Monthly Archives: February 2017

Solar + Li-Ion Power Supply Unit

This week we started  testing our brand new solar charger PSU design. The solar PSU powers a WiSense WSN1120L node operating in full function device (FFD) mode. FFDs are involved in mesh  routing and therefore need to keep their radio on at all times.  The WSN1120L’s current consumption in this mode is around 31 mA  in normal receive mode and  around 50 mA  when transmitting at +13 dBm.

The solar PSU also supports continuous measurement of the panel voltage and current as well as the battery voltage and current. This data is being sent every 5 minutes to the  cloud. See the graphs at the end.

The  PSU supports panels with voltage output up to 10.5V. The PSU allows charge current of  up to 2 A which allows high capacity Li-Ion batteries to be charged by high wattage panels.



Test Panel Specs

  • Peak  power: 3  Watts
  • Voltage output at peak power point: 8.5V
  • Current output at peak power point: 300 mA

Test Battery Specs

  • Chemistry:  Single  cell Lithium-Ion Battery
  • Capacity: 1100  mAh
  • Output voltage: 3.7 V  (nominal), 4.2 V (full charge)

Charger PSU Board Specs

  • Max  input voltage:  10.5 V
  • Max charge current: 2  A
  • Max  battery discharge  current: 4 A
  • Li-Ion battery charged in 3 phases (trickle charge,  pre-charge, constant  current and constant voltage).
  • Battery under-voltage lockout supported as load is not connected directly to battery.
  • Charger IC can power the load and charge the battery simultaneously.
  • Multiple output voltages
    • 3.3V  (Max 1 A)
    • 4.9V (Max 50 mA)
    • Li-Ion battery output (Max 4A). This supply is gated by a load switch which can be controlled by a signal external to the PSU (for example – by an external micro-controller).
  • On board  current and  voltage sensors which measure the following  parameters:
    • Solar  panel output voltage (Available over I2C)
    • Solar  panel output current  (Available over I2C)
    • Battery voltage  (Available over I2C)
    • Battery current  (Available over I2C)
  • PCB specs
    • Layers: 4
    • Dimensions:  53 mm x 48 mm
    • Mounting holes:  4
    • Finish:  ENIG

Here is  a pic of  the  setup. You can  see the 3W  panel lying flat and connected to a weather proof enclosure containing the PSU, battery and an WSN1120L. This location is not the best with tall structures/buildings in the vicinity.


Snapshot of battery voltage and current (captured from WiSight running on AWS).



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RS-485 interface for water meters

This week we tested a new RS-485 board to be used for wiring water meters for apartments. This is in addition to the wireless water meter solution we developed last year using the WSN1120L wireless mesh nodes.

In apartment complexes, water meters are usually one below the other in one or more plumbing/utility shafts. Wiring meters using a multi-drop bus such as RS-485 is a  more  cost effective and reliable solution in this case.

In India, it is common for apartment complexes to have multiple water supply entry points for each apartment. This means we need multiple water meters for each apartment. These entry points are usually not close to each other. Multiple RS-485  strings are therefore required to connect all these meters. All this adds to the cost of the overall solution.

We have so far tested 1  master  RS-485 node and 2 slave RS-485 nodes.  Each master node can collect metering data from up to 31 slave nodes.  A slave node can  accommodate two water  meters. The master node will have a wireless interface (WiSense Sub-GHz radio or GSM/GPRS module) for cloud connectivity. Even if there are multiple RS-485  strings, only one master  node needs to have GSM/GPRS connectivity.  All the other master nodes will connect to this one over the WiSense Sub-GHz radio network. Broadband internet connection (if available) can be used  instead of GSM/GPRS.

Here are a  few pictures of our initial test setup. You  can see the master connected to a  slave through 50 feet of shielded twisted pair  CAT-5E cable.


Here is a closer  look at our RS-485 interface board.  Each board has a micro-controller, an RS-485 transceiver providing half-duplex communication and a 5 bit dip switch for assigning address (1 to 31) with 0 reserved for each string’s master node. All the nodes are daisy  chained. Power is supplied through the master node over the CAT-5E cable.


Here is a pic of  a water meter connected to a slave board.


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WiSense Wireless Node for Agriculture

We have developed a multi-sensor wireless node for agriculture related monitoring and control applications.

This sensor node is  based on the WiSense  WSN1120L sub GHz wireless mesh node.  This “agri node” operates in the license free 865-867 MHz in India. The nodes have  a max transmit power of  +14  dBm and a range of about 1 KM (LOS) and around 0.5 KM  (non LOS).

The node includes the sensors listed  below:

  • Relative Humidity (CC2D33S)
  • Temperature (CC2D3SS,  LM75B and an NTC thermistor)
  • Ambient  light (TEPT5700)
  • Atmospheric pressure (MS5637)
  • Soil temperature (based on an NTC thermistor)
  • Soil Moisture (Capacitive)

We can add  more sensors based  on customer requirement and energy budget. The selected sensors are relatively low cost.

The node can be  operated as an  FFD (takes part in mesh routing) or as an RFD (no routing). In the RFD configuration, the sleep mode current consumption  is  around 2 microamps which enables the node to run on limited energy sources such as a pair of AA/AAA batteries, small solar panels and low capacity  rechargeable batteries etc.

The WSN1120L,  battery and related electronics is protected against the elements by a weather proof  enclosure  (IP65/ABS).  The humidity, pressure  and temperature sensors are protected by a radiation sensor. These external sensors connect to the  WSN1120L through a weather proof 4 wire cable.  Similarly the soil temperature and soil moisture sensors connect to the WSN1120L through a 2 and 4  wire cable respectively.

In addition, the “agri node” can be used to control valves, motors etc using add on relay boards.

The coordinator node (WSN1120CL) can be interfaced to the available back-haul mechanism such as GSM, 3G, WiFi etc.

We are currently testing a solar  + single cell lithium-ion power supply  unit  which can keep these nodes running  for at least 1 year without any  need to  replace the  battery.

Here is a pic of  a WiSense coordinator node and an “agri node”.



Here is a pic of  another  “agri node” with an enclosure having a transparent hinged lid. You can see the white colored radiation shield enclosing the three sensors mentioned  above.



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