Monthly Archives: April 2015

Interfacing the INA219 current sensor to a WiSense Node

The INA219 (from TI) is a high side bi-directional current sensor with I2C output. It measures the voltage across a shunt resistance. It can also measure the supply voltage.

We are using the INA219 to monitor the battery pack in a solar powered light. During the day , the solar panel charges the battery. At night, the battery powers the light (string of LEDs). This requires bi-directional current measurement.

The INA219 can measure both the current (into the battery or out of the battery) as well as the battery voltage. The INA219 allows battery current to be measured using high side sensing. High side sensing and low side sensing are two different concepts.

We have a customer who wants to monitor the battery on a group of street lights. These lights will be part of a WiSense mesh network. The voltage/current data from each light will be forwarded to a gateway node comprising a Raspberry Pi and a GSM modem (there is no internet in the area where the lights are going to be deployed). Software on the Pi will upload the data to the cloud.

solar_batt_current_sensing

In the figure above, the WiSense node and the INA219 are shown to be powered from a different power source (couple of AAA batteries for instance).  The Battery pack ground and the INA219 ground need to be connected only of the INA219 is used to measure the battery voltage. It is not required to tie the two grounds together if only shunt voltage (across the shunt resistance) needs to be measured.

Here is a pic of the PCB with a 0.1 ohm shunt resistor mounted.

ina219_pcb_pic

We tested the PCB at very low currents (around 20 mA). The INA219 is reporting the expected shunt voltage value. We will be testing the PCB with one of the solar street lights when we get the equipment. The battery current is expected to be around 300 mA to 600 mA.

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Hardware Update – WiSense Gateway Box

We built a couple of gateway boxes for a customer. Each gateway has a WiSense node and a Raspberry Pi. The box can connect to a router through ethernet. The box is powered through a USB mini-B connector. The USB interface also provides serial connectivity to the Raspberry Pi. This gateway has a CC2D33S temperature and humidity sensor.

Here are some pics.

wi_gw_box_1

ws_gw_box_4

ws_gw_box_3

ws_gw_box_2

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Soil moisture monitor using the WSN1101L (Low profile mesh node)

The low profile of our new offering (WSN1101L) allows it to be easily integrated with the Chirp soil moisture sensor. I glued the WSN1101L to the top of the moisture sensor and connected the two with 5 wires (Vcc, ground, I2C-SDA, I2C-SCL and RESET). The moisture sensor “stick” has a coin cell retainer which can hold a 20 mm CR-2032 coin cell. This coin cell powers both the moisture sensor and the WSN1101L. The WSN1101L has a flexible whip antenna connected to a U.FL connector on the radio board.

Here are some pics.

lcn_chirp_5

lcn_chirp_3

lc_chirp_2