JSN-SR04T / AJ-SR04M Waterproof Ultrasonic Distance Sensor

The JSN-SR04T (also sold as the AJ-SR04M) is a waterproof ultrasonic distance sensor with a sealed probe on a cable, so you can mount the transducer outdoors or in wet spots while the control board stays protected. It works like the classic HC-SR04 using trigger and echo pins, but also supports UART serial output, making it a drop-in choice for Arduino, ESP32 and Raspberry Pi projects.

Popular for water tank level monitoring, parking sensors, bin fill detection, robot obstacle avoidance and outdoor presence detection.

Features

• Waterproof sealed probe for wet and outdoor environments
• Measures 20 to 600 cm with 1 mm resolution
• Wide 3.0-5.5V supply, so it runs directly from 3.3V boards like the ESP32
• Low power, under 8 mA working current
• Three selectable output modes: trigger/echo pulse, automatic UART, or UART on request
• Onboard LED shows when a measurement is taken

Wiring

• 5V: power supply positive (3.0-5.5V)
• Trig: trigger input (RX in serial modes)
• Echo: echo output (TX in serial modes)
• GND: power supply negative

Output logic level matches the supply voltage, so power it from 3.3V when connecting to a 3.3V microcontroller.

Output Modes

The mode is set by the R27 resistor position on the board.

Using Pulse Mode

Distance = (echo high time x speed of sound) / 2. In practice, distance in cm is roughly the echo time in microseconds divided by 58. Allow at least 50 ms between measurements so echoes from the previous pulse have faded. If nothing is detected, the echo pin times out and returns low.

Using Serial Mode

UART runs at 9600 baud, 8N1. See our Arduino, ESP32 and Raspberry Pi pinout reference for which UART pins to use on your board. Each reading is a 4 byte frame:

• 0xFF: start byte
• H_DATA: distance high byte
• L_DATA: distance low byte
• SUM: checksum, the low 8 bits of 0xFF + H_DATA + L_DATA

Combine H_DATA and L_DATA for the distance in millimetres. For example, the frame FF 07 A1 A7 gives 0x07A1 = 1953 mm. A reading of 0 means nothing was measured or the target is out of range.

Notes

• Only the probe is waterproof. Keep the control board in a sealed enclosure.
• Objects closer than 20 cm fall in the blind zone and can't be measured. For short range measurement, use a VL53L0X time of flight sensor instead.
• The 600 cm maximum assumes a large, flat, hard target. Soft, angled or small objects return weaker echoes and reduce range.

Specifications