Category Archives: Solar Powered Router

SolarWind updates: MPPT, Carambola2

During the past month, I mainly concentrated on combining my CurrentMonitor with a DIY MPPT charge controller. Work is done at the CurrentMonitor repository.

My design goals for the MPPT charge controller are:

  • 10A (sufficient to cover 250W panels)
  • MPPT charge controller acts as an I²C slave
    • It can be configured via I²C
    • Values like power consumption or power harvested can be queried via I²C as well
    • The MPPT doesn’t have a user interface or LCD by default
  • Uses APR/ATC/ATO blade fuses instead of solar fuses due to the reduced cost (and regular glass fuse holders only are widely available up to 6.3A)
  • +12V Output can be disconnected via software
  • Additional +3.3V 800mA output which also can be disconnected via software

Below is a picture of the prototype, which is likely to heavily change.

IMG_20130716_002117 IMG_20130716_002108

DIY Directional WiFi Antenna + Tests

IMG_20130528_020617IMG_20130528_195107Yesterday I stumbled over the IKEA Baren Toilet Brush – yes, I do keep an eye on toilet brush holders since nazco and myself are planning a directional WiFi link. The box said 10cm diameter, which is a bit too much according to this article, however, once I arrived at RaumZeitLabor I was surprised that it is “only” 92mm in diameter – the optimal size.

The setup consisted of a TL-WR741ND on our balcony as well as an external Logilink WiFi adaptor which I used to measure stuff along my 2 hour trip.


Test Point 1


The TL-WR741ND sat on our balcony, approx 8m above ground level. It was equipped with the stock stub antenna. All tests were done using an external USB WiFi stick (Logilink, unknown model, rt73usb driver). For all test locations, I did the test with the stock stub antenna and my directional antenna.

I’ve used iperf to measure the throughput (iperf server on the TL-WR741ND, and iperf client on my notebook). I could have done send/receive stats, but at first I thought that it wouldn’t work so well if I had only one directional antenna – I was wrong! Performance was much better than expected, even at over half a kilometer I had nearly the same performance using only one directional antenna as I had when I was at home).


Test Point 3

Big Win: With only one directional antenna, I was able to achieve 18.3 Mbit/s over half a kilometer with a non-optimal line of sight, which is only 4 Mbit/s less than placing both antennas next to each other.

You can find my measurements in the following file: Messwerte.ods (20kb or so)

Additionally, I have visualized the test points on Google Maps.

Note that performance is sometimes less because of trees in the line of sight, and the antenna isn’t constructed in an optimal way. Additionally, my hand is a bit shaky, so there’s room for some additional percent of performance.


Test Point 4


Test Point 6

Autonomous WLAN Router Experiment: Part 3 – Up and Running

After hacking the software for the CurrentMonitor, I finalized the case as well as the wiring. I’ve built a frame which holds the solar cell at 33°. I might need to move the cell to get the best performance, but I already got 300mA out of it when the sky was cloudy. I’m curious which performance I’ll get on a sunny day.

I also held a talk at RaumZeitLabor (Slides) and I implemented a cosm feed to publish all data of the cell. If you have questions, contact me.

Solar Powered Router CaseSolar Powered Router: Panel



Autonomous WLAN Router Experiment: Part 2

I’ve bought some hardware in order to build up the experiment:

  • One monocrystalline 20Wp solar cell (25€)
  • A solar charge controller (15€)

I’ve also built a current/voltage monitoring board, which allows monitoring of the power generated by the solar cell as well as power drawn by the peripherials. Because it also monitors voltage, we can make assumptions of the battery charge status. It uses two INA138 current shunt monitors, together with two 0.1Ω 0.5W 1% precision shunt resistors and a ATMEGA168 (no low power, that’s the one I got at hand). The power monitoring circuit draws less than 10mA at 5V. I have to measure the exact power consumption,   I only recall that the bench PSU was showing 0.00A and my Fluke 27 did show something like 5mA, but I haven’t written down the exact numbers.

So much cool things and then: kaboom! I accidently connected the step-down converter backwards (12V to the output and the 5V router to the input), which delivered 12V straight to the router, which then refused to work. As I couldn’t find any fuse on the router, I assume it is totally bricked and I’ll most likely go with a Carambola2 board, which also requires only 0.5W power.

In the meanwhile, I’ll use a carambola board which uses 1.5W power, but that should be okay until my Carambola2 board arrives.

The current monitoring board can be found at

Autonomous WLAN Router Experiment: Part 1

I’ve been wanting to build an autonomous WLAN router for quite some time. That WLAN router should be powered by a solar cell and a battery. I have spoken about the TL-WR702N at RaumZeitLabor a while ago, and it is very small, draws 150mA max and is therefore suitable for such a project.

Some time has passed, and yesterday Doozer gave two Lead–acid batteries to me, rated 12V 5Ah each. So now I have to do this project!

I’ll use one battery for the project. I have made the following assumptions:

Ed: Average daily electricity production (kWh)
Em: Average monthly electricity production (kWh)

  • The router will permanently draw 150mA at 5V, or 0.75W (worst-case)
  • The solar charger circuit will have a power requirement of 15mA at 12V (0.075W)
  • The DC-DC converter I’ll be using has an efficiency of 80% (worst-case assumption, will probably be much higher)
  • To be on the safe side, all further calculations assume that our overall power usage is 2W
  • Overall yearly power: ~18kWh, 1.5kWh per month
  • Using the interactive photovoltanic calculator (linked below) I was able to calculate that a 20Wp cell is not sufficient from October till February.
  • As we assumed the worst-case scenario, and this is an experiment only, I decided to go with two 20W panels, where I’ll experiment with one panel, then two if it’s not sufficient.

I now have to buy the solar panels and the charge controller, which I’ll do in may.

Useful resources: