AJ-SR04M Waterproof Ultrasonic Distance Sensor - 2.5m Cable, 20cm to 8m Range

Specifications

• Model: AJ-SR04M

• Operating Voltage: 3V to 5.5V DC

• Operating Current: ~30mA to 40mA

• Standby Current: < 2mA

• Measuring Range: 20cm to 800cm (8 meters)

• Blind Zone (Dead Zone): < 20cm

• Operating Frequency: 40 kHz

• Cable Length: 2500mm (2.5 meters)

• Probe Diameter: 24mm (Housing outer width)

• PCB Dimensions: 41mm x 28mm x 15mm

• Interface Pins: 5V, Trig (RX), Echo (TX), GND

Key Applications and Projects

• Sump Pump / Tank Level Monitor: Mounting the waterproof probe inside the lid of a septic or water tank to continuously measure the liquid level.

• Automotive Proximity Alarms: Creating custom reversing sensors for large trucks or tractors that are exposed to rain and mud.

• Outdoor Perimeter Security: Detecting objects or individuals approaching a driveway or gate entry point.

Unlock New Capabilities (Project Evolution)

The Smart Rainwater Harvesting Hub: Stop manually checking your outdoor water barrels. Evolve your property's plumbing! Mount the waterproof AJ-SR04M probe inside the top of your main rainwater harvesting tank. Connect the blue control board to an ESP8266 or ESP32 microcontroller inside a waterproof junction box. Write a simple ESPHome script to translate the ultrasonic distance into a percentage representing tank volume. You can now push this data directly to your Home Assistant server via MQTT, allowing you to view your water reserves on your smartphone and set up automated alerts when the tank drops below 10%!

PACKAGE INCLUDED

• 1 x AJ-SR04M Ultrasonic Control Board

• 1 x Enclosed Waterproof Ultrasonic Transducer Probe (with 2.5m / 2500mm wire)

FREQUENTLY ASKED QUESTIONS (FAQ)

1. What is the difference between the AJ-SR04M and a standard HC-SR04?The HC-SR04 uses two exposed, non-waterproof transducers (one sends, one receives). The AJ-SR04M uses a single, fully enclosed waterproof transducer on a 2.5m cable that handles both sending and receiving, making it suitable for wet and outdoor environments.

2. Is the blue circuit board waterproof?No. Only the black transducer probe and its cable are waterproof. The blue control board must be kept dry inside a protective enclosure.

3. What is the minimum measuring distance?The module has a "blind zone" or "dead zone" of 20cm. It cannot accurately measure objects closer than 20cm.

4. What happens if an object is closer than 20cm?If an object enters the 20cm blind zone, the sensor will return inaccurate data, or simply report a flat 20cm / 0cm reading. Ensure your target is always at least 20cm away from the probe face.

5. What is the maximum measuring range?Under optimal conditions against a large, flat, hard surface, the sensor can measure distances up to 800cm (8 meters). Soft or angled objects will reduce this maximum range.

6. How long is the probe's cable?The black cable attached to the waterproof probe is exactly 2.5 meters (2500mm) long.

7. Can I cut, shorten, or extend the probe cable?It is highly recommended not to modify the cable length. Ultrasonic transducers are precisely tuned to the impedance and resistance of their factory cable. Modifying it can completely ruin the module's accuracy or stop it from working entirely.

8. What voltage does it require?The AJ-SR04M accepts a working DC voltage between 3V and 5.5V.

9. Can I use this with a 3.3V microcontroller like an ESP32 or Raspberry Pi?Yes. Because it supports down to 3V, you can power it directly from the 3.3V pin of an ESP32 or Raspberry Pi, and the Echo pulse will safely return a 3.3V logic level, protecting your board.

10. How do I wire this to an Arduino in default mode?Connect the module's VCC to the Arduino's 5V, GND to GND, Trig to a digital output pin, and Echo to a digital input pin. You can then use standard HC-SR04 libraries (like NewPing) to read the distance.

11. How do I change the operating mode?You change the operating mode by soldering a specific value resistor across the surface mount pads labeled "R19" on the blue circuit board.

12. What is the default mode if I don't solder anything to R19?If R19 is left open (factory default), the board operates in Mode 1: HC-SR04 compatible trigger/echo pulse mode.

13. How does the Automatic Serial Port mode (120k resistor) work?When a 120KΩ resistor is placed on R19, the board enters Mode 3. It will automatically and continuously measure the distance and send the data out over the TX (Echo) pin formatted as serial data at 9600 baud.

14. What is the Low Power Serial mode (47k resistor)?With a 47KΩ resistor on R19 (Mode 4), the device sleeps to save power. When your microcontroller sends a specific byte (0x01) to the RX (Trig) pin, the sensor wakes up, takes one measurement, sends the result out over the TX (Echo) pin, and goes back to sleep.

15. How wide is the ultrasonic beam?The module emits an ultrasonic pulse with a beam angle of approximately 50 to 75 degrees.

16. Can this measure water levels inside a tank?Yes, this is one of its primary use cases. Mount the probe at the absolute top of the tank facing straight down at the water.

17. Will condensation on the sensor face affect readings?While the probe is waterproof, large beads of water, heavy condensation, or ice forming directly on the flat face of the transducer can scatter the sound waves and cause erratic readings. Periodic cleaning or angled mounting to encourage run-off may be required.

18. What is the white connector on the board for?The white JST-style connector on the top of the blue PCB is the port where the 2.5m sensor probe cable plugs into the control circuit.

19. Are the Trig/RX and Echo/TX pins shared?Yes. In the default Mode 1, they act as the standard Trigger and Echo pins. If you solder a resistor to R19 to enter a Serial mode, the Trig pin becomes the RX (Receive) pin, and the Echo pin becomes the TX (Transmit) pin.

20. Does this sensor work through glass or plastic walls?No. Ultrasonic sensors rely on high-frequency sound waves bouncing off physical objects. If you place the sensor behind a pane of glass or a solid plastic wall, it will simply measure the distance to the glass (which will likely be within its 20cm dead zone, causing failure). The probe needs an unobstructed path through the air to the target.