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Hardware

Supported Sensors

Raspi-Sump works with the HC-SR04 ultrasonic distance sensor and compatible variants. For sump pit use, the JSN-SR04T v2.0 waterproof version is strongly recommended.

Sensor Notes
HC-SR04 Standard sensor, not waterproof
JSN-SR04T v2.0 Waterproof, recommended for sump pit use
JSN-SR04T v3.0 Not compatible — different protocol

JSN-SR04T v3.0 — Do not use

Version 3.0 uses a different communication protocol and is not compatible with Raspi-Sump. Verify the version before purchasing.

Voltage Divider Requirement

The HC-SR04 and JSN-SR04T sensors operate at 5V and return a 5V signal on the echo pin. The Raspberry Pi GPIO pins are 3.3V tolerant only — a 5V echo signal connected directly will damage your Pi.

A voltage divider on the echo pin is required to reduce the signal to 3.3V.

Simple Two-Resistor Divider

Voltage Divider

A 470 Ohm and 1K Ohn resitor divides the 5V echo signal to approximately 3.3V.

A 1K Ohm and 2K Ohm would accomplish the same things. There are many online voltage divider calculators you can use to test various settings.

Real world example

Real World Voltage Divider

The 470 Ohm resister is on top and the 1K resistor is on the bot tom. The wires to the right go to the Pi and the ones to the left go to the sensor. The orange wire is the Echo wire and you can see it is connected right at the center of the two connected resistors. The yellow wire goes to Ground on the Pi.

There are plenty of diagrams online explaining voltage dividers, but this picture provided by github user @rhiller was very helpful to see in practice.

Mounting Inside a PVC Pipe

Mounting the JSN-SR04T within a guide pipe significantly improves reading precision and reproducibility. This is known as a stilling well, a design borrowed from industrial tank-level sensing.

The pipe acts as a waveguide: it doesn't focus the sound so much as strip away everything except the wave energy traveling nearly parallel to the pipe's axis. The sensor's ultrasonic pulse normally radiates as a wide cone (30°–75° depending on spec/version) that, in a crowded pit, reflects off the walls, the pump, and surrounding pipes, producing false-target echoes. Confined to a pipe, only the near-axial portion of the wavefront survives, traveling straight down the bore to the water and straight back, so the return echo is dominated by one clean, perpendicular reflection instead of a noisy mix of others. As a side benefit, the pipe also calms the water surface it opens onto, which helps since still water reflects specularly rather than diffusely and any ripple would otherwise deflect the echo away from the sensor.

Note that this setup will cause the max distance read to always be the length of the pipe. This is due to the off-axis wave energy which gets stripped away due to internal reflections off the pipe wall back toward the transducer, arriving out of phase with the direct echo. Destructive interference degrades the return signal enough that the sensor effectively can't range beyond the bottom of the pipe.

In the photos below, a 26" section of 1-1/2" PVC was used together with a corresponding PVC cap. A 7/8" paddle drill bit is the perfect size for the sensor. Friction alone is used to hold the sensor in place.

Sensor recessed at the top of the pipe bore

Interior of the mounted sensor in the PVC cap.

Full pipe assembly

The finished assembly.

Sensor secured in a basin coupling fitting

The sensor probe is passed through the sump pit cover via a pre-drilled hole which holds it in place.

GPIO Wiring

Any available GPIO pins can be used for trig and echo. The defaults are:

Signal Default GPIO Pi Header Pin
Trig 17 Pin 11
Echo 27 Pin 13
VCC (5V) — Pin 2 or 4
GND — Pin 6

BCM numbering

Raspi-Sump uses BCM GPIO numbering, not the physical pin numbers printed on the Pi header. Set your chosen pins in raspisump.conf under [gpio_pins].

Tip - Use pinsource to test your sensor

Test your sensor with the pinsource utility;

pinsource -t 17 -e 27

Sensor Placement

  • Mount the sensor at the top of the sump pit, facing downward
  • Set pit_depth in raspisump.conf to the distance from the sensor face to the bottom of the pit
  • Ensure the sensor has a clear line of sight to the water surface — avoid mounting near pipes or walls that could cause false reflections
  • The JSN-SR04T v2.0 has a minimum sensing distance of approximately 20cm — do not mount closer than this to the maximum expected water level

Raspi-Sump runs on any Raspberry Pi 2 or greater with Raspberry Pi OS Bookworm (v12) or Trixie (v13).