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a blog of things that I (eventually) make work

Wednesday 9 March 2016

Smart Mirror - Custom composite cable

Yesterday I mentioned that the composite video cable for the Raspberry Pi that I bought was far too long. It serves its purpose well, but at 3m long it would be a bit ridiculous to include it in the final build of the frame.

The newest models of the Pi (including the 3, which I recently bought but have yet to even power up...) do not include a full size RCA jack for composite video like previous models did, but instead use a three-ring 3.5mm jack, providing combined stereo audio and video, similar to some old camcorders or the original iPod video.

To make my own cable, I was going to need to source one of these jacks from somewhere. I didn't want to cut up my iPhone headphones, so went off to Poundworld in the hope they'd have a cheap set that included a microphone (which use the same jack, except the extra connections are for the mic). I didn't find any of these, but as luck would have it, I did find a remote shutter release for smartphones, a bit like this one. This had a three ring jack attached to it, so I bought one and cannibalised it.

Here's the cable I built during testing:

(Yes it's a bodge, but it works!)

I later epoxied the crap out of both ends of the cable to allow for some strain relief.

As ever with projects like this, I've thought of a bunch of other features I want to add (feature creep again...) so the new to do list is even longer than yesterday:
  • Build a custom short composite video cable for the RPi. The one I bought is far too long for this and could be the source of some of the signal problems I've been having with the screen, so a shorter custom one should fix this. DONE
  • Add in a 19V -> 5V converter module. This is already ordered but is shipping directly from China. Because the screen needs 12-24V DC for power, I'm using an old universal laptop power supply for it. I'd like to use the same source to power the Pi, so that only one cable has to enter the frame, so a small DC-DC buck converter module should do the trick.
  • Software updates as necessary. I've not modified the software since the last blog post about this, so I need to add in the PIR module (so that the screen isn't always on) and make a couple of other improvements.
  • Switch out the Raspberry Pi 2 for a Raspberry Pi 3 for a performance boost and to save using dongles for bluetooth and wifi.
  • Figure out a way of turning the LCD backlight on and off (probably a relay or optoisolator?) so that it is completely black and inconspicuous when in "mirror mode".

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