Avian’s Blog

Speaking of Munin, here's a thing that I've made recently: A small stand-alone display that cycles through a set of measurements from a Munin installation.

(Click to watch Munin display video)

Back when ESP8266 chip was the big new thing I ordered a bag of them from eBay. The said bag then proceeded to gather dust in the corner of my desk for a year or so, as such things unfortunately tend to do these days. I also had a really nice white transflective display left over from another project (suffice to say, it cost around 20 £ compared to ones you can get for a tenth of the price with free shipping on DealExtreme). So something like this looked like a natural thing to make.

The hardware is not worth wasting too many words on: an ESP8266 module handles radio and the networking part. The display is a 2-line LCD panel using the common 16-pin interface. An Arduino Pro Mini acts as glue between the display and the ESP8266. There are also 3.3 V (for ESP8266) and 5 V (for LCD and Arduino) power supplies and a transistor level shifter for the serial line between ESP8266 and the Arduino.

ESP8266 runs stock firmware that exposes a modem-like AT-command interface on a serial line. I could have omitted the Arduino and ran the whole thing from the ESP8266 alone, however the lack of GPIO lines on the module I was using meant that I would have to use some kind of GPIO extender or multiplexer to run the 16-pin LCD interface. Arduino with the WeeESP8266 library just seemed less of a hassle.

From the software side, the device basically acts as a dumb display. The ESP8266 listens on a TCP socket and Arduino pushes everything that is received on that socket to the LCD. All the complexity is hidden in a Python daemon that runs on my CubieTruck. The daemon uses PyMunin to periodically query Munin nodes, renders the strings to be displayed and sends them to the display.

Speaking of ESP8266, my main complaint would be that there is basically zero official documentation about it. Just getting it to boot means reconciling conflicting information from different blog and forum posts (for me, both CH_PD and RST/GPIO16 needed to be pulled low high). No one mentioned that RX pin has an internal pull-up. I also way underestimated the current consumption (it says 1 mA stand-by on the datasheet after all and the radio is mostly doing nothing in my case). It turns out that a linear regulator is out of the question and a 3.3 V switch-mode power supply is a must.

My module came with firmware that was very unreliable. Getting official firmware updates from a sticky forum post felt kind of shady and it took some time to get an image that worked with 512 kB flash on my module. That said, the module has been working without resets or hangs for a couple of weeks now which is nice and not something that all similar radio modules are capable of.

Finally, this is also my first 3D printed project and I learned several important lessons. It's better to leave too much clearance than too little between parts that are supposed to fit together. This box took about four hours of careful sanding and cutting before the top part could be inserted into the bottom since the 3D printer randomly decided to make some walls 1 mm thicker than planned. Also, self-tapping screws and automagically hollowed-out plastic parts don't play nice together.

With all the careful measuring and planning required to come up with a CAD drawing, I'm not sure 3D printing saved me any time compared to a simple plywood box which I could make and fit on the fly. Also, relying on the flexibility and precision of a 3D print made me kind of forget about the mechanical design of the circuit. I'm not particularly proud of the way things fit together and how it looks inside, but most of it is hidden away from view anyway and I guess it works well enough for a quick one-off project.