Saturday, November 5, 2016

Smaller & lighter

While Totally not a bomb Mk II was useful for testing, it was a little too bulky even for the 1:10 car, so it was time to go shopping again...


  • 1x Raspberry Pi Zero 1.3
  • 1x 0.5mm pitch to 1mm pitch 15pin FPC cable
  • 1x TL-WN722N
  • 1x male RSMA connector with pigtail
  • some Kapton tape
  • enough 0.5mm sheet aluminium to make a box
  • ?x M2 screws
  • 1x XL6009 adjustable step up down Converter (DSN6000AUD)
The procedure:

1) Liberate the TL-WN722N out of it's plastic casing
2) perform UglyHack™ No.1 by desoldering the USB connector and solder on wires instead. Solder the other end fo the data lines into the USB port of the Rpi and solder the power to the power USB port of the Pi. PP1 is +5V, PP6 is gnd, PP22 is D+ and PP23 is D-. Twist the data lines to avoid weird shit from happening.
Grade-A hacking here!
the Kapton tape really is necessary...
Make liberal use of Kapton tape, covering all the exposed pads to avoid short circuits later.
3) Perform UglyHack™ No.2 by desoldering the RSMA connector and soldering on the pigtail one.
Again, Kapton tape is a must, not only as an electrical insulator, but also strain relief.

4) add the DC-DC power module (forgot pics, sry)
5) make an aluminium enclosure like so:
The other half is just a rectangular "U" shape, some of the holes are actually threaded.
6) Jam everything in, making sure that nothing is touching where it shouldn't.
In my implementation, the Pi is held a few mm away from the wall with screws and nuts on flexible washers. Some day I'll probably add a thermal pad between the main IC and the box, it does get warm.
The metal shield of the WiFi dongle is in direct contact with the wall to aid cooling, as it gets fairly hot when running with EZ-wifibroadcast. The DC-DC module is not even warm to the touch, so it can just float in the middle.
7) connect camera, strain-relive the cable so it doesn't get damaged.
Bottom scale is mm, top is inches.
Definitely could be made even smaller!

8) attach to desired mode of transport
Totally not a bomb Mk III
Camera holder needs to be adjustable...
Antennas for WiFi and RC
A few notes:
  • The antenna arrangement here is not ideal, the WiFi and RC systems interfere with each other, they should probably at least be further apart... (or I should use 5 GHz WiFi).
  • EZ-wifibroadcast on default runs outside of allowed WiFi bands (shhh, don't tell anybody...) and I keep it that way, there really isn't anything critical or even important using this band and it keeps the mutual interference with other WiFi to a minimum.
  • There are 5 GHz modules that are known to work with wifibroadcast if you want to avoid interference in the 2.4 GHz band.
  • The XL6009 module should safely operate between 4V to 30V. Pushing the voltage to the limits is asking for it.
  • The XL6009 is superior to LM2577 in that it runs at 400kHz instead of 50kHz, meaning the inductors can be much smaller.
  • If you feel lucky or actually know how to design a better DC-DC module, you can run both the dongle and Rpi directly on 3.3V. Be however aware, that if you fuck up, you'll fry them both.
  • If I ever make this smaller, I'll use one of the fancier DC-DC controllers that run at 1MHz, so the inductors are even smaller and the ripple is easier to filter out.
  • I advise using EZ-wifibroadcast as it boots much faster.
  • Last but not least, keep in mind that the sytem draws slightly over 2.5W and most of this is dumped as heat between the Rpi and the WiFi dongle, you have to allow air to flow around the box, otherwise it might overheat.

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