Candle at the Pool

Thunderbolt and Lightning

 The Kaminari Lightning Detector Pyramid I recently found an article about the AS3935 Franklin Lightning Sensor. As I am already recording some weather data, it immediately raised my interest.

The sensor module can be found at many online shops selling products from China. It is not really cheap, but still affordable. I decided to use an ESP8266 as microcontroller, so I can read the sensor data by WLAN. The sensor is connected to the ESP via SPI. There was also some space left for a SK6812 RGBW LED indicating the sensor status.

The result of this project can be found at GitHub. It's called Kaminari (which is Japanese for lightning), and also comes with OpenSCAD files for a 3D printed, pyramid shaped case with illuminated tip. In this article I will explain a bit about how I developed the Kaminari firmware.

The first problem was the calibration. The sensor is roughly pre-calibrated, but must be fine-tuned to 500 kHz ±3.5% via the TUN_CAP register. For this purpose, the antenna frequency can be routed to the IRQ pin and then be measured by the ESP. I chose to prescale the frequency by a ratio of 128, giving an IRQ frequency of 3,906.25 Hz. For measurement, I've set an IRQ handler that is just counting up a variable on each interrupt. I then reset the counter, wait for 1000 ms, then read the counter, and get the IRQ frequency in Hz units. It's not 100% accurate, but good enough for this purpose.

The TUN_CAP register offers 16 calibration steps. Just incrementing it until the frequency matches, would take up to 16 seconds. Instead I used an algorithm called successive approximation to find the correct calibration value in only 4 iterations, taking a quarter of the time.

 The AS3935 connected to an ESP8266 To my disappointment, it turned out that the manufacturer of my module (CJMCU) has used nonstandard components, so my module could only reach a maximum frequency of about 491 kHz. I first suspected that the ESP might be too slow for this kind of measurement, but a scope connected to the IRQ pin confirmed the frequency. Well, it is still within the required tolerance, but it gives a suboptimal tuning result and renders the TUN_CAP register useless.

The next problem is finding a good noise floor level. This is some kind of background radio noise filter. If the level is too low, the sensor cannot operate properly because of interfering background noise. If it is set too high, the lightning detection quality declines.

The noise floor level cannot be calibrated just once at the start. Radio noise sources come and go, may it by turning on an electronic device or just by a change in the weather. I did some experiments, and the most promising solution is a kind of tug-of-war. When the AS3935 detects too much noise, it triggers an interrupt, and the noise floor level is raised to the next higher step. If the last level change was 10 minutes ago, the ESP attempts to reduce the level by one step.

In order to reduce the number of level changes, I have added a counter. Each noise interrupt increments the counter, and every 10 minutes the counter is decremented. The level is raised when the counter reaches 2, and lowered when the counter reaches -2.

Sometimes I noticed a "noise level runaway", where the AS3935 triggers a lot of noise interrupts in a very short time, raising the noise floor level to its maximum value immediately. To stop that behavior, further noise interrupts are being ignored for one minute after a noise interrupt has been processed.

Now the noise floor level has settled to an average of 95 µVrms here. In the graph, one can see that the level is raised at some time, but then reduced again after a while. One can also see the frequent attempts to lower the level a bit further, immediately followed by a raise back to the average level. It seems that the AS3935 and the ESP have negotiated a good compromise. 😉

The AS3935 seems to be set up in an optimal way now, but I still get some false lightning events from time to time. There are a few registers left to experiment with, namely WDTH (watchdog threshold), SREJ (spike rejection) and MIN_NUM_LIGH (minimum number of lightning). I have raised the watchdog threshold to 2, and did not have a false lightning event since then.

Now I have to wait for some real lightnings… 😄

Premium Wall Bias Lighting, Part 3

I haven't forgotten about you. Some private stuff kept me from completing this project for a while. To make it up, I have added OpenSCAD files for a 3D printed case.

The controller was a little tricky to complete, mostly because of the very different component heights. I decided to use two circuit boards that are stacked onto each other by headers.

On the upper board, there are only the two buttons and the LCD, as well as the transistor and resistor for the LCD backlight. As I only used one-layer TriPad strip boards, I had to use this one upside down for the male headers to point downward. This rather unconventional use made it a little tricky to solder the buttons and LCD headers on the actual bottom side of the board.

The soldered controller boards. The lower board contains all the other components, as well as the wiring. The rotary encoder also made it to the lower board, because it is much taller than the other buttons. This way, the top of the button caps are almost level and nice to look at.

The result is surprisingly compact for a DIY solution. The button caps and the LCD are just perfectly positioned for a case.

With plastic feet attached, you can use the controller as it is. You can also get a plastic case with transparent top, drill three holes in it for the button caps, and mount the sandwich with spacers. But if you have the chance, you should definitely go for a 3D printed case.

I have set up a project at GitHub. It contains the circuit diagram, the bill of materials, the firmware source code, and OpenSCAD files for a printed case. There is no firmware binary yet, as you need to adapt the source code to the length of your LED strip anyway.

You will find the OpenSCAD files for the case in the GitHub project. There are bonus OpenSCAD files in the project, for printing a customized case. Due to the absence of properly layouted PCBs, I am aware that each controller is going to look differently when finished. In the parameter.scad file, you can change all kind of parameters, so you should be able to make your individual case in, well, almost any case (silly pun intended). 😄

The SPI flash memory of the Feather M0 Express is not used yet. In a future release, I may add a settings menu for the LED strip size. The controller is also forgetting all its settings when disconnected from the power. This needs to be addressed in a future release as well.

But after all, this is a start for your own DIY wall bias lighting. Feel free to send pull requests for enhancements!

Again, remember that you must remove the jumper before connecting the Feather to an USB port, otherwise your computer will be damaged.