Raspi-Sump

Voltage Divider

The purpose of a voltage divider is to reduce the amount of current sent from one component to another. As shown in Figure 1, I soldered a 470R Ohm resistor on the Echo wire and bridged a 1K Ohm resistor between the Echo and Ground wires. This prevents blasting 5V to a pin that is rated only for 3.3V. With these resistors, voltage is actually a touch higher at 3.4V, which is within a tolerable level. All soldered connections are covered with heat-shrinking tube to avoid electrical shorts.

Calculating resistor types required is beyond the scope of this article, but there are many handy Web-based voltage divider calculators available to determine your requirements. In this example, a 1K and 2K Ohm resistor would reduce the current to 3.333V.

Wiring

The Raspberry Pi is connected to the sensor with a five-foot length of CAT5 cable. Because there are four connections, only four of the eight twisted wires are used. On each end of the selected wires, I soldered connectors that were compatible with the sensor pins and the pins on the Pi. An old 3.5" floppy drive power connector works great for the sensor connection (Figure 2). I used a couple two-pin PC case-fan connectors, salvaged from an old PC, for the connections on the Pi's pins. These connectors are available on-line, but anything you can salvage from an old PC works great.

Figure 2. Floppy Connector

Mounting

The HC-SR04 is attached to a plastic case and screwed onto a piece of wood strapping. The wood strapping is inserted into the sump pit facing downward and is easily adjustable and removable if needed. The Cat5 wire is securely taped to the sump pump's ABS pipe and an open wall stud to prevent tangling and disconnection of the wire when removing the sump pit lid.

Figure 3. Open Sump Pit View

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