Interfacing SW-420 Vibration Switch Module with Comparator with Arduino Uno R3

The SW-420 vibration sensor module is a tiny, low-cost board that tells your microcontroller when something has been shaken, knocked or bumped. It has three pins, no library and no configuration: when vibration crosses a threshold you set with a small screw trimmer, the digital output changes state.

That simplicity is what makes it useful. Typical projects include a knock-activated doorbell, an alarm that fires when a box, bike or cabinet is moved, a counter that registers each time a machine thumps, and a "wake up" trigger that lets a battery project sleep until something happens.

It is important to know what the module is not. It does not measure how hard or how fast something shakes, and it cannot tell you the direction or frequency. It answers one question only: did the vibration cross my threshold or not? If you need numbers, use an accelerometer such as the ADXL345 or MPU-6050 instead.

How the module works

The SW-420 sensor is a small spring-type switch that is normally closed. When the board is still, the switch sits in one state. When it is shaken or knocked, the contacts flicker for a moment, producing a brief electrical disturbance.

That disturbance alone is too small and ragged for a microcontroller to use, so an LM393 comparator cleans it up. It compares the signal from the sensor against a reference voltage set by the blue trimmer. When the signal crosses the reference, the comparator output flips and the DO pin changes state. The second LED on the board lights to show this.

That is why the trimmer is called a threshold control rather than a calibration. Turn it one way and a gentle tap triggers the output; turn it the other way and only a hard knock does. The module is inherently a yes or no detector, not a measuring instrument.

Which level means vibration?

Documentation for this module generally describes the output as low at rest and high when vibration is detected, but cheap clones do not all follow that. Some boards are inverted, and the behaviour can change with the trimmer setting. Never assume. Upload the sketch below, watch the Serial Monitor and the output LED, and note which level appears when you tap the board. The code uses a single constant so you can flip it in one place.

Setting the sensitivity

1. Mount the module in its final position first. How it is fixed changes how vibration reaches the sensor, so tune it where it will live.
2. Power it up and let the output settle at rest.
3. Turn the trimmer slowly with a small screwdriver until the output LED just stays in its resting state.
4. Tap the surface where you expect the event. The LED should flash. If it does not, back the trimmer off a little at a time.
5. Check that normal background vibration, such as a fan or footsteps, does not trigger it.

Small turns make a big difference, and the direction that raises sensitivity differs between boards, so go gently and watch the LED.

Basic sketch: count vibration events

This sketch checks the pin in the main loop and ignores repeats for half a second, so a single knock is counted once rather than dozens of times. Change ACTIVE_LEVEL to LOW if your module is inverted. On an ESP32, change SENSOR_PIN to the GPIO you wired.

Interrupt version for fast or low-power projects

Polling can miss a very short pulse if your loop is busy, and it keeps the processor awake. An interrupt reacts the instant the pin changes and lets you do other work, or sleep, in between. On an Uno or Mega use pin 2 or 3 (the Mega also has 18 to 21). On an ESP32 any GPIO works, but add IRAM_ATTR before the handler. Use RISING for an active-high module and FALLING if yours is inverted.

Using it well

1. Fix it firmly. A loose board makes its own vibration and triggers falsely.
2. Tune in place. Always set the trimmer in the finished enclosure, not on the desk.
3. Debounce in software. One knock produces several quick transitions, so ignore repeats for a short time.
4. Match the logic voltage. Power it at 3.3 V on 3.3 V boards.
5. Combine it with an output. An active buzzer turns it into a simple alarm in a few lines of code.
6. Use an interrupt for battery projects. Let the processor sleep until the sensor wakes it.

Frequently asked questions

Does it have an analog output? No. Only the digital DO pin is provided on this module, so you cannot read the strength of the vibration.

Can it detect an earthquake? It can react to strong shaking, but it is far too crude and unreliable to be used as an earthquake detector.

Why is the output inverted on my board? Different manufacturers wire the comparator differently. Check the resting level with the Serial Monitor and set ACTIVE_LEVEL to match.

Can I power it from 3.3 V? Yes. The module is rated for 3.3 to 5 V, and 3.3 V is the right choice for ESP32 and Raspberry Pi projects.

How sensitive is it? It depends on the trimmer and on how the module is mounted. There is no calibrated figure, so test it against the event you care about.