For now, this post is going to contain a few notes regarding a solar powered MySensors node that I put together last weekend. This may not work, so the post will contain a basic circuit and readings from the setup over the next couple of weeks.

Basic Idea

Hooking together Solar (PV) Panel + Supercapacitor + MCU1703 + Arduino

The idea is to power a MySensors node (Arduino Mini Pro with NRF24L01+ radio) using a small solar panel. A Supercapacitor will be used in place of a rechargeable battery. The Solar Panel and Supercapacitor must be carefully balanced as voltage into the Supercapitor shouldn’t exceed the rated value.

The circuit will include a really basic resistor divider so that the arduino can measure the input voltage to the regulator. The sketch is setup to send back the voltage on the AO pin every 5 mins (more about this later). Here is a basic rundown of the parts:

Solar (PV) Panel: 6v 1W rating, 8cm x 5cm, v.cheap!

Supercapacitor: 5.5v rating, 4F.

Regulator MCP1703: Converts input voltage to regulated 3.3v output. Uses only 2uA (micro amps) when there is no load!

Resistor Divider: Simple resistor divider using spare 100K resistors. Converts 5.3v to 3.3v. Sensed on A0 pin on Arduino

Arduino Mini Pro 3.3v: Arduino Mini Pro with Connected NRF24L01+ radio. Running 2.1.0 MySensors code. Radio links to my gateway which pushes messages to MQTT broker and then this blog using MqttCogs

It’s midwinter here. Very cloudy, no sun and lots of rain. Ideal time to check out a solar panel powered Ardiuno!

Prototype

….and this is what it looks like. All pushed together on a breadboard.

Here’s a list of questions that I want answered over the next few days/weeks.

• Solar Panel voltage after diode must be less than rating for Supercapacitor. Need to check voltage drop across diode when no current is drawn
• Need to check Arduino current draw with radio on, radio off and when sleeping
• Need to check Solar Panel current and voltage in different conditions
• Need to record how quickly Super Capacitor discharges when no solar panel attached
• Need to record charging rate of Super Capacitor in different conditions
• Need to test behaviour of system when Super Capacitor is unable to drive the Arduino properly

Live Super Capacitor Voltage

Here’s a couple of graphs using MqttCogs. The first shows the Super Capacitor voltage in the last 2.4 hours, the second the voltage in the last 24 hours.

[mqttcogs_drawgoogle options="{series: {0:{labelInLegend: 'Capacitor Voltage(mV)'}}, animation:{startup:true,duration:5},curveType:'function',title:'(98) Supercap voltage mV (last 2 hours)',height:200, hAxis: { title: 'DateTime(UTC)',format:'dd MMM HH:mm' }, vAxis: { title: 'Voltage (mv)' }}" charttype="LineChart" ]]
[[mqttcogs_data from="-0.2"  limit="999" topics="mysensors_out/98/0/1/0/38"/]][[/mqttcogs_drawgoogle]

[mqttcogs_drawgoogle options="{series: {0:{labelInLegend: 'Capacitor Voltage(mV)'}}, animation:{startup:true,duration:5},curveType:'function',title:'(98) Supercap voltage mV (last 24 hrs)',height:200, hAxis: { title: 'DateTime(UTC)',format:'dd MMM HH:mm' }, vAxis: { title: 'Voltage (mv)' }}" charttype="LineChart" ]][ [mqttcogs_data from="-1"  limit="999" topics="mysensors_out/98/0/1/0/38"/]][[/mqttcogs_drawgoogle]