometer-set over-voltage and under-voltage thresholds, and drives a relay module plus a power contactor to connect or disconnect the demo load. A digital voltmeter displays the line voltage live, a red lamp annunciates the tripped state, and a reset button re-arms the guard after a trip.
Transformer-isolated module senses the mains; its output is scaled into the Arduino ADC range.
Follow the working sequence
How it works
- The isolated sensor module is wired across the mains input; its low-voltage AC output is scaled and biased into the Arduino's ADC range.
- The firmware samples the waveform at high rate, computes the true RMS voltage each mains cycle, and smooths it with a short moving average.
- The RMS value is compared against the over-voltage and under-voltage thresholds set by the two trimmers, with hysteresis bands around each.
- If the voltage leaves the safe window for longer than the confirmation time, the Arduino drops the relay, the contactor opens, and the load is disconnected; the red trip lamp lights.
- When the voltage returns inside the window, the auto-reconnect timer runs; only after the full stabilization delay does the contactor re-close and restore the load.
- The student demonstrates all of this with a variable autotransformer (variac) sweeping the input voltage while the audience watches the voltmeter and the lamp.
Tech stack:
- Arduino Nano (ATmega328P)
- Isolated AC voltage sensor module (transformer-based)
- 5 V relay module + 230 V AC power contactor (20 A class)
- Digital panel voltmeter for live display
- Trimmer potentiometers for threshold setting
- Red trip-indicator lamp and reset push-button
- C/C++ firmware with RMS computation and hysteresis logic
- Enclosed prototype housing with shrouded mains terminals
| Parameter | Value |
|---|---|
| Input | 230 V AC nominal, 50 Hz (design) |
| Under-voltage trip | Adjustable, approximately 160–200 V (design) |
| Over-voltage trip | Adjustable, approximately 240–280 V (design) |
| Load rating | Up to approximately 20 A resistive (contactor datasheet) |
| Reconnect delay | Adjustable approximately 5–60 s (design) |
| Trip confirmation | Approximately 0.5–1 s sustained excursion (design) |
| Display | 3-digit digital voltmeter, approximately ±1% (expected) |
Project features
- [Isolated voltage sensing] A transformer-isolated sensing module measures the mains without any direct high-voltage connection to the low-voltage electronics — the safe way to sense AC line voltage.
- [True RMS computation] The firmware samples the waveform and computes RMS voltage, so the thresholds respond to the real heating value of the supply, not a peak estimate.
- [Adjustable over/under thresholds] Two trimmer potentiometers set the trip points (e.g. trip below ~180 V and above ~260 V), letting the student configure the guard for different appliance ratings.
- [Hysteresis on thresholds] Separate pick-up and drop-out levels stop the relay chattering when the voltage hovers at a threshold — a real protection-relay concept implemented in firmware.
- [Auto-reconnect timer] After the voltage returns to the safe band, the guard waits a configurable stabilization delay before reconnecting the load — protecting compressor motors from rapid cycling.
- [Live voltage display] A digital voltmeter shows the measured line voltage continuously, so the demo audience sees exactly what the guard is reacting to.
- [Manual reset and trip lamp] A red indicator shows the tripped state and a reset button re-arms the unit, demonstrating protection lockout behavior.
What is included
- Working automatic voltage guard prototype (sensing, control, switching, display)
- Arduino firmware with RMS computation, thresholds, hysteresis and auto-reconnect
- Threshold-calibration procedure using a variac and multimeter
- Wiring diagram and bill of materials
- Project report PDF (protection principles, sensing design, calibration results procedure)
- PPT presentation for final review
- Viva Q&A preparation document (RMS vs peak, hysteresis, contactor vs relay, auto-reconnect)
Limitations & prerequisites
- A single-phase guard for household-class loads; three-phase protection is listed as future scope.
- It protects against sustained over/under voltage, not against lightning surges or nanosecond transients — a surge protection device is a different product.
- Threshold calibration needs a variable autotransformer (variac), which the student's lab provides; the procedure documents the setup.
- The contactor has a finite electrical life (datasheet-rated operations); it is a demo unit, not a lifetime appliance accessory.
- Mains wiring is involved: the manual mandates qualified supervision and the prototype uses shrouded terminals and an enclosed housing.
- The voltmeter display is indicative (approximately ±1% expected), not a calibrated metering instrument.
Frequently Asked Questions
How is this different from a stabilizer?
A stabilizer corrects the voltage (buck/boost) and keeps the appliance running; a guard disconnects the appliance when voltage is unsafe and reconnects when it recovers. Guards are cheaper, simpler, and the standard protection for compressor loads.
Why does the reconnect need a delay?
Refrigerator and AC compressors are damaged by rapid on-off cycling (short-cycling). The stabilization delay — typically tens of seconds — lets pressures equalize before restart.
What is hysteresis and why is it in the firmware?
Without it, a voltage hovering exactly at the threshold would make the relay chatter on and off. Hysteresis uses separate trip and restore levels, a standard protection-relay technique.
Is it safe for a student to build with mains voltage?
The design keeps all mains inside a shrouded, enclosed section and senses through an isolated transformer module; the manual still mandates qualified supervision for any mains work.
What load can it switch?
The contactor is 20 A class (datasheet), which covers typical household appliances — fridge, TV, washing machine — one at a time in the demo.
Is this project suitable for a final-year project?
Yes — for Electrical and Electronics programs. It demonstrates protection principles, isolated sensing, embedded firmware and a real calibration procedure. Suitable for B.E./B.Tech final-year projects in Electrical and Electronics.
Components & software requirements
- Arduino Nano (ATmega328P)
- Isolated AC voltage sensor module (transformer-based)
- 5 V relay module + 230 V AC power contactor (20 A class)
- Digital panel voltmeter for live display
- Trimmer potentiometers for threshold setting
- Red trip-indicator lamp and reset push-button
- C/C++ firmware with RMS computation and hysteresis logic
- Enclosed prototype housing with shrouded mains terminals
Delivery information
Built-to-order project. Delivery timeline is shared after order confirmation based on current queue.
Support terms
Complete documentation, setup guide, and viva preparation included. Support for setup and explanation provided.
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